Summary
# The basics of animal cells and cell division
This section provides a foundational understanding of animal cell structures, distinguishes between prokaryotic and eukaryotic cells, and details the processes of mitosis and meiosis.
### 1.1 Basic animal cell components and cell types
The development of a cell membrane was a crucial event in cell formation, enabling the containment of cellular components within an aqueous solution, separated from the external environment. This membrane is composed of molecules with both fat-soluble and water-soluble parts. Animal cells contain various organelles, which are discussed in detail. A primary distinction in cell types is made between prokaryotic and eukaryotic cells. The term "karyon" is derived from Greek and means "kernel" or "nucleus". Thus, prokaryotic cells are without a nucleus, while eukaryotic cells possess one where genetic material (DNA) is stored [5](#page=5).
#### 1.1.1 Prokaryotic vs. eukaryotic cells
The DNA in prokaryotic cells is located in the cytoplasm, whereas in eukaryotic cells, it is segregated within the nucleus. Metabolic reactions (anabolic and catabolic) occur in the cytoplasm and are regulated by various organelles [6](#page=6).
| Feature | Prokaryotes | Eukaryotes |
|---|---|---|
| Organisms | Bacteria, cyanobacteria | Protists, fungi, plants, animals |
| Cell size | 1 to 10 µm | 5 to 100 µm |
| Metabolism | Aerobic and anaerobic | Aerobic |
| Organelles | Few to none (nucleus, mitochondria, chloroplasts, endoplasmic reticulum are absent) | Nucleus, mitochondria, chloroplasts, endoplasmic reticulum are present |
| DNA | Circular, in cytoplasm | Linear, in nucleus |
| RNA and protein synthesis | Occurs in cytoplasm | DNA to RNA in nucleus; RNA to protein in cytoplasm |
| Cytoplasm | No cytoskeleton | Cytoskeleton composed of protein filaments |
One of the most important organelles in animal cells is the mitochondrion, responsible for energy production through the synthesis of adenosine triphosphate (ATP) from oxygen and nutrients like sugars and fats. Mitochondria are surrounded by a double membrane, distinct in composition from the nuclear membrane. The widely accepted theory for the origin of mitochondria is endosymbiosis, involving the engulfment of a bacterial cell, given their similar size. Chloroplasts, involved in photosynthesis, are analogous structures but are found only in plant cells [6](#page=6) [7](#page=7).
The nucleus is another characteristic organelle, enclosed by a double membrane called the nuclear envelope, separating its contents from the cytoplasm. The Golgi apparatus in animal cells modifies, transports, and secretes macromolecules, functioning as a collection of membrane structures. The endoplasmic reticulum (ER) is also a network of membrane structures, often an extension of the nuclear membrane. Rough ER, studded with ribosomes, is involved in the synthesis of membrane proteins [7](#page=7) [8](#page=8).
Key differences between animal and plant cells include:
* **Cell wall:** Absent in animal cells; present in plant cells (made of cellulose) [8](#page=8).
* **Centrosome:** Present in animal cells (involved in spindle formation during cell division); absent in plant cells (though analogous structures called polar caps may exist) [8](#page=8).
* **Plastids:** Absent in animal cells; present in plant cells (e.g., chloroplasts, chromoplasts, amyloplasts) [8](#page=8).
### 1.2 Nuclear division
Cell division involves the splitting of a cell into two daughter cells, with organelles distributed between them. However, nuclear material must be distributed identically to ensure each daughter cell receives the same genetic information. This is achieved through nuclear division, which occurs via two types: mitosis and meiosis [9](#page=9).
#### 1.2.1 Mitosis
Mitosis occurs in somatic (body) cells and results in an exact copy of the nuclear material. Prior to division, the centrosome duplicates, and these centrosomes migrate to opposite poles, forming a network of fibers that will move chromosomes. During this phase, the nuclear membrane dissolves, and chromosomes become visible, consisting of tightly coiled DNA strands bound to proteins (histones). Each chromosome is composed of two identical arms, called chromatids, joined at the centromere. This preparatory phase is called **prophase** [9](#page=9).
* **Metaphase:** The spindle apparatus is fully formed, and all chromosomes align at the cell's equatorial plane [9](#page=9).
* **Anaphase:** Centromeres split, and each chromatid is pulled towards opposite poles of the cell [9](#page=9).
* **Telophase:** The spindle apparatus disappears, and nuclear envelopes form around the two sets of daughter nuclei. The cell membrane begins to constrict, initiating cytokinesis. Each daughter cell receives an identical number of chromatids, allowing for the formation of a complete chromosome in each. Chromosomes decondense back into chromatin [10](#page=10) [9](#page=9).
> **Tip:** Chromosomes are only visible during cell division; at other times, the DNA exists as uncondensed chromatin [9](#page=9).
#### 1.2.2 Meiosis
Meiosis is a reductional division occurring in germ cells (for sexual reproduction) where the chromosomal number is halved. In somatic cells, chromosomes exist in homologous pairs, making these cells diploid ($2n$). A diploid cell contains $n$ paternal chromosomes and $n$ maternal chromosomes. The daughter cells resulting from meiosis contain only one chromosome from each homologous pair, making them haploid ($n$). This halving is essential for sexual reproduction, as the fusion of male and female gametes restores the diploid number in the fertilized egg [10](#page=10) [11](#page=11) [9](#page=9).
Meiosis is divided into Meiosis I and Meiosis II, with each phase further divided into prophase, metaphase, anaphase, and telophase. Meiosis I is where the reduction in chromosome number occurs, while Meiosis II is analogous to mitosis [11](#page=11).
##### 1.2.2.1 Meiosis I
* **Prophase I:** The spindle apparatus and chromosomes form, and the nuclear envelope disappears. Homologous chromosomes align opposite each other at the equatorial plane. During this stage, **crossing-over** can occur, which is the exchange of DNA between homologous chromosomes, resulting in non-identical chromatids and genetic recombination [11](#page=11).
* **Metaphase I:** All homologous chromosome pairs are aligned at the equatorial plane. The orientation of these pairs is random, meaning the distribution of maternal and paternal chromosomes relative to the equator is not fixed [11](#page=11).
* **Anaphase I:** Homologous chromosomes are pulled to opposite poles of the cell (unlike mitosis, where chromatids separate) [12](#page=12).
* **Telophase I:** Nuclear envelopes reform, and cytokinesis begins through cell constriction [12](#page=12).
> **Example:** Crossing-over in Prophase I leads to genetic diversity by creating new combinations of alleles on the chromatids [11](#page=11).
##### 1.2.2.2 Meiosis II
Meiosis II follows Meiosis I.
* **Prophase II:** The spindle apparatus and chromosomes form, and the nuclear envelope disappears [12](#page=12).
* **Metaphase II:** Chromosomes align at the equatorial plane [12](#page=12).
* **Anaphase II:** Chromatids of each chromosome separate and are pulled to opposite sides of the cell by the centrosomes [12](#page=12).
* **Telophase II:** Chromosomes decondense into chromatin, and nuclear envelopes form. In each daughter cell, a copy of each chromatid is made, so each chromosome once again has two chromatids [12](#page=12).
The outcome of meiosis is the production of four genetically distinct haploid cells from one diploid cell [13](#page=13).
---
# Classification of the animal kingdom
This section delves into the principles of zoological systematics, encompassing taxonomy, diagnostics, classification, and nomenclature, while exploring various historical and modern kingdom classification systems, including cladistics [16](#page=16).
### 2.1 Zoological systematics
Zoological systematics is the study of the classification of the animal kingdom. It comprises several subdisciplines [16](#page=16):
#### 2.1.1 Taxonomy
Taxonomy is the study of hierarchical units, or taxonomic categories, such as species, genus, and class, which are collectively termed taxa. In zoology, the standard hierarchical categories are [16](#page=16):
* **Kingdom:** Animalia
* **Phylum:** Chordata
* **Class:** Mammalia
* **Order:** Lagomorpha
* **Family:** Leporidae
* **Genus:** *Oryctolagus*
* **Species:** *cuniculus*
A taxonomist determines the appropriate taxonomic rank for a group of organisms (e.g., placing mammals within the "Class" taxon) based on their characteristics and value within the hierarchical system [16](#page=16).
#### 2.1.2 Diagnostics
Diagnostics is the science of describing organisms [16](#page=16).
#### 2.1.3 Classification
Classification is the arrangement of taxa according to their degree of relatedness, based on shared characteristics. The concept that extinction only separates groups, rather than creating new ones, implies that a natural classification would be possible if all extinct forms reappeared, though distinguishing subtle variations would be challenging [16](#page=16).
#### 2.1.4 Nomenclature
Nomenclature is the system of rationally naming organisms, with the aim of assigning scientific (Latin) names. The nomenclature for living organisms is binomial, consisting of a genus name and a species name. For example, a dog is scientifically named *Canis familiaris*, a universally understood designation, unlike its common names in different languages (e.g., "dog," "cane," "Hund"). Scientific names are always written in italics, with only the genus name capitalized. The name of the author who first described the species may follow, not in italics, often abbreviated (e.g., L. for Linnaeus). When a specific species within a genus is not identified, "spp." (not italicized) is used [17](#page=17).
> **Example:**
> * *Anguilla anguilla* Linnaeus
> * *Canis familiaris* L.
> * *Hippocampus europaeus* Ginsberg
> * *Canis spp.* indicates an unspecified species within the genus *Canis* [17](#page=17).
### 2.2 Kingdom classification systems
Throughout history, various systems have been proposed for classifying living organisms into kingdoms:
#### 2.2.1 Two-kingdom system
This early system divided life into two kingdoms: Plants and Animals [17](#page=17).
#### 2.2.2 Three-kingdom system
The two-kingdom system struggled to classify unicellular organisms (protists) as the distinction between plants and animals was often unclear. Ernst Haeckel proposed a third kingdom, Protista, for these single-celled organisms [17](#page=17).
#### 2.2.3 Four-kingdom system
In the three-kingdom system, bacteria and blue-green algae were grouped with protists. However, due to significant cellular differences, a new kingdom, Monera (for bacteria and blue-green algae), was established, separating them from the lower eukaryotes (Protista) [17](#page=17).
#### 2.2.4 Five-kingdom system (Whittaker, 1969)
Robert Whittaker's five-kingdom system, proposed in 1969, was based on differences in feeding strategies among higher eukaryotes [18](#page=18).
* **Animals:** Ingest food through a mouth and process it in a digestive system [18](#page=18).
* **Plants:** Obtain nourishment through photosynthesis [18](#page=18).
* **Fungi:** Absorb organic food from their environment [18](#page=18).
#### 2.2.5 Cladistics
With the advent of DNA analysis, it became possible to determine evolutionary relationships between organisms. This method of classifying species based on their relatedness is called cladistics. Cladistics utilizes cladograms, which are graphical representations of evolutionary lineages. This approach can challenge traditional classifications based on morphology, as seen with birds, which share a common ancestor with reptiles [18](#page=18).
