Kingdom Animalia | Class 11 Biology

1. Basis of Classification

The Animal Kingdom includes a huge variety of animals, from tiny insects to large mammals. To study them scientifically, we need to classify them based on certain characteristics. This classification helps us understand their body structure, function, and evolutionary relationships.

Scientists use the following major criteria to classify animals:

1. Levels of Organization

Animals show different levels of structural organization. These levels indicate how cells are organized into tissues, organs, and systems.

🔹 a. Cellular Level

Seen in sponges (Phylum Porifera).
The animal body is made of loosely arranged cells.
Cells perform different functions but are not organized into tissues.

🔹 b. Tissue Level

Found in Cnidarians (Phylum Coelenterata).
Cells performing the same function group together to form tissues.
For example, Hydra shows simple muscle and nerve tissues.

🔹 c. Organ Level

Found in Platyhelminthes (flatworms).
Tissues combine to form organs with specific functions.
Example: The digestive system in flatworms is well-developed.

🔹 d. Organ System Level

Found in Annelida to Chordata (from earthworms to humans).
Organs form well-developed organ systems (e.g., digestive, respiratory, circulatory).
These systems show greater complexity and specialization.

2. Symmetry

Symmetry refers to the arrangement of body parts around a central line or point.

🔹 a. Asymmetry

Body cannot be divided into two equal halves.
Example: Sponges (Porifera).
Their body shape is irregular.

🔹 b. Radial Symmetry

Body can be divided into equal halves through any plane passing through the central axis.
Seen in animals like Cnidaria (Hydra, Jellyfish) and Echinoderms (Sea stars) (in adult form).
Suitable for sessile or slow-moving animals.

🔹 c. Bilateral Symmetry

Body can be divided into two equal halves only in one plane (left and right sides).
Found in most advanced animals (from Platyhelminthes to Chordates).
Helps in streamlined movement and better coordination.

3. Germ Layer Organization (Diploblastic vs. Triploblastic)

During embryonic development, animals form layers called germ layers, which later develop into different tissues and organs.

🔹 a. Diploblastic Animals

Body develops from two germ layers:
- Ectoderm (outer)
- Endoderm (inner)
A non-cellular mesoglea is present between them.
Found in Cnidarians (Hydra, Jellyfish).

🔹 b. Triploblastic Animals

Body develops from three germ layers:
- Ectoderm
- Mesoderm (middle)
- Endoderm
Mesoderm gives rise to muscles, skeleton, blood, etc.
Found in Platyhelminthes to Chordata (i.e., most animals).

4. Coelom (Body Cavity)

The coelom is a fluid-filled space between the body wall and the internal organs. It allows independent movement of organs and better organ development.

🔹 a. Acoelomates

No coelom (body cavity).
Space between body wall and gut is filled with solid tissue.
Found in Platyhelminthes (e.g., flatworms).

🔹 b. Pseudocoelomates

Body cavity is not fully lined with mesoderm.
Mesoderm is scattered in between ectoderm and endoderm.
Found in Aschelminthes (roundworms) like Ascaris.

🔹 c. Coelomates (True Coelom)

Coelom is completely lined with mesoderm.
Organs are suspended in the cavity and protected by mesodermal linings.
Found in Annelids, Molluscs, Arthropods, Echinoderms, and Chordates.

5. Segmentation (Metamerism)

Segmentation refers to the division of the body into repeated units (segments). It helps in movement and specialization of body parts.

🔹 a. No Segmentation

Body is not divided into segments.
Example: Platyhelminthes, Nematodes, Molluscs.

🔹 b. True Segmentation

Body is clearly divided into repeating segments, both externally and internally.
Found in Annelids (earthworm), Arthropods (insects), and Chordates (vertebral column in humans).
It is called metameric segmentation.

🔹 c. Pseudosegmentation

Segments appear repeated but are not real or internally divided.
Example: Tapeworm (Taenia).

6. Notochord

The notochord is a flexible, rod-like structure present in the embryonic stage of all chordates. It provides support and structure.

🔹 Characteristics of Notochord:

Lies between the nerve cord and the gut.
Acts as a primitive backbone.
Present throughout life in some animals, and replaced by vertebral column in others.