> **Tip:** Cladistics emphasizes evolutionary descent and shared derived characteristics (synapomorphies) to define groups, which can lead to reclassifications compared to older systems.
Classification in cladistics aims to group organisms into branching lineages based on shared characteristics. However, this can lead to the formation of paraphyletic groups, which do not include all descendants of a common ancestor. The class Reptilia is an example of a paraphyletic group because it excludes birds, despite their shared ancestry [18](#page=18) [19](#page=19).
> **Example:** A cladogram might show birds branching off from within the reptile lineage, suggesting they should not be classified as a separate class if strictly adhering to monophyletic groups [19](#page=19).
#### 2.2.6 Modern Classification Systems (e.g., Catalogue of Life)
Since 2001, the publicly accessible Catalogue of Life (CoL) database has been mapping the evolutionary relationships of all living organisms, containing 1.6 million names and the work of 3000 taxonomists. As CoL does not strictly adhere to Linnaean taxa, an overarching classification system based on CoL was needed. The classification used in this course is based on Ruggiero et al. starting with two super-kingdoms (or domains): Eukaryotes and Prokaryotes. Within these, there are seven kingdoms: Prokaryotes are divided into Archaebacteria and Bacteria. The five eukaryotic kingdoms are Protozoa, Chromista, Fungi, Plantae, and Animalia. This course focuses on the animal and protozoa kingdoms, and some chromista [19](#page=19) .
---
# Invertebrate phyla and their characteristics
This section provides a comprehensive overview of various invertebrate phyla, detailing their general characteristics, classification, structure, life cycles, and notable examples.
### 3.1 Unicellular organisms (Protozoa and Chromista)
Unicellular organisms, or protozoa, consist of a single cell, which typically possesses one nucleus, though multinucleated forms exist. These organisms can be free-living in various aquatic and soil environments, or they can be parasitic. Unicellular organisms containing chloroplasts are classified as plants, not protozoa. However, some unicellular organisms acquire organelles through symbiosis with other eukaryotes, enabling them to perform photosynthetic activity without possessing chloroplasts themselves; these are classified under Chromista. The Chromista also includes unicellular organisms that have lost their organelles and, consequently, their photosynthetic capabilities [20](#page=20).
#### 3.1.1 Relevant phyla within Protozoa and Chromista
* **Ciliophora (Ciliates):** Characterized by cilia for locomotion. Examples include the slipper animalcule and kidney bean animalcule [20](#page=20).
* **Amoebozoa (Amoebas):** Move using cytoplasmic extrusions called pseudopodia. Many species possess an internal or external skeleton. Examples include *Amoeba* and *Arcella* [20](#page=20) [30](#page=30).
* **Euglenozoa (Flagellates):** Move using one or two long flagella. Heterotrophic flagellates are considered part of the animal kingdom. Examples include *Trypanosoma* [20](#page=20) [31](#page=31).
* **Metamonada:** Lack mitochondria but possess grouped flagella, typically in fours. They are thought to have lost mitochondria during evolution, with evidence of mitochondrial-related organelles. Examples include *Giardia duodenalis* and *Trichomonas gallinea* [20](#page=20) [33](#page=33) [37](#page=37).
* **Sporozoa (Miozoa):** Parasitic organisms with complex life cycles, forming spores at some stage. Examples include *Eimeria spp.* and *Toxoplasma gondii* [20](#page=20) [38](#page=38) [42](#page=42).
#### 3.1.2 Slipper animalcule (*Paramecium spp.*)
*Paramecium* species are commonly found in water containing decaying plant material, feeding on bacteria involved in decomposition [21](#page=21).
* **Structure and Locomotion:** *Paramecium* has an oval shape, with the anterior end being more rounded and the posterior end more pointed. The ventral side features a oral groove leading to the cytostome (cell mouth). The cytoplasm is enclosed by a pellicle, a complex membrane structure. The entire organism is covered in cilia, arranged in oblique rows, which beat rhythmically for locomotion and can also be used for backward movement. Internally, it possesses a large macronucleus (regulating metabolic processes) and a small micronucleus (involved in sexual reproduction) [21](#page=21) [22](#page=22).
* **Feeding and Digestion:** Food particles, such as bacteria and other small organisms, are directed by cilia in the oral groove towards the cytostome and then into a cytopharynx. Food is ingested into food vacuoles, which circulate within the cytoplasm, where digestion occurs via enzymes secreted into the vacuoles. Undigested waste is eliminated through specific points on the pellicle called cytoprocts [22](#page=22) [23](#page=23).
* **Respiration:** Oxygen diffuses directly into the cytoplasm from the surrounding water across the plasmalemma. Carbon dioxide exits the cell in the same manner [23](#page=23).
* **Excretion:** Excess water enters the cell by osmosis, and contractile vacuoles regulate water balance by periodically expelling the fluid through a pore in the pellicle. Nitrogenous waste products also diffuse out of the cell [23](#page=23) [24](#page=24).
* **Nervous System:** While lacking a true nervous system, *Paramecium* exhibits coordinated responses to stimuli, such as avoiding obstacles and certain chemicals, suggesting a rudimentary coordination mechanism involving interconnected bases of cilia [24](#page=24) [25](#page=25).
* **Reproduction:** Reproduction can be asexual (binary fission) or sexual (conjugation). Asexual reproduction involves the division of nuclei and the cell, with the macronucleus dividing amitotically and the micronucleus mitotically. Conjugation involves the exchange of nuclear material between two individuals, leading to genetic recombination and the formation of new individuals [25](#page=25) [26](#page=26).
* **Encystment:** Under unfavorable conditions, *Paramecium* can encyst, forming a protective shell, allowing it to survive harsh environments until favorable conditions return [26](#page=26).
#### 3.1.3 Other ciliates
Other examples of ciliates include kidney animalcules (*Colpoda spp.*), *Colpiudium spp.*, *Coleps spp.*, *Lacrymaria olor*, *Litonotus cygnus*, stylonychia (*Stylonichia spp.*), *Spirostomum spp.*, trumpet animalcules (*Stentor spp.*), and bell animalcules (*Vorticella spp.*). Bell animalcules are sessile and attach to substrates via a stalk that can retract spirally. Trumpet animalcules are commensals in the digestive tracts of ruminants and other ungulates [27](#page=27).
#### 3.1.4 Amoebas (*Amoebozoa*)
Amoebas are characterized by their constantly changing shape and the formation of lobed or thread-like extrusions called pseudopodia, used for locomotion and feeding [28](#page=28).
* **Structure and Locomotion:** The cell is a form of cytoplasm enclosed by a plasmalemma. It contains ectoplasm (clear) and endoplasm (granular), with organelles such as the nucleus, food vacuoles, mitochondria, and contractile vacuoles within the endoplasm. Amoeboid movement involves the extension and anchoring of pseudopodia, with the rest of the cell flowing forward [28](#page=28) [29](#page=29).
* **Feeding and Digestion:** Amoebas engulf prey (bacteria, algae, other protozoa) through phagocytosis, forming food vacuoles where digestion occurs. Undigested material is expelled via the cytoproct [29](#page=29).
* **Respiration and Excretion:** Oxygen diffuses across the plasmalemma for respiration. Carbon dioxide exits the cell similarly. Water balance is regulated by contractile vacuoles [29](#page=29).
* **Reproduction and Encystment:** Reproduction is by simple binary fission. Under unfavorable conditions, amoebas encyst, forming a protective shell to survive [30](#page=30).
#### 3.1.5 Other Amoeboids
Many naked amoebas are free-living, but some are parasitic, inhabiting the digestive tract of various animals. *Entamoeba coli* is a harmless intestinal inhabitant, while *Entamoeba histolytica* can cause amoebic dysentery. Shelled amoebas, such as *Arcella vulgaris* and *Difflugia elegans*, secrete a shell, often composed of silica [30](#page=30) [31](#page=31).
#### 3.1.6 Flagellates (*Euglenozoa*)
Flagellates use one or two long flagella for locomotion. Photosynthetic flagellates, like *Euglena spp.*, are classified as plants. Heterotrophic flagellates are considered part of the animal kingdom [31](#page=31).
* ***Trypanosoma spp.:*** These flagellates are parasitic in the blood of vertebrates, often transmitted by biting insects. Species like *T. brucei gambiense* and *T. brucei rhodesiense* cause African sleeping sickness in humans, transmitted by the tsetse fly. The disease progresses through stages affecting the nervous system and is often fatal if untreated. *Trypanosoma* has a spindle-shaped body with a undulating membrane formed by the flagellum running along its side [31](#page=31) [32](#page=32).
* **Other Flagellates:** *Leishmania donovani*, transmitted by sandflies (*Phlebotomus spp.*), causes kala-azar (black fever) in humans, a chronic and potentially fatal disease [33](#page=33).
#### 3.1.7 Metamonada
This group of unicellular organisms lacks mitochondria but possesses grouped flagella, typically in fours. Evidence suggests they once had mitochondria, which were lost during evolution, supported by the presence of mitochondrial-related organelles and genetic remnants [33](#page=33).
* ***Giardia duodenalis:*** A common intestinal parasite in humans and other vertebrates, it is a major cause of parasitic diarrhea worldwide. Infection occurs through the ingestion of resistant cysts. *Giardia* has two forms: the motile trophozoite, which attaches to intestinal cells, and the infective cyst. Reproduction is asexual (binary fission). A unique feature is its semi-open mitosis [33](#page=33) [34](#page=34) [35](#page=35) [36](#page=36).
* ***Trichomonas gallinae:*** This parasite causes "canker" or "frounce" in birds, forming a yellowish, cheesy substance in the throat. Unlike *Giardia*, it does not form cysts and cannot survive in the environment. It possesses a characteristic cytoskeleton, including an axostyle, pelta, and costa, and has four anterior flagella and a recurrent flagellum with an undulating membrane [37](#page=37).
#### 3.1.8 Sporozoans (*Miozoa, superclass Sporozoa*)
Sporozoans are obligate parasites with complex life cycles, often involving spore formation [38](#page=38).
* ***Eimeria spp.:*** These coccidian parasites cause coccidiosis in various animals, notably chickens and rabbits, leading to significant economic losses due to reduced nutrient absorption and illness. Their life cycle involves asexual reproduction (schizogony) forming merozoites and sexual reproduction forming gametes, culminating in zygote and oocyst formation [38](#page=38) [39](#page=39) [40](#page=40) [41](#page=41).
* ***Toxoplasma gondii:*** A globally distributed parasite with cats as definitive hosts, it infects warm-blooded animals as intermediate hosts, including humans, causing toxoplasmosis. Infection during pregnancy can have severe consequences for the fetus. The life cycle involves sexual reproduction in cats and asexual reproduction in intermediate hosts, forming tachyzoites and bradyzoites within tissue cysts [42](#page=42) [43](#page=43).