🔹 Classification Based on Notochord:

Type of Animal	Notochord Status
Non-chordates	Absent
Chordates	Present at least in embryonic stage
Urochordates	Notochord only in larval tail
Cephalochordates	Notochord present throughout life
Vertebrates	Notochord is replaced by vertebral column (backbone)

2. Phylums Of Kingdom Animalia or Animal Kingdom

Kingdom Animalia divided into these phylums-

Phylum Porifera
Phylum Coelenterata (also called Cnidaria)
Phylum Ctenophora
Phylum Platyhelminthes
Phylum Aschelminthes
Phylum Annelida
Phylum Arthropoda
Phylum Mollusca
Phylum Echinodermata
Phylum Hemichordata
Phylum Chordata
- Class Cyclostomata
- Class Chondrichthyes
- Class Osteichthyes
- Class Amphibia
- Class Reptilia
- Class Aves
- Class Mammalia

2.1 Porifera: The Sponge Group

Phylum Porifera includes animals commonly known as sponges. These animals are mostly found in the sea and are usually asymmetrical, meaning their body cannot be divided into two equal halves. Sponges are very simple and are considered primitive multicellular organisms. They show a cellular level of organisation, which means that their body is made of loosely arranged cells, but there is no tissue or organ system. A unique feature of sponges is their canal system or water transport system. Water enters their body through small pores on the surface called ostia. This water moves into a central cavity called the spongocoel, and then flows out through a large opening called the osculum. This constant flow of water helps in collecting food, exchanging gases for respiration, and removing waste. The inner lining of the canal system has special cells called choanocytes or collar cells. These help trap food particles. The digestion of food takes place inside the cells (intracellular digestion). The body of the sponge is supported by either spicules or spongin fibres, which form a basic internal skeleton. Sponges are usually hermaphrodites, meaning the same individual produces both eggs and sperms. They can reproduce in two ways: asexual reproduction by fragmentation (breaking into pieces which grow into new individuals), and sexual reproduction by the formation of gametes. In sexual reproduction, fertilisation takes place inside the body (internal fertilisation), and the development of the new organism goes through a larval stage, which looks completely different from the adult (this is called indirect development).

Common examples of sponges are Sycon (also called Scypha), Spongilla (a freshwater sponge), and Euspongia (also known as bath sponge).

2.2 Phylum Coelenterata (Jellyfish and Corals)

Phylum Cnidaria, also known as Coelenterata, includes aquatic organisms that are mostly marine and can be either sessile (fixed) or free-swimming. These animals exhibit radial symmetry, meaning their body can be divided into similar halves by multiple planes passing through the central axis. They are diploblastic, having two germ layers—ectoderm and endoderm—with a jelly-like mesoglea in between. The body is organized at the tissue level, and they have a central gastrovascular cavity with a single opening that functions both as a mouth and anus, located on a raised structure called the hypostome.

One of the most distinguishing features of cnidarians is the presence of specialized cells called cnidoblasts or cnidocytes, which contain nematocysts (stinging capsules) that help in prey capture, defense, and attachment. Digestion is both extracellular (in the gastrovascular cavity) and intracellular (within the cells). Some cnidarians, like corals, secrete calcium carbonate to form exoskeletons. They exist in two basic body forms—polyp (sessile and cylindrical, e.g., Hydra, Adamsia) and medusa (umbrella-shaped and free-swimming, e.g., Aurelia or jellyfish). Some cnidarians such as Obelia exhibit alternation of generations (also known as metagenesis) in which asexual polyps give rise to sexual medusae, and vice versa.

Common examples of cnidarians include Physalia (Portuguese man-of-war), Adamsia (sea anemone), Pennatula (sea pen), Gorgonia (sea fan), and Meandrina (brain coral).

2.3 Marine Animals: Phylum Ctenophora

Phylum Ctenophora, commonly known as sea walnuts or comb jellies, includes exclusively marine organisms that are radially symmetrical and diploblastic, with a soft, transparent, gelatinous body. They show tissue-level organization and are very similar to cnidarians in body structure, but lack cnidoblasts. One of their most unique features is the presence of eight external rows of ciliated comb plates, arranged radially around the body, which beat in a coordinated manner to help in locomotion. This characteristic makes ctenophores the only animals that use cilia for locomotion as their primary mode of movement. Digestion in ctenophores is both extracellular and intracellular.

A striking feature of many ctenophores is bioluminescence, the natural ability to emit light, which is often seen as glowing flashes in the water.

Unlike cnidarians, ctenophores are hermaphroditic, meaning sexes are not separate and each individual produces both eggs and sperm. Reproduction occurs sexually, with external fertilization. Their development is indirect, involving a larval stage before becoming an adult. They do not exhibit alternation of generations.

Examples of ctenophores include Pleurobrachia and Ctenoplana.