* ***Plasmodium spp.:*** These protozoans cause malaria in humans, requiring both a mosquito (Anopheles) and a human host for their life cycle. Transmission occurs via mosquito bites, with sporozoites infecting the liver and then red blood cells, causing cyclical fevers upon lysis. The life cycle involves both asexual reproduction (schizogony) and sexual reproduction (gametogony) in different hosts [44](#page=44) [45](#page=45).
### 3.2 Sponges (*Porifera*)
Sponges are sessile, invertebrate animals with a simple body plan consisting of a mass of cells rather than true tissues. They lack specialized organ systems like muscles or nervous systems [46](#page=46).
* **General Characteristics:** Sponges have a porous body structure that facilitates water flow, bringing in food and oxygen while expelling waste. They have no defined front or back, mouth, or anus [46](#page=46) [47](#page=47).
* **Classification:** Sponges are classified into three classes based on their skeletal spicules:
* **Calcarea (Calcareous sponges):** Possess spicules made of calcium carbonate, typically found in shallow, warm coastal waters [46](#page=46).
* **Hexactinellida (Glass sponges):** Have spicules fused into a silica skeleton, often found at great depths [46](#page=46).
* **Demospongiae (Common or horn sponges):** Have a skeleton of spongin fibers and are often colorful. This class also includes freshwater sponges [46](#page=46).
* **Structure:** The simplest sponges have a sac-like form with an osculum (excurrent opening) and numerous pores (formed by porocytes) for water intake. The inner surface is lined with choanocytes (collar cells), each with a flagellum surrounded by a collar, which create the water current and capture food particles. Food particles are transferred to amoebocytes (wandering cells) for digestion and distribution. The skeletal support comes from spicules made of calcium or silica, formed by scleroblasts. The natural sponge used for household purposes is the spongin skeleton [47](#page=47) [48](#page=48).
* **Life Cycle and Reproduction:** Sponges are hermaphroditic, producing both sperm and eggs. Fertilization is external, with a free-swimming parenchymula larva developing from the zygote. The larva eventually settles and metamorphoses into a sessile adult sponge [49](#page=49) [50](#page=50).
* **Peculiarities:** Some sponge species exhibit commensalism, such as the Venus flower basket (*Euplectella aspergillum*), which houses two young shrimp (*Spongicola venusta*) within its body cavity [50](#page=50).
### 3.3 Cnidarians (Coelenterates)
Cnidarians are relatively simple aquatic animals characterized by a radially symmetrical body with a gastrovascular cavity and a single opening serving as both mouth and anus. They possess tentacles armed with stinging cells (cnidocytes) for capturing prey. Cnidarians exist in two forms: the sessile polyp and the free-swimming medusa (jellyfish) [51](#page=51) [53](#page=53).
* **Classification:**
* **Hydrozoa (Hydroids):** Typically exhibit alternation of polyp and medusa generations. Examples include *Obelia spp.* and the freshwater polyp (*Hydra spp.*), which only exists in the polyp form [51](#page=51) [52](#page=52).
* **Scyphozoa (True jellyfish):** Primarily exist as large medusae with a prominent mesoglea. Examples include the moon jelly (*Aurelia aurita*) [51](#page=51) [60](#page=60).
* **Anthozoa (Anemones and Corals):** Lack a medusa stage and exist only as polyps. Sea anemones have soft bodies, while corals secrete a calcium carbonate skeleton [51](#page=51) [60](#page=60) [61](#page=61).
#### 3.3.1 Freshwater polyp (*Hydra spp.*)
*Hydra* are cylindrical freshwater organisms, typically 10-15 mm in length, attached to a substrate by a foot at one end and possessing 4-8 tentacles around the mouth at the other end [52](#page=52).
* **Structure:** The body wall consists of three layers: the ectoderm (outer layer), endoderm (inner layer), and mesoglea (support layer) between them. The ectoderm contains cnidocytes, each housing a coiled, harpoon-like cnidocil that discharges a venomous thread upon stimulation. The endoderm contains cells for phagocytosis and gland cells that secrete digestive enzymes into the gastrovascular cavity [53](#page=53) [54](#page=54).
* **Locomotion:** *Hydra* can change shape and move slowly by somersaulting or inching along a substrate [54](#page=54) [55](#page=55).
* **Feeding and Digestion:** Prey is paralyzed by cnidocytes and brought to the mouth by tentacles. Digestion begins in the gastrovascular cavity with secreted enzymes, and food particles are absorbed by endodermal cells via phagocytosis. Undigested waste is expelled through the mouth [55](#page=55).
* **Reproduction:** Reproduction can be asexual (budding) or sexual. Sexual reproduction occurs under unfavorable conditions, with the formation of either sperm or eggs. Fertilization is external, and the zygote develops into a protected embryo within a capsule [55](#page=55) [56](#page=56).
* **Regeneration:** *Hydra* exhibits remarkable regenerative capabilities, with each fragment capable of regenerating into a complete organism [57](#page=57).
#### 3.3.2 Sea-fringe (*Obelia spp.*)
*Obelia* colonies are sessile and typically exhibit alternation of generations, with a polyp stage forming the colony and a medusa stage for reproduction [57](#page=57) [58](#page=58).
* **Structure:** The polyp colony has a plant-like structure with a base (hydrorhiza), a vertical stem (hydrocaulus), and branches. The colony is covered by a protective chitinous cuticle (perisarc). Two types of polyps are present: hydrants (feeding polyps) with tentacles and gonangia (reproductive polyps) that produce medusae [57](#page=57) [58](#page=58).
* **Reproduction:** Medusae are typically unisexual, with external fertilization occurring in the water. The resulting planula larva settles and develops into a new polyp colony [59](#page=59).
#### 3.3.3 Other Cnidarians
* **True Jellyfish (Scyphozoa):** These are typically large jellyfish with a well-developed mesoglea. Examples include the moon jelly (*Aurelia aurita*) [60](#page=60).
* **Anemones (Anthozoa):** Sea anemones are solitary polyps, often attached to substrates. They have a muscular foot for slow movement and numerous tentacles with cnidocytes. Some anemones engage in symbiotic relationships, such as with hermit crabs [61](#page=61) [62](#page=62).
* **Corals (Anthozoa):** Corals are colonial polyps that secrete a hard calcium carbonate skeleton, forming reefs. Examples include stony corals and precious coral (*Corallium rubrum*) [61](#page=61) [62](#page=62).
### 3.4 Flatworms (*Platyhelminthes*)
Flatworms are bilaterally symmetrical, unsegmented worms that are typically dorsoventrally flattened. They lack a coelom and a circulatory system, with gas exchange occurring across the body surface. Many are hermaphroditic and parasitic [63](#page=63).
* **Classification:**
* **Turbellaria (Free-living flatworms):** Many are aquatic and possess cilia for movement. Examples include planarians (*Planaria spp.*) [63](#page=63) [64](#page=64).
* **Trematoda (Flukes):** Parasitic flatworms, often with suckers for attachment, with complex life cycles involving intermediate hosts. The liver fluke (*Fasciola hepatica*) is a classic example [63](#page=63) [67](#page=67).
* **Cestoda (Tapeworms):** Long, ribbon-like intestinal parasites with a scolex (head) for attachment and proglottids (segments) containing reproductive organs. The beef tapeworm (*Taenia saginata*) is a human parasite with cattle as an intermediate host [63](#page=63) [73](#page=73).
#### 3.4.1 Turbellarians (e.g., Planarians)
Planarians are free-living flatworms found in freshwater and marine environments. They are characterized by their flattened bodies, the presence of eyespots, and a muscular pharynx that can be extended for feeding [64](#page=64) [65](#page=65).
* **Structure and Locomotion:** Their bodies are covered in cilia and a mucus layer, facilitating gliding movement. Larger species also use muscle contractions for locomotion [64](#page=64) [65](#page=65).
* **Feeding and Digestion:** Planarians are scavengers, feeding on dead organic matter. They possess a three-branched gastrovascular cavity, and food is ingested through a ventral mouth and pharynx. Digestion occurs extracellularly, and nutrients are absorbed by cells in the gut lining via phagocytosis [65](#page=65) [66](#page=66).
* **Regeneration:** Planarians have an exceptional capacity for regeneration, with fragments capable of developing into complete individuals [64](#page=64).
#### 3.4.2 Flukes (*Trematoda*)
Flukes are parasitic flatworms with suckers for attachment and complex life cycles involving at least one intermediate host, typically a mollusk [67](#page=67).
* ***Fasciola hepatica* (Liver fluke):** This fluke infects the liver of herbivores, primarily sheep and cattle. Its life cycle involves a freshwater snail as the intermediate host, progressing through miracidium, sporocyst, redia, and cercaria larval stages before encysting as metacercaria on vegetation, which is then ingested by the definitive host [67](#page=67) [68](#page=68) [69](#page=69).
* ***Schistosoma spp.:*** These flukes infect humans, causing schistosomiasis. They inhabit blood vessels of the intestines or bladder and have separate sexes. The life cycle involves freshwater snails as intermediate hosts, with cercariae penetrating the human skin to initiate infection [71](#page=71) [72](#page=72).
#### 3.4.3 Tapeworms (*Cestoda*)
Tapeworms are endoparasites of vertebrates, lacking a digestive system and absorbing nutrients directly through their body surface [73](#page=73).
* ***Taenia saginata* (Beef tapeworm):** This tapeworm infects humans, with cattle serving as intermediate hosts. The adult worm consists of a scolex for attachment and numerous proglottids containing reproductive organs. Infection occurs by consuming undercooked beef containing cysticerci (bladder worms) [73](#page=73) [74](#page=74) [75](#page=75).
* ***Taenia solium* (Pork tapeworm):** Similar to the beef tapeworm, but pigs are the intermediate hosts. Humans can also act as intermediate hosts, developing cysticerci in their tissues [77](#page=77).
* ***Dipylidium caninum* (Dog tapeworm):** Fleas are the intermediate hosts for this tapeworm, which infects dogs and cats [77](#page=77).
* ***Echinococcus granulosus* (Fox tapeworm):** Humans can be intermediate hosts, developing large hydatid cysts containing numerous daughter cysts and protoscoleces, leading to echinococcosis [78](#page=78).
### 3.5 Nematodes (*Nematoda*)
Nematodes are unsegmented, roundworms with a tough cuticle that must be molted for growth. They possess a complete digestive system and separate sexes. Many are free-living in soil and water, while others are parasitic to plants, animals, and humans [79](#page=79) [80](#page=80).
* **General Characteristics:** Nematodes are characterized by a cylindrical, unsegmented body covered in a thick, non-cellular cuticle that is periodically molted. They have a pseudocoelom and a complete digestive tract [79](#page=79).
* **Free-living Forms:** Soil nematodes play a crucial role in nutrient cycling, breaking down organic matter and making nutrients available to plants [80](#page=80).