2.4 Phylum Platyhelminthes: Flat-bodied Worms

Phylum Platyhelminthes consists of organisms commonly known as flatworms, because their bodies are dorsoventrally flattened. These animals are mostly parasitic, living inside hosts such as humans and animals. They exhibit bilateral symmetry and are triploblastic, having three embryonic layers: ectoderm, mesoderm, and endoderm. However, they are acoelomate, meaning they do not have a true body cavity. Their bodies are organized at the organ level, and many parasitic forms possess hooks and suckers to attach themselves to the host’s internal organs. A unique adaptation in some species is the ability to absorb nutrients directly through their body surface, especially in parasitic types like Taenia. The excretory system contains special cells called flame cells, which help in osmoregulation and excretion. Reproduction in Platyhelminthes is mostly sexual, and most species are hermaphrodites, possessing both male and female reproductive organs. Fertilization is internal, and development is indirect, often involving one or more larval stages. Some free-living members like Planaria have the remarkable ability of regeneration. Examples of Platyhelminthes include Taenia (tapeworm) and Fasciola (liver fluke), which are medically important parasites in humans.

2.5 Phylum Aschelminthes: Pseudocoelomate Worms

Phylum Aschelminthes, commonly referred to as roundworms, includes animals whose bodies are cylindrical in cross-section, hence the name. These organisms can be free-living in aquatic or terrestrial habitats, or parasitic in plants and animals. They exhibit organ-system level of body organisation, which means their organ systems are well-developed to perform specific functions. Roundworms are bilaterally symmetrical, triploblastic (having three germ layers), and pseudocoelomate, meaning they possess a body cavity that is not completely lined with mesoderm. The alimentary canal is complete, starting from the mouth and ending at the anus, and includes a well-developed muscular pharynx that aids in food intake. Excretion occurs through a specialized excretory tube that discharges waste via an excretory pore. These animals are dioecious (sexes are separate), with distinct male and female individuals, and females are often larger than males. Fertilisation is internal, and development can be either direct, where the young resemble adults, or indirect, involving a larval stage. Common examples of this phylum include Ascaris (roundworm), Wuchereria (filaria worm), and Ancylostoma (hookworm) – all of which are of medical significance and often included in NEET-based zoology questions.

2.6 Phylum Annelida: Metamerically Segmented Worms

Phylum Annelida includes animals that can live in water, both marine and freshwater, or on land. These animals are mostly free-living, though some can be parasitic. They have an organ-system level of body organization and show bilateral symmetry, meaning their body can be divided into two equal halves. Annelids are triploblastic, which means they have three layers of cells in their body, and their bodies are segmented into multiple parts called metameres. This segmentation is very clear on their body surface, which is why they are called Annelida, derived from the Latin word ‘annulus’ meaning little ring. They have both longitudinal and circular muscles that help them move. Some aquatic annelids like Nereis have side appendages called parapodia which help them swim. They possess a closed circulatory system where blood flows inside vessels. For excretion and maintaining water balance, they have structures called nephridia. Their nervous system consists of paired ganglia connected by lateral nerves to a double ventral nerve cord. In terms of reproduction, aquatic annelids like Nereis have separate sexes (dioecious), while earthworms and leeches have both male and female reproductive organs in the same individual (monoecious). Reproduction is sexual.

Common examples of annelids are Nereis, Earthworm (Pheretima), and the blood-sucking leech (Hirudinaria).

2.7 Phylum Arthropoda: Jointed-Limbed Animals

Phylum Arthropoda is the largest phylum in the animal kingdom and includes insects. More than two-thirds of all named species on Earth belong to this group. Arthropods have an organ-system level of organization and show bilateral symmetry. They are triploblastic, segmented, and coelomate animals. Their bodies are covered by a hard chitinous exoskeleton which protects them and provides support. The body is divided into three main parts: head, thorax, and abdomen. They possess jointed appendages (the word “arthro” means joint and “poda” means appendages), which help in movement and handling objects. Their respiratory organs vary and include gills, book gills, book lungs, or a tracheal system depending on the species. Arthropods have an open circulatory system, where the blood flows freely in body cavities. Their sensory organs are well-developed and include antennae, compound and simple eyes, and statocysts which help in balance. For excretion, they use malpighian tubules. Most arthropods are dioecious (having separate sexes), with internal fertilization, and they are mostly oviparous (lay eggs). Their development can be direct or indirect depending on the species.

Some important examples include economically valuable insects like Apis (Honey bee), Bombyx (Silkworm), and Laccifer (Lac insect); disease vectors like Anopheles, Culex, and Aedes (mosquitoes); the gregarious pest Locusta (Locust); and the living fossil Limulus (King crab).