* **Parasitic Forms:**
* ***Ascaris lumbricoides* (Human roundworm):** A large intestinal parasite of humans, transmitted by ingesting infective eggs. Larval migration through the lungs can cause respiratory symptoms [80](#page=80) [81](#page=81) [82](#page=82).
* ***Enterobius vermicularis* (Pinworm):** Primarily infects children, with females laying eggs around the anus, causing itching [82](#page=82).
* ***Toxocara canis* (Dog roundworm):** Puppies are primarily infected via the placenta or milk. Humans can be paratenic hosts [83](#page=83).
* ***Trichinella spiralis*:** Causes trichinellosis, a serious disease contracted by consuming undercooked meat containing larvae encysted in muscle [83](#page=83) [84](#page=84).
* **Plant Parasites:** Eelworms, such as the potato cyst nematode (*Globodera rostochiensis*), parasitize plant roots, causing significant crop damage [85](#page=85).
### 3.6 Annelids (*Annelida*)
Annelids are segmented worms with a coelom and a circulatory system. Segmentation is evident both externally and internally, with each segment containing coelomic fluid and organs [87](#page=87) [91](#page=91).
* **Classification:**
* **Clitellata:** Possess a clitellum (saddle) for reproduction.
* **Oligochaeta (Earthworms):** Have few or no setae (bristles), are mostly terrestrial or freshwater. Example: *Lumbricus terrestris* [87](#page=87) [88](#page=88).
* **Hirudinea (Leeches):** Flattened bodies, lack setae, and possess suckers for locomotion and feeding. Example: *Hirudo medicinalis* [87](#page=87).
* **Polychaeta (Bristle worms):** Have numerous bristles (chaetae) on fleshy appendages called parapodia, are predominantly marine, and often live in burrows or tubes. Example: lugworm (*Arenicola marina*) [87](#page=87).
#### 3.6.1 Earthworms (*Oligochaeta*)
Earthworms, like *Lumbricus terrestris*, are segmented worms important for soil aeration and nutrient cycling [88](#page=88) [89](#page=89).
* **Structure and Locomotion:** They have a cylindrical body with a pointed anterior and blunt posterior end. Each segment bears setae (bristles) used for anchoring and locomotion. Movement is achieved by the coordinated action of circular and longitudinal muscles and the body fluid within the coelom [89](#page=89) [97](#page=97) [98](#page=98).
* **Digestive System:** A complete digestive tract includes a mouth, pharynx, esophagus with calciferous glands, crop for storage, gizzard for grinding, and intestine. Chloragogen cells in the intestine store nutrients and play a role in detoxification [91](#page=91) [92](#page=92).
* **Circulatory and Respiratory Systems:** Annelids have a closed circulatory system with dorsal and ventral blood vessels and "hearts" (muscular vessels). Gas exchange occurs through the moist body surface via capillaries in the skin. Blood contains respiratory pigments like erythrocruorin [93](#page=93) [94](#page=94).
* **Nervous System:** A simple nervous system consists of a brain (cerebral ganglia), a ventral nerve cord with segmental ganglia, and sensory cells in the epidermis [95](#page=95) [96](#page=96).
* **Reproduction:** Earthworms are hermaphroditic, with cross-fertilization occurring between two individuals. A clitellum produces a mucus cocoon that receives eggs and sperm, where fertilization and development take place [99](#page=99).
#### 3.6.2 Leeches (*Hirudinea*)
Leeches are segmented annelids, often flattened, lacking setae but possessing suckers for locomotion and feeding. They are hermaphroditic and play a role in aquatic ecosystems .
* **Structure:** Leeches have a fixed number of segments though external annulation may not correspond to internal segmentation. Anterior and posterior suckers facilitate movement and attachment [33](#page=33).
* **Feeding:** Many leeches are blood-sucking, secreting hirudin to prevent blood clotting. They require symbiotic bacteria in their gut to digest blood .
* **Locomotion:** Movement is typically by an inching motion using the suckers, similar to a caterpillar .
#### 3.6.3 Bristle worms (*Polychaeta*)
Polychaetes are marine annelids characterized by numerous bristles on parapodia, used for locomotion and respiration. Many live in tubes or burrows [87](#page=87).
* ***Arenicola marina* (Lugworm):** Lives in U-shaped burrows on the coast, ingesting sand and organic matter. They have external gills for respiration .
* **Other Polychaetes:** Examples include the velvet worm (*Aphrodita aculeata*) and the palolo worm (*Eunice spp.*), known for its synchronized spawning behavior .
### 3.7 Mollusks (*Mollusca*)
Mollusks are a diverse phylum characterized by a soft body, typically protected by a calcareous shell. They exhibit a wide range of forms, from snails to clams to squids .
* **Classification:**
* **Bivalvia (Bivalves):** Possess a laterally compressed body enclosed by a two-part shell. Examples: mussels (*Mytilus edulis*), clams (*Cardium edule*), oysters .
* **Gastropoda (Snails):** Typically have a coiled shell, a distinct head with tentacles and eyes, and a radula for feeding. Examples: garden snail (*Cepaea nemoralis*), slug (*Arion rufus*), whelk (*Buccinum undatum*) .
* **Cephalopoda (Cephalopods):** Marine mollusks with a ring of arms around the mouth. Examples: squid, octopus, cuttlefish (*Sepia officinalis*) .
* **Polyplacophora (Chitons):** Marine mollusks with an eight-part shell .
* **Scaphopoda (Tusk shells):** Marine mollusks with a tusk-shaped shell .
#### 3.7.1 Bivalves (e.g., Mussel)
Bivalves are aquatic mollusks adapted for burrowing or attachment, feeding on plankton filtered from the water .
* **Structure:** The body is laterally flattened and enclosed by two hinged shells, held together by an elastic ligament and closed by adductor muscles. They lack a distinct head, with mantle lobes performing sensory functions. Gills are used for respiration and filter feeding .
* **Shell:** The shell is composed of three layers: the periostracum (outer protein layer), the prismatic layer (calcium carbonate crystals), and the nacreous layer (mother-of-pearl) .
* **Feeding and Respiration:** Water circulates through the mantle cavity, bringing oxygen and food particles to the gills. Filtered food is directed to the mouth via labial palps .
* **Circulation and Nervous System:** Mollusks have an open circulatory system with a heart and blood sinuses. The nervous system is decentralized with three pairs of ganglia .
* **Reproduction:** Most bivalves are unisexual with external fertilization, producing trochophore and veliger larvae. Freshwater mussels have a parasitic glochidium larval stage on fish .
* **Locomotion and Attachment:** Bivalves can burrow using their muscular foot or attach to substrates with byssal threads secreted by a byssal gland .
#### 3.7.2 Snails (*Gastropoda*)
Snails are characterized by a coiled shell and a muscular foot for locomotion. They possess a radula for scraping food and have adapted to various environments, including terrestrial ones .
* **Structure:** The body undergoes torsion, rotating the visceral mass and mantle cavity 180 degrees relative to the head and foot. Terrestrial snails have a lung (modified mantle cavity) for aerial respiration, connected to the outside by a pneumostome .
* **Feeding and Digestion:** Snails use a radula to scrape food, and their digestive system includes salivary glands, a esophagus, crop, stomach, and intestine. The hepatopancreas aids in digestion and nutrient absorption .
* **Reproduction:** Most snails are hermaphroditic, with cross-fertilization common. They produce spermatophores and may use "love darts" during mating .
* **Whelk (*Buccinum undatum*):** A carnivorous marine snail that has gills for respiration and an operculum to seal its shell .
#### 3.7.3 Cephalopods
Cephalopods are highly active marine mollusks with a well-developed nervous system and complex behaviors. They have a ring of arms around the mouth and typically possess a shell (internal or external) for support or protection .
* **Structure:** Cephalopods have a closed circulatory system with three hearts: two branchial hearts pumping blood through the gills and a systemic heart pumping oxygenated blood to the body. They have a mantle cavity with a siphon for jet propulsion .
* **Locomotion:** Jet propulsion is achieved by expelling water from the mantle cavity through the siphon. Fins and arms aid in steering and stability .
* **Reproduction:** Cephalopods have separate sexes, with males using a specialized tentacle (hectocotylus) to transfer spermatophores to the female's mantle cavity .
* **Cuttlefish (*Sepia officinalis*):** Possesses an internal shell (cuttlebone) for buoyancy, well-developed eyes, and ten arms with suckers. They can expel ink from an ink sac to escape predators .
* **Squid:** Often have a gladius (pen-like internal shell) and a streamlined body with fins .
### 3.8 Arthropods (*Arthropoda*)
Arthropods are characterized by a segmented body, a chitinous exoskeleton, and jointed appendages. This phylum is the most diverse and abundant in the animal kingdom .
* **Classification:** Arthropods are divided into four subphyla:
* **Crustacea (Crustaceans):** Primarily aquatic, with a exoskeleton hardened by calcium carbonate. Examples: crabs, shrimp, lobsters (*Nephrops norvegicus*), barnacles, water fleas (*Daphnia spp.*) .
* **Hexapoda (Insects):** Terrestrial or freshwater arthropods with three body segments (head, thorax, abdomen) and six legs. Examples: beetles, butterflies, flies .
* **Chelicerata (Chelicerates):** Includes spiders, scorpions, and horseshoe crabs.
* **Myriapoda (Myriapods):** Includes centipedes and millipedes.
#### 3.8.1 Crustaceans (*Crustacea*)
Crustaceans typically have a exoskeleton containing chitin and calcium carbonate, and their bodies are segmented with jointed appendages. They breathe via gills or directly through the skin and are found in aquatic and terrestrial habitats .
* **Structure:** Crustaceans possess a body divided into distinct regions, often a cephalothorax and abdomen. Appendages are specialized for various functions, including locomotion, feeding, and sensory perception .
* **Exoskeleton:** The exoskeleton is composed of chitin and protein and is molted to allow for growth .
* ***Nephrops norvegicus* (Norway lobster):** A decapod crustacean with a distinct cephalothorax and abdomen, covered by a carapace. It has five pairs of walking legs, with the first pair modified into claws for grasping. Respiration occurs through plumose gills located in gill chambers .
* ***Daphnia spp.* (Water flea):** A small freshwater crustacean with a bivalve carapace enclosing most of its body. They move using their large antennae. Reproduction is primarily parthenogenetic in favorable conditions, with males and resting eggs produced during unfavorable periods .
* **Other Crustaceans:** Examples include shrimp, hermit crabs, isopods (e.g., woodlice), amphipods (e.g., sand fleas), copepods, and cirripeds (e.g., barnacles, goose barnacles) .
#### 3.8.2 Insects (*Hexapoda*)
Insects have a body divided into three segments: head, thorax, and abdomen, with three pairs of legs attached to the thorax. Most insects possess wings attached to the second and third thoracic segments. They respire through a tracheal system and often undergo metamorphosis .
* **Structure:** The head bears antennae and compound eyes. The thorax has three segments, each with a pair of legs, and most insects have two pairs of wings. The abdomen contains most of the internal organs .
* **Respiration:** A tracheal system of air-filled tubes delivers oxygen directly to tissues, with openings (spiracles) on the thorax and abdomen .
* **Senses:** Insects have compound eyes (facets) and simple eyes (ocelli). The compound eye is composed of numerous ommatidia, each with a lens, crystal cone, rhabdom, photoreceptor cells, and pigment sheath .
* **Flight:** Wing movement is powered by thoracic muscles, either directly or indirectly .
* **Metamorphosis:** Many insects undergo complete or incomplete metamorphosis, involving larval stages that differ significantly from the adult form .
### 3.9 Echinoderms (*Echinodermata*)
Echinoderms are marine invertebrates characterized by radial symmetry, a water vascular system, and an internal calcareous skeleton (ossicles) .
* **Classification:** Echinoderms are divided into five classes: Asteroidea (starfish), Ophiuroidea (brittle stars), Echinoidea (sea urchins), Holothuroidea (sea cucumbers), and Crinoidea (sea lilies) .
* **Starfish (*Asteroidea*):** Possess a central disc with five or more arms. The underside of the disc has a mouth surrounded by grooves containing tube feet used for locomotion, attachment, and feeding. They have a water vascular system for movement and gas exchange, and a decentralized nervous system. Starfish are typically carnivorous, capable of everting their stomach to digest prey externally. They exhibit remarkable regenerative abilities .
* **Other Echinoderms:** Brittle stars have distinct arms separated from the central disc and move by arm flexion. Sea urchins have a spherical or flattened test (skeleton) covered in movable spines used for locomotion and defense. Sea cucumbers have elongated, leathery bodies and use tentacles around the mouth to ingest food particles from the substrate. Sea lilies are sessile, with a stalk attached to the substrate and feathery arms for filter feeding .
---
# Vertebrate classification and characteristics
This section details the evolutionary journey and defining features of chordate subphyla, with a deep dive into the diverse classes of vertebrates, highlighting their anatomical, physiological, and reproductive adaptations .
### 12.1 Chordate characteristics and embryology
Chordates are characterized by four key features present at some stage of their life cycle: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. These features manifest differently across the three subphyla: Urochordata (tunicates), Cephalochordata (lancelets), and Vertebrata. The pharyngeal gut, initially for respiration and filter-feeding, evolves in terrestrial forms for internal secretion .
#### 12.1.1 Embryonic development
Chordate embryos typically undergo several developmental stages: zygote, morula, blastula, and gastrula .
* **Cleavage:** Cell division without growth, resulting in smaller blastomeres .
* **Morula:** A solid ball of cells .
* **Blastula:** A hollow ball of cells with a central cavity (blastocoel) .
* **Gastrulation:** Invagination of cells to form the archenteron (primitive gut), establishing the three germ layers: ectoderm, endoderm, and mesoderm. The mesoderm splits to form the coelom (body cavity) .
* **Neurulation:** Formation of the neural tube from the ectoderm, which develops into the central nervous system .
> **Tip:** While the embryonic development stages are similar, variations exist due to the amount of yolk present in the embryo .
The three germ layers give rise to specific tissues and organs:
* **Ectoderm:** Epidermis (outer skin), nervous system .
* **Mesoderm:** Skeleton, muscle tissue, reproductive system, dermis (inner skin), blood vessel system .
* **Endoderm:** Respiratory system, digestive system and associated glands, bladder .
### 12.2 Subphylum: Lancelets (Cephalochordata)
Lancelets are marine, fish-like creatures with a persistent notochord running the length of their body and a dorsal nerve cord .
* **Mouth:** Funnel-shaped with cirri for sensory and filter-feeding functions .
* **Pharyngeal slits:** Numerous, located on both sides of the pharynx, facilitating respiration. Water enters through the mouth, passes through the pharynx, enters a cavity, and exits via an atriopore anterior to the anus .
* **Circulatory system:** Closed, with a simple pulsating "gill heart" formed by contractile swellings .
* **Locomotion:** Achieved by V-shaped muscles (myomeres) and a fin fold along the back, tail, and belly .
* **Diet:** Microorganisms and organic debris filtered from the water .
* **Reproduction:** Sexual with external fertilization; larvae undergo metamorphosis. The most known species is *Branchiostoma lanceolatum* .
### 12.3 Subphylum: Tunicates (Urochordata)
Tunicates, once mistaken for mollusks, are characterized by a tough outer membrane called a tunic, composed of tunicin .
* **Body form:** Varies from sessile to free-swimming; bilaterally symmetrical .
* **Siphonophores:** Possess inflow and outflow siphons for water circulation .
* **Pharyngeal slits:** Large with numerous openings, serving for both respiration and filter-feeding .
* **Notochord:** Present only in the embryonic and larval stages, located in the tail region .
* **Circulatory system:** Open, consisting of a network of cavities rather than true vessels .
### 12.4 Subphylum: Vertebrates (Vertebrata)
Vertebrates are distinguished by the presence of a vertebral column (replacing the notochord during embryonic development, except in jawless fish) and a more complex nervous system .
#### 12.4.1 Infraphylum: Jawless fishes (Agnatha)
This group lacks jaws and possesses a notochord in adults .
* **Appearance:** Cylindrical, eel-like with unpaired fins .
* **Mouth:** Round, with rasping horny teeth that are continuously replaced. Larvae are blind, toothless, and microphagous, filtering food with a "gill basket." .
* **Skin:** Mucous and lacking scales .
* **Classes:**
* **Lampreys (Cephalaspidomorphi):** Have seven gill openings, a nostril, and eyes. Respiration occurs through gill openings. They attach to prey, rasping flesh with their tongue and secreting anti-coagulant substances .
* **Hagfishes (Myxini):** Breathe through their nostril and are hermaphroditic. They create a slit in prey with their tongue and burrow into the flesh. Their skin glands secrete large amounts of mucus .
#### 12.4.2 Superclass: Cartilaginous fishes (Chondrichthyes)
Characterized by a skeleton made of cartilage .
* **Skeleton:** Cartilaginous, with the notochord replaced by cartilaginous vertebrae .
* **Fins:** Supported by fin rays; pectoral and pelvic fins are paired, while dorsal, anal, and caudal fins are unpaired .
* **Skin:** Covered with placoid scales (dentin and enamel), similar in structure to mammalian teeth .
* **Thermoregulation:** Poikilothermic/ectothermic (cold-blooded) .
* **Swim bladder:** Absent .
* **Spiracles:** Present between the eye and the first gill slit .
* **Teeth:** Two rows, continuously replaced due to wear .
* **Reproduction:** Males have a copulatory organ formed from the posterior part of the pelvic fins .
* **Habitat:** Primarily marine, preying on other vertebrates .
* **Circulatory system:** Single, closed circulation with a two-chambered heart (atrium and ventricle) .
* **Respiration:** Water passes over the gills and exits through separate gill slits .
> **Example:** Sharks and rays belong to this group .
#### 12.4.3 Class: Bony fishes (Teleostei)
Evolved in freshwater, many primitive bony fish possessed "lungs" for breathing in shallow or dry conditions .
* **Skeleton:** Bony .
* **Fins:** Divided into ray-finned (skin membrane supported by bony rays) and lobe-finned (fleshy fins supported by bones embedded in muscle) .
* **Respiration:** Through gills covered by an operculum (gill cover) .
* **Skin:** Protected by scales embedded in the dermis. Scales grow with the fish, showing growth rings .
* **Swim bladder:** Typically present, aiding in buoyancy .
* **Skeleton:** Lacks a neck. Consists of the vertebral column, skull, and fins (paired and unpaired) .
* **Senses:** Eyes are similar to mammals but lack eyelids. The lens is spherical and moves for focusing. Possess a lateral line for sensing water movement and temperature .
* **Respiration efficiency:** Uses the countercurrent principle for efficient gas exchange in gills .
> **Tip:** The oxygen-binding capacity of hemoglobin varies depending on the oxygen levels in the fish's environment .
#### 12.4.4 Class: Amphibians (Amphibia)
Amphibians are the earliest land vertebrates, adapted for a dual life in water and on land .
* **Skin:** Thin, permeable, with mucus glands to prevent water loss; functions as a respiratory surface .
* **Skeleton:** Robust to support the body on land .
* **Limbs:** Pentadactyl (five-fingered) limbs evolved from paired fins .
* **Respiration:** Primarily through lungs (often "storage lungs"), but also via skin and gills (especially in larval stages) .
* **Heart:** Two atria and one ventricle .
* **Thermoregulation:** Poikilothermic (cold-blooded) .
* **Reproduction:** Lack amnion and allantois; undergo metamorphosis .
* **Metamorphosis:** Larval stage (tadpole) transforms into an adult form with significant anatomical changes, regulated by the pituitary and thyroid glands .
* **Circulatory system:** Closed, double, incomplete circulation due to mixing of oxygenated and deoxygenated blood in the single ventricle .
> **Example:** Frogs, toads, salamanders, and caecilians .
#### 12.4.5 Class: Reptiles (Reptilia)
Reptiles are the first vertebrates fully adapted to terrestrial life .
* **Skin:** Dry, keratinized, and scaly, preventing water loss. Lacks glands, leading to shedding (ecdysis) .
* **Limbs:** Two pairs of limbs with five toes (exceptions exist, e.g., snakes) .
* **Skeleton:** Fully ossified .
* **Skull:** Single occipital condyle .
* **Heart:** Two atria and two partially divided ventricles .
* **Thermoregulation:** Cold-blooded .
* **Respiration:** Exclusively by lungs throughout all life stages .
* **Reproduction:** Internal fertilization; possess all embryonic membranes (yolk sac, amnion, allantois, chorion). Lay amniotic eggs on land .
* **Urogenital system:** Kidneys have their own ducts; uric acid is the primary excretory product in terrestrial reptiles. Bladder is an outgrowth of the cloaca .
> **Tip:** Birds are the direct descendants of dinosaurs, not modern reptiles .
#### 12.4.6 Subclass: Birds (Aves)
Birds are direct descendants of dinosaurs, sharing many ancestral traits .
* **Homoiothermy:** Maintain a constant body temperature independent of the environment, requiring higher energy intake .
* **Plumage:** Feathers are epidermal in origin and serve various functions, including insulation, display, and flight .
* **Forelimbs:** Modified for flight, with a unique skeletal structure .
* **Teeth:** Absent .
* **Respiration:** Highly efficient system with air sacs that facilitate unidirectional airflow through the lungs, allowing for continuous gas exchange .
* **Heart:** Four-chambered .
* **Skeleton:** Lightweight due to pneumatic bones (hollow bones) .
* **Digestive system:** Characterized by a crop for food storage and softening .
* **Urogenital system:** Lack a urinary bladder. Ureters empty into the cloaca. Most lack a copulatory organ .
* **Reproduction:** Oviparous, with complex egg formation including albumen, shell membranes, and a calcium carbonate shell .
> **Example:** Penguins, pelicans, eagles, ducks, and songbirds represent a diverse array of avian orders .
#### 12.4.7 Class: Mammals (Mammalia)
Mammals are a diverse group characterized by several unique features .
* **Homoiothermy:** Maintain a constant internal body temperature .
* **Integument:** Possess hair and sweat glands .
* **Mammary glands:** Produce milk to nourish young .
* **Diaphragm:** A muscular diaphragm aids in respiration .
* **Heart:** Four-chambered .
* **Auditory ossicles:** Three middle ear bones (malleus, incus, stapes) .
* **Dentition:** Specialized teeth (incisors, canines, premolars, molars), with only one set of permanent teeth (diphyodonty) .
* **Placenta:** In most mammals, a placenta facilitates nutrient and gas exchange between mother and fetus .
* **Viviparity:** Give birth to live young (exceptions are monotremes) .
* **Urogenital system:** Cloaca is absent in most; reproductive and urinary tracts are separated. Testes are external in males .
* **Skin:** Composed of epidermis, dermis, and hypodermis. Hair is a defining feature, with various modifications like scales, claws, nails, hooves, and horns .
* **Skeleton:** Generally similar structure across species, with 7 cervical vertebrae (except sloths), and a vertebral column with intervertebral discs .
* **Reproductive strategies:** Classified into Prototheria (egg-laying, e.g., platypus), Metatheria (marsupials with a pouch, e.g., kangaroo), and Eutheria (placental mammals with prolonged gestation, e.g., humans, rodents) .
* **Digestive system:** Includes a complex stomach, especially in ruminants .
* **Circulatory and Respiratory systems:** Four-chambered heart ensures complete separation of oxygenated and deoxygenated blood. Bidirectional airflow in lungs .
> **Example:** Mammals include diverse groups such as rodents, carnivores, primates, cetaceans, and ungulates .
---
## Common mistakes to avoid
- Review all topics thoroughly before exams
- Pay attention to formulas and key definitions
- Practice with examples provided in each section
- Don't memorize without understanding the underlying concepts
Glossary
| Term | Definition |
|------|------------|
| Animal Cell | A type of eukaryotic cell that is a fundamental component of animal tissues and organs, characterized by the absence of a cell wall and chloroplasts, and the presence of a centrosome. |
| Cell Membrane | A selectively permeable barrier composed of lipid molecules, which encloses the cell and separates its internal environment from the external surroundings, maintaining cellular integrity. |
| Prokaryotic Cell | A type of cell that lacks a membrane-bound nucleus and other membrane-bound organelles; its genetic material (DNA) is located in the cytoplasm. Examples include bacteria. |
| Eukaryotic Cell | A type of cell that possesses a membrane-bound nucleus containing its genetic material (DNA) and various other membrane-bound organelles, such as mitochondria and the endoplasmic reticulum. |
| Nucleus | The central organelle in eukaryotic cells that houses the cell's genetic material (DNA) and controls the cell's growth and reproduction. |
| Cytoplasm | The jelly-like substance filling the cell, enclosed by the cell membrane, which contains various organelles and is the site of many metabolic reactions. |
| Mitochondrion (plural: Mitochondria) | An organelle found in eukaryotic cells responsible for generating most of the cell's supply of adenosine triphosphate (ATP), used as a source of chemical energy. |
| ATP (Adenosine Triphosphate) | A molecule that serves as the primary energy currency of the cell, produced through cellular respiration. |
| Endoplasmic Reticulum (ER) | A network of membranes found throughout the cytoplasm of eukaryotic cells, involved in protein and lipid synthesis. The rough ER is studded with ribosomes. |
| Golgi Apparatus | An organelle that modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles. |
| Mitosis | A type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus, typical of ordinary tissue growth. |
| Meiosis | A type of cell division that reduces the number of chromosomes in the parent cell by half and produces four gamete cells. This process is essential for sexual reproduction. |
| Chromosome | A thread-like structure of nucleic acids and protein found in the nucleus of most living cells, carrying genetic information in the form of genes. |
| Chromatin | The complex of DNA and proteins that forms chromosomes within the nucleus of eukaryotic cells. It is the uncoiled form of DNA. |
| Chromatid | One of two identical halves of a duplicated chromosome, connected at the centromere. |
| Centromere | The region of a chromosome that links sister chromatids, and to which the spindle fibers attach during cell division. |
| Centrosome | An organelle that serves as the main microtubule-organizing center (MTOC) of the animal cell. It plays a role in cell division by organizing the spindle fibers. |
| Spindle Fiber | Microtubule structures that form during mitosis and meiosis, responsible for separating chromosomes. |
| Prophase | The first stage of mitosis and meiosis, during which chromosomes condense, the nuclear envelope breaks down, and the spindle apparatus forms. |
| Metaphase | The stage of mitosis or meiosis in which chromosomes are aligned at the metaphase plate (equator) of the cell. |
| Anaphase | The stage of mitosis or meiosis when sister chromatids (in mitosis and meiosis II) or homologous chromosomes (in meiosis I) are separated and pulled towards opposite poles of the cell. |
| Telophase | The final stage of mitosis or meiosis, in which chromosomes decondense, nuclear envelopes reform around the two sets of chromosomes, and cytokinesis usually begins. |
| Diploid (2n) | A cell or organism containing two complete sets of chromosomes, one from each parent. |
| Haploid (n) | A cell or organism containing a single set of unpaired chromosomes. |
| Homologous Chromosomes | A pair of chromosomes that have the same genes in the same order, one inherited from each parent. |
| Cytokinesis | The division of the cytoplasm to form two separate daughter cells, typically occurring at the end of mitosis or meiosis. |
| Crossing Over | The exchange of genetic material between non-sister chromatids of homologous chromosomes during meiosis. |
| Zoosystematics | The scientific discipline concerned with the principles of classifying and naming living organisms, specifically within the animal kingdom. It encompasses taxonomy, diagnostics, classification, and nomenclature. |
| Taxonomy | The science of systematic categories, dealing with the principles of classification and the hierarchical arrangement of organisms into groups such as species, genus, and family. These groups are referred to as taxa. |
| Taxon | A taxonomic unit, representing a group of organisms at any level of the hierarchical classification system, such as a species, genus, or family. |
| Diagnostics | The branch of zoology that focuses on the description of organisms, detailing their characteristics and features. |
| Classification | The arrangement of taxa according to their degree of relatedness, based on shared characteristics and evolutionary history. |
| Nomenclature | The system for assigning scientific names to organisms, typically in Latin. For living beings, this is often binomial, consisting of a genus name and a species name, written in italics with the genus capitalized. |
| Binomial Nomenclature | A scientific naming system where each organism is given a two-part name, comprising the genus name followed by the species name. This system ensures a universally understood scientific designation. |
| Cladistics | A method of classification based on evolutionary relationships between organisms, often visualized using cladograms. It focuses on identifying common ancestors and branching patterns of descent. |
| Cladogram | A branching diagram that illustrates the evolutionary relationships between different groups of organisms, showing their hypothesized lines of descent from common ancestors. |
| Parafyletic Group | A group in a classification system that includes a common ancestor but does not include all of its descendants. This occurs when a classification is based primarily on shared characteristics rather than complete evolutionary lineage. |
| Catalogue of Life (CoL) | A publicly accessible online database that aims to document the evolutionary lineage of all living organisms, encompassing millions of species names and taxonomic information. |
| Kingdoms (Two-kingdom system) | An early classification system dividing all life into two primary groups: plants and animals. |
| Kingdoms (Three-kingdom system) | An expanded classification system that introduced a third kingdom, Protista, to accommodate single-celled organisms that did not fit neatly into the plant or animal kingdoms. |
| Kingdoms (Four-kingdom system) | A further refinement of the kingdom system, separating bacteria and blue-green algae into a distinct kingdom (monomers) from other single-celled eukaryotes (Protista). |
| Kingdoms (Five-kingdom system) | Proposed by Whittaker, this system classifies life into five kingdoms: Monera, Protista, Fungi, Plantae, and Animalia, based on differences in nutrition and cellular organization. |
| Eukaryotes | Organisms whose cells contain a nucleus and other membrane-bound organelles. |
| Prokaryotes | Organisms whose cells lack a nucleus and other membrane-bound organelles. |
| Protozoa | A kingdom of unicellular organisms, typically microscopic and free-living in soil, fresh water, and salt water, but also including parasitic forms. Those possessing chloroplasts are classified with plants, while others that acquired symbiotic organelles capable of photosynthesis are grouped with Chromista. |
| Chromista | A kingdom that includes certain unicellular organisms, some of which possess chloroplasts and can photosynthesize, and others that have lost their organelles and photosynthetic capabilities. |
| Ciliophora | A phylum of protozoa characterized by the presence of cilia (trilharen), which are short, hair-like structures used for locomotion and feeding. Examples include Paramecium (pantoffeldiertje) and Colpoda (nierdiertje). |
| Paramecium | A genus of unicellular ciliates commonly found in freshwater environments rich in decaying plant matter. They are characterized by their slipper-like shape and movement via cilia. |
| Pellicula | A flexible, yet relatively rigid, outer covering of a protozoan cell, composed of multiple membranes, including the plasma membrane, which provides structural support and protection. |
| Macronucleus | The larger of the two nuclei found in ciliates like Paramecium, responsible for regulating metabolic processes and cellular functions. |
| Micronucleus | The smaller of the two nuclei found in ciliates like Paramecium, which is primarily involved in genetic recombination during sexual reproduction. |
| Food vacuole | A membrane-bound sac within a protozoan's cytoplasm that contains ingested food particles. Digestion occurs within these vacuoles through the action of enzymes. |
| Contractile vacuole | A specialized organelle found in many protozoa, particularly those living in freshwater, that regulates water balance by periodically expelling excess water from the cell. |
| Amoebozoa | A phylum of protozoa characterized by the use of pseudopods (schijnvoetjes) for locomotion and feeding. Many species possess an internal or external skeleton. Examples include Amoeba (amoebe) and Arcella vulgaris (horlogeglaasje). |
| Pseudopodia | Temporary, arm-like projections of the cytoplasm of amoeboid protozoa, used for movement (amoeboid movement) and engulfing food particles (phagocytosis). |
| Phagocytosis | The process by which a cell engulfs large particles, such as bacteria or other cells, by extending its plasma membrane around them to form a food vacuole. |
| Euglenozoa | A phylum of protozoa characterized by the presence of one or two flagella (zweepharen) used for locomotion. This group includes parasitic forms like Trypanosoma, which causes sleeping sickness. |
| Flagellum | A long, whip-like appendage used for locomotion in many protozoa and other microorganisms. |
| Metamonada | A group of flagellated unicellular organisms that lack mitochondria, though evidence suggests they may have lost them during evolution. They typically have flagella arranged in groups of four. Examples include Giardia duodenalis and Trichomonas gallinea. |
| Sporozoa | A superclass of parasitic protozoa within the phylum Miozoa, known for their complex life cycles that often involve the formation of spores. The malaria parasite is a notable example. |
| Eimeria spp. | A genus of spore-forming protozoa belonging to the subclass Coccidea, known for causing coccidiosis in various animal hosts, particularly poultry and rabbits. Their life cycle involves asexual and sexual reproduction phases. |
| Oöcyst | A resistant, thick-walled structure produced by certain protozoa, such as Eimeria and Toxoplasma, which contains the infective sporozoites and allows for transmission. |
| Schizont | An asexual stage in the life cycle of certain parasitic protozoa (e.g., Eimeria, Plasmodium) where the nucleus undergoes multiple divisions, producing numerous daughter cells (merozoites). |
| Merozoites | Asexual daughter cells produced within a schizont during the life cycle of certain parasitic protozoa. These are typically motile and capable of infecting new host cells. |
| Gametogony | The phase in the life cycle of certain parasitic protozoa where schizonts differentiate into male (microgametes) and female (macrogametes) gametes, which then fuse to form a zygote. |
| Zygote | The product of the fusion of male and female gametes (fertilization) in sexually reproducing organisms. In protozoa like Eimeria, the zygote develops into an oöcyst. |
| Sporogony | The process of spore formation in certain parasitic protozoa, typically involving nuclear and cytoplasmic divisions within an oöcyst, leading to the production of infective sporozoites. |
| Toxoplasma gondii | A ubiquitous intracellular parasite that infects warm-blooded animals, including humans. It is a significant cause of toxoplasmosis, particularly dangerous if acquired during pregnancy. Cats are the definitive hosts. |
| Plasmodium spp. | A genus of parasitic protozoa belonging to the subclass Hematozoa, responsible for causing malaria in humans and other vertebrates. Their life cycle involves both a mosquito vector and a vertebrate host. |
| Porifera | The phylum of sponges, which are sessile, multicellular aquatic animals characterized by their porous bodies and the absence of true tissues, organs, or systems. |
| Spongocoel | The central cavity within a sponge's body, through which water circulates. It is lined with choanocytes (collar cells). |
| Choanocytes | Specialized flagellated cells lining the spongocoel of sponges. Their flagellar action creates water currents, and their collar-like structure filters food particles from the water. |
| Amoebocytes | Mobile cells found in the mesenchyme of sponges, responsible for distributing nutrients, collecting waste, and differentiating into other cell types, including those that form spicules. |
| Spicules | Microscopic, skeletal elements found in sponges, composed of calcium carbonate or silica. They provide structural support and protection and can be of various shapes. |
| Spongin | A fibrous protein material that forms the skeleton of some sponges, particularly the common bath sponge. It is flexible and supportive. |
| Cnidaria | A phylum of aquatic invertebrates that possess specialized stinging cells called cnidocytes, which contain nematocysts. They typically have a radially symmetrical body with a central gastrovascular cavity. Examples include hydroids, jellyfish, anemones, and corals. |
| Polyp | One of the two basic body forms of cnidarians, characterized by a sessile, cylindrical structure with the mouth and tentacles facing upwards. Hydra (zoetwaterpoliep) is an example. |
| Medusa | The free-swimming, bell-shaped body form of cnidarians, with the mouth and tentacles facing downwards. Many cnidarians exhibit a life cycle with alternating polyp and medusa stages. |
| Cnidocytes | Specialized stinging cells found in cnidarians, containing a harpoon-like organelle called a nematocyst, which is discharged to capture prey or deter predators. |
| Nematocysts | The stinging organelles within cnidocytes, containing coiled, barbed tubules that are explosively discharged upon stimulation. |
| Gastrovascular cavity | A central cavity in cnidarians that serves as both a digestive and circulatory system. It has a single opening that functions as both mouth and anus. |
| Mesoglea | A gelatinous, non-cellular layer located between the ectoderm and endoderm in cnidarians, providing support and helping to maintain the body shape. |
| Platyhelminthes | The phylum of flatworms, characterized by their flattened, unsegmented bodies and the absence of a coelom and circulatory system. Many are parasitic. Examples include planarians, trematodes, and cestodes. |
| Triploblastic | Having a body derived from three embryonic germ layers: ectoderm, mesoderm, and endoderm. |
| Bilateral symmetry | A body plan in which an organism can be divided into two mirror-image halves along a single plane. This symmetry is associated with cephalization (development of a head). |
| Protonephridia | Primitive excretory organs found in many invertebrates, including flatworms, that regulate water and solute balance. They consist of flame cells and associated tubules. |
| Flame cell | A specialized cell in the protonephridial system of flatworms, containing a tuft of cilia that beat rhythmically, creating a current to draw waste products into the tubule. |
| Tricladida | An order of flatworms characterized by a three-branched digestive tract. Many are free-living and exhibit remarkable regenerative abilities, such as Planaria. |
| Cestoda | The class of tapeworms, which are endoparasitic flatworms with long, ribbon-like bodies divided into segments (proglottides). They lack a digestive system and absorb nutrients directly through their body surface. |
| Trematoda | The class of flukes, which are parasitic flatworms with typically leaf-shaped bodies. Most have suckers for attachment to their hosts and complex life cycles often involving multiple hosts. Fasciola hepatica (great liver fluke) is an example. |
| Proglottides | The segments of a tapeworm's body, each containing a complete set of reproductive organs. They are produced at the neck and mature towards the posterior end. |
| Scolex | The anterior end of a tapeworm, equipped with suckers and sometimes hooks, used for attachment to the host's intestinal wall. |
| Cysticercus | The larval stage of certain tapeworms, consisting of a fluid-filled bladder containing an invaginated scolex. It is the infective stage for the intermediate host. |
| Nematoda | The phylum of roundworms, characterized by their unsegmented, cylindrical bodies, a complete digestive system, and a pseudocoelom. Many are free-living in soil and water, while others are parasitic in plants and animals. |
| Cuticle | A tough, protective outer layer secreted by the epidermis of many invertebrates, including nematodes and arthropods. It provides support and protection but requires molting for growth. |
| Pseudocoelom | A body cavity found in nematodes and rotifers, located between the endoderm and mesoderm but not completely lined by mesoderm. It plays a role in hydrostatic support and organ function. |
| Molting (Ecdysis) | The process by which arthropods and nematodes shed their exoskeleton or cuticle to allow for growth. |
| Ancylostomatidae | A family of parasitic roundworms, commonly known as hookworms, that infect the intestines of vertebrates. They are characterized by hook-like mouthparts used to attach to the intestinal wall and feed on blood. |
| Ascaris lumbricoides | A large roundworm that infects the small intestine of humans. It is a common intestinal parasite worldwide, causing ascariasis. |
| Enterobius vermicularis | The pinworm, a common intestinal nematode parasite of humans, particularly children. It resides in the large intestine and lays eggs around the anus, causing anal itching. |
| Trichinella spiralis | A parasitic roundworm that causes trichinosis in humans and other mammals. Humans become infected by consuming undercooked meat containing the larvae, which encyst in muscle tissue. |
| Annelida | The phylum of segmented worms, characterized by bodies divided into repeating segments both internally and externally. They possess a true coelom and a closed circulatory system. Examples include earthworms, leeches, and polychaetes. |
| Coelom | A true body cavity, lined by mesoderm, that develops within the mesoderm. It is found in coelomate animals like annelids, arthropods, and mollusks. |
| Clitellata | A class of annelids that includes earthworms (Oligochaeta) and leeches (Hirudinea). They are characterized by a clitellum, a glandular, saddle-like structure involved in reproduction. |
| Oligochaeta | A subclass of annelids that includes earthworms. They are characterized by having few or no bristles (setae) and living in soil or freshwater. Lumbricus terrestris (earthworm) is a representative. |
| Hirudinea | A subclass of annelids that includes leeches. They are characterized by flattened bodies, the absence of bristles, and the presence of suckers for locomotion and feeding. |
| Polychaeta | A class of annelids that are predominantly marine. They are characterized by having numerous bristles (setae) arranged on fleshy, paddle-like appendages called parapodia. |
| Setae | Bristles or stiff hairs found on the segments of many annelids, used for locomotion and anchoring. |
| Clitellum | A thickened, glandular, saddle-like section of the body wall near the head in earthworms and leeches, that secretes a viscid cocoon for egg deposition. |
| Nephridia | The excretory organs of annelids, which are typically paired in each segment and function in filtering waste products from the coelomic fluid and blood. |
| Hemoglobin | The oxygen-carrying protein found in the blood of many vertebrates and some invertebrates, including annelids like earthworms. It is located within red blood cells or dissolved in the plasma. |
| Mollusca | A large and diverse phylum of invertebrate animals, characterized by a soft body that is often protected by a calcareous shell. They typically have a muscular foot, a mantle, and a radula. Examples include snails, clams, and cephalopods. |
| Bivalvia | A class of mollusks characterized by a laterally compressed body enclosed within a two-part hinged shell. They are typically filter feeders. Mussels (Mytilus edulis) and clams are examples. |
| Gastropoda | The largest class of mollusks, characterized by a coiled shell (in most species), a distinct head with sensory tentacles, and a muscular foot for locomotion. Snails and slugs are examples. |
| Cephalopoda | A class of highly intelligent marine mollusks characterized by a well-developed head, a ring of tentacles or arms surrounding the mouth, and a reduced or internalized shell. Squids, octopuses, and nautiluses are examples. |
| Mantle | The outer layer of tissue in mollusks that secretes the shell. It also encloses the mantle cavity, which contains the gills. |
| Radula | A ribbon-like structure in the mouth of most mollusks, bearing rows of chitinous teeth used for scraping or cutting food. |
| Periostracum | The outermost, protective organic layer of a mollusk shell, composed of conchiolin, which prevents dissolution of the underlying calcareous layers. |
| Prism layer | The middle layer of a mollusk shell, composed of calcareous prisms oriented perpendicular to the shell surface, providing strength. |
| Nacreous layer (Pearly layer) | The innermost layer of a mollusk shell, composed of thin, aragonitic platelets layered with conchiolin, creating an iridescent effect. |
| Foot | A muscular appendage of mollusks used for locomotion, attachment, or burrowing. Its form varies greatly among different classes. |
| Siphons | Tubular extensions of the mantle in some bivalves (e.g., clams, razor clams), used for drawing in and expelling water for feeding, respiration, and waste removal. |
| Operculum | A calcareous or horny plate that seals the aperture of the shell in some gastropods when the animal retracts into its shell. |
| Arthropoda | The largest phylum of animals, characterized by a segmented body, jointed appendages, and a chitinous exoskeleton. This phylum includes insects, arachnids, crustaceans, and myriapods. |
| Exoskeleton | The hard, external covering of arthropods, composed primarily of chitin and proteins, which provides structural support, protection, and attachment points for muscles. It must be molted to allow for growth. |
| Chitin | A tough, flexible polysaccharide that is a primary component of the exoskeleton of arthropods and the cell walls of fungi. |
| Crustacea | A subphylum of arthropods that includes a diverse group of mostly aquatic animals like crabs, lobsters, shrimp, and barnacles. They typically possess a hard exoskeleton reinforced with calcium carbonate and two pairs of antennae. |
| Hexapoda | A subphylum of arthropods that includes insects and their wingless relatives (collembolans, diplurans, proturans). Insects are characterized by a body divided into head, thorax, and abdomen, and three pairs of legs. |
| Mandibles | The primary pair of mouthparts in insects and crustaceans, used for biting, cutting, or grinding food. |
| Maxillae | The second pair of mouthparts in insects and crustaceans, often used for manipulating food or as accessory mouthparts. |
| Antennae | Paired sensory appendages located on the head of arthropods, used for detecting touch, smell, taste, and vibration. |
| Thorax | The middle body section of insects and other hexapods, consisting of three segments (prothorax, mesothorax, metathorax), to which the legs and wings (if present) are attached. |
| Abdomen | The posterior body section of insects and other arthropods, typically containing the digestive, excretory, and reproductive organs. |
| Trachea (Insect) | A network of fine, branching tubules that extend throughout the body of insects, delivering oxygen directly to the tissues from the external openings (spiracles). |
| Spiracle | External openings to the tracheal system of insects, located along the sides of the thorax and abdomen, which allow for gas exchange. They are typically equipped with valves to control airflow. |
| Hemolymph | The circulatory fluid of arthropods, analogous to blood, which bathes the organs and transports nutrients, waste products, and hormones. It does not typically transport oxygen. |
| Compound eye | An eye composed of many individual visual units called ommatidia, found in insects and crustaceans, providing a wide field of vision and excellent motion detection. |
| Ommatidium | A single visual unit of a compound eye, consisting of a lens, crystalline cone, light-sensitive cells (retinula), and pigment cells. |
| Echinodermata | A phylum of exclusively marine invertebrates characterized by radial symmetry (usually pentaradial), a water vascular system, and an endoskeleton of calcareous plates. Examples include starfish, sea urchins, and sea cucumbers. |
| Water vascular system | A unique hydraulic system found in echinoderms, used for locomotion, feeding, respiration, and sensory perception. It consists of a network of canals filled with seawater. |
| Tube feet (Podia) | Small, muscular, extensible appendages of the water vascular system of echinoderms, typically ending in suction cups, used for locomotion and manipulating objects. |
| Radial symmetry | A body plan in which the body parts are arranged around a central axis, such that any plane passing through this axis divides the organism into mirror-image halves. |
| Madreporite | A porous plate that serves as the entrance for seawater into the water vascular system of echinoderms. |
| Pedicellariae | Small, pincer-like appendages found on the surface of many echinoderms, used for cleaning the body surface and defense. |
| Asterias rubens | The common starfish or sea star, a well-known example of the class Asteroidea within the phylum Echinodermata. It possesses five arms radiating from a central disk and uses tube feet for locomotion. |
| Aristotle's lantern | The complex jaw-like apparatus found in sea urchins, used for grazing on algae and other food sources. |
| Sea urchin | A globular or flattened echinoderm characterized by a test (internal skeleton) composed of fused calcareous plates, covered with movable spines. |
| Sea cucumber | An elongated, cylindrical echinoderm that lies on one side, with a mouth surrounded by feeding tentacles and an anus at the opposite end. |
| Sea lily | A crinoid echinoderm that is typically attached to the substrate by a stalk, with a calyx bearing feathery arms that filter food from the water. |
| Planula larva | The free-swimming, ciliated larval stage of cnidarians, which settles and develops into a polyp. |
| Hydroid | A polyp stage of a hydrozoan cnidarian, often colonial and sessile, with a base attached to a substrate and tentacles surrounding the mouth. |
| Jellyfish | The medusa stage of scyphozoan cnidarians, characterized by a bell-shaped body and trailing tentacles. |
| Anemone | A polyp-form cnidarian, typically solitary and sessile, with a column-like body attached to a substrate and a crown of tentacles surrounding the mouth. |
| Coral | Colonial polyps of anthozoan cnidarians that secrete a hard, calcium carbonate exoskeleton, forming reefs. |
| Turbellarians | Free-living flatworms, typically aquatic, with ciliated bodies. Planarians are a well-known example. |
| Trematodes | Parasitic flatworms (flukes) with complex life cycles, often involving intermediate hosts. |
| Cestodes | Tapeworms, parasitic flatworms that lack a digestive system and absorb nutrients from their host's intestine. |
| Nematodes | Roundworms, unsegmented worms with a cylindrical body and a pseudocoelom. Many are free-living, while others are parasitic. |
| Annelids | Segmented worms with a true coelom and a closed circulatory system. Earthworms, leeches, and polychaetes belong to this phylum. |
| Oligochaetes | Earthworms, characterized by few bristles and a segmented body. |
| Leeches | Segmented worms belonging to the Hirudinea subclass, typically flattened and lacking bristles, with suckers for locomotion and feeding. |
| Polychaetes | Marine annelids with numerous bristles (setae) on parapodia. |
| Mollusks | A phylum of soft-bodied invertebrates, often with a shell, including snails, clams, squids, and octopuses. |
| Bivalves | Mollusks with a shell consisting of two hinged valves, such as clams and mussels. |
| Gastropods | Mollusks with a single, typically coiled shell, such as snails and slugs. |
| Cephalopods | Marine mollusks with a prominent head and a ring of tentacles or arms surrounding the mouth, such as squids and octopuses. |
| Arthropods | Invertebrates with segmented bodies, jointed appendages, and a chitinous exoskeleton. Insects, arachnids, crustaceans, and myriapods are examples. |
| Crustaceans | A group of arthropods that includes crabs, lobsters, shrimp, and barnacles, typically aquatic and possessing a hard exoskeleton reinforced with calcium carbonate. |
| Insects | Hexapod arthropods characterized by a body divided into head, thorax, and abdomen, three pairs of legs, and usually one or two pairs of wings. |
| Echinoderms | Marine invertebrates with radial symmetry, a water vascular system, and a calcareous endoskeleton. Starfish, sea urchins, and sea cucumbers are examples. |
| Starfish | A type of echinoderm with a typically five-armed, star-shaped body, using tube feet for locomotion and feeding. |
| Chordates | A diverse phylum of animals that at some stage of their life cycle possess a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. |
| Vertebrates | A subphylum of Chordata characterized by the presence of a vertebral column (backbone) that protects the spinal cord and supports the body. |
| Lancelets | Small, marine invertebrates belonging to the subphylum Cephalochordata, known for their distinct fish-like appearance and a persistent notochord throughout their life. |
| Tunicates | Marine invertebrates belonging to the subphylum Urochordata, also known as sea squirts, where the notochord is present only in the larval stage. |
| Jawless fish | Primitive aquatic vertebrates belonging to the infra-class Agnatha, characterized by a lack of jaws and a cartilaginous skeleton; they include hagfish and lampreys. |
| Cartilaginous fish | Aquatic vertebrates belonging to the class Chondrichthyes, possessing a skeleton made entirely of cartilage rather than bone, such as sharks and rays. |
| Bony fish | Aquatic vertebrates belonging to the class Osteichthyes, characterized by a skeleton made of bone. This is the largest class of vertebrates. |
| Amphibians | Vertebrates belonging to the class Amphibia, which typically live a dual life, partly in water and partly on land. They possess moist skin and undergo metamorphosis. |
| Reptiles | Terrestrial vertebrates belonging to the class Reptilia, characterized by scaly skin, lungs for breathing, and internal fertilization, enabling them to lay eggs on land. |
| Birds | Warm-blooded vertebrates belonging to the class Aves, distinguished by feathers, wings, a beak, and the ability to fly (though some species are flightless). |
| Mammals | Warm-blooded vertebrates belonging to the class Mammalia, characterized by the presence of hair or fur, mammary glands for producing milk, and typically giving birth to live young. |
| Notochord | A flexible, rod-shaped structure that provides skeletal support in chordates, running along the dorsal side. It is a defining characteristic of the phylum Chordata. |
| Pharyngeal slits | Openings in the pharynx of chordates that in aquatic forms are used for filter-feeding or respiration, and in terrestrial forms typically disappear or are modified. |
| Blastula | An early stage of embryonic development in animals, typically a hollow ball of cells formed from the morula. |
| Gastrulation | The process in embryonic development where the blastula reorganizes into a multilayered structure known as the gastrula, forming the primary germ layers (ectoderm, endoderm, mesoderm). |
| Ectoderm | The outermost of the three primary germ layers of an embryo, which gives rise to the epidermis, nervous system, and sensory organs. |
| Endoderm | The innermost of the three primary germ layers of an embryo, which develops into the lining of the digestive tract and associated glands. |
| Mesoderm | The middle germ layer of an embryo, located between the ectoderm and endoderm, which develops into muscles, bone, cartilage, blood, and other connective tissues. |
| Neuraula | An embryonic stage characterized by the formation of the neural tube, the precursor to the central nervous system. |
| Agnatha | An infra-class of jawless vertebrates, characterized by the absence of jaws and typically retaining a notochord throughout life, including hagfish and lampreys. |
| Gnathostomata | A super-class of vertebrates that possess jaws, contrasting with Agnatha. This group includes cartilaginous and bony fishes, amphibians, reptiles, birds, and mammals. |
| Chondrichthyes | The class of cartilaginous fishes, which includes sharks, rays, and skates, defined by their skeletons made of cartilage. |
| Osteichthyes | The class of bony fishes, characterized by skeletons made of bone. This is the most diverse group of vertebrates. |
| Tetrapoda | A super-class of vertebrates that includes all four-limbed terrestrial vertebrates: amphibians, reptiles, birds, and mammals. |
| Homoiothermy | The ability of an organism to maintain a stable internal body temperature regardless of external influences; characteristic of birds and mammals (warm-bloodedness). |
| Poikilothermy | The characteristic of an organism whose body temperature varies with the temperature of its environment; characteristic of amphibians, reptiles, and fish (cold-bloodedness). |
| Cloaca | A common chamber in the hindgut of certain vertebrates (including amphibians, reptiles, birds, and monotremes) that serves as the terminal section of the digestive, urinary, and reproductive tracts. |
| Oviparous | Reproducing by laying eggs. |
| Viviparous | Giving birth to live young that have developed inside the body of the parent. |
| Placenta | An organ formed in the uterus of pregnant mammals that connects the developing fetus to the uterine wall, providing nourishment and oxygen and removing waste products. |
| Metamorphosis | A process of transformation from an immature form to an adult form in two or more distinct stages, commonly observed in amphibians and insects. |