2.8 Soft-bodied Animals (Phylum Mollusca)

Phylum Mollusca is the second largest animal phylum and includes animals that can live on land (terrestrial) or in water (marine or freshwater). Molluscs have an organ-system level of organization and show bilateral symmetry. They are triploblastic and coelomate animals. Their body is usually covered by a hard calcareous shell and is unsegmented, consisting of a distinct head, a muscular foot used for movement, and a soft rounded part called the visceral hump. Over the visceral hump, a soft, spongy layer of skin called the mantle is present. The space between the mantle and the visceral hump is called the mantle cavity, which contains feather-like gills that serve both respiratory and excretory functions. The front part of the head has sensory tentacles to help detect the environment. Their mouth contains a special rasping organ called the radula, which is like a file and helps in feeding by scraping food. Molluscs are usually dioecious (having separate sexes) and oviparous (lay eggs), and they develop through indirect development, meaning their young go through larval stages before becoming adults.

Some common examples are Pila (Apple snail), Pinctada (Pearl oyster), Sepia (Cuttlefish), Loligo (Squid), Octopus (Devil fish), Aplysia (Sea hare), Dentalium (Tusk shell), and Chaetopleura (Chiton).

2.9 Spiny-Skinned Animals (Phylum Echinodermata)

Phylum Echinodermata includes animals that have an internal skeleton called an endoskeleton, made up of small calcareous (calcium-based) plates called ossicles, which give them a spiny body, hence the name Echinodermata meaning “spiny skin.” All echinoderms live in the marine environment and have an organ-system level of organization. While the adult echinoderms show radial symmetry (body parts arranged around a central axis), their larvae are bilaterally symmetrical. They are triploblastic and coelomate animals. Their digestive system is complete with the mouth located on the lower (ventral) side and the anus on the upper (dorsal) side. One of the most important and unique features of echinoderms is the water vascular system, which is used for movement, food capture and transport, and respiration. They do not have a separate excretory system.

The sexes are separate, and reproduction is sexual with external fertilization. Their development is indirect, meaning the young go through a free-swimming larval stage before becoming adults.

Some common examples are Asterias (Starfish), Echinus (Sea urchin), Antedon (Sea lily), Cucumaria (Sea cucumber), and Ophiura (Brittle star).

2.10 hylum Hemichordata: Proboscis, Collar, and Trunk Animals

Phylum Hemichordata was earlier considered a sub-phylum under Chordata but is now recognized as a separate phylum within non-chordates. Hemichordates are worm-like marine animals with an organ-system level of organization. They show bilateral symmetry, are triploblastic, and coelomate. Their body is cylindrical and divided into three parts: an anterior proboscis, a collar, and a long trunk. A unique feature of hemichordates is the presence of a rudimentary structure in the collar called the stomochord, which is similar to the notochord found in chordates. They have an open circulatory system, and respiration occurs through gills. Their excretory organ is called the proboscis gland. The sexes are separate, fertilization is external, and their development is indirect, meaning their larvae go through free-swimming stages before maturing. Examples include Balanoglossus and Saccoglossus.

2.11 Phylum Chordata

Phylum Chordata is mainly characterized by the presence of a notochord, a dorsal hollow nerve cord, and paired pharyngeal gill slits. These animals show bilateral symmetry, are triploblastic, coelomate, and have an organ-system level of organization. They also possess a post-anal tail and a closed circulatory system. Phylum Chordata is divided into three subphyla: Urochordata (Tunicata), Cephalochordata, and Vertebrata. The first two subphyla, Urochordata and Cephalochordata, are called protochordates and live exclusively in the marine environment. In Urochordata, the notochord is present only in the larval tail, whereas in Cephalochordata, it extends from head to tail and persists throughout life. Examples of Urochordata include Ascidia, Salpa, and Doliolum, while Branchiostoma (Amphioxus or Lancelet) represents Cephalochordata.

Members of the subphylum Vertebrata have a notochord during embryonic development, which is replaced by a vertebral column made of cartilage or bone in adults. This means all vertebrates are chordates, but not all chordates are vertebrates. Besides the basic chordate features, vertebrates have a ventral muscular heart with two, three, or four chambers, kidneys for excretion and osmoregulation, and paired appendages like fins or limbs.

Class Cyclostomata

Class Cyclostomata includes living animals that are ectoparasites on some fishes. They have an elongated body with 6 to 15 pairs of gill slits used for respiration. Cyclostomes possess a circular, sucking mouth without jaws, which helps them attach to their host. Their bodies lack scales and paired fins. The cranium and vertebral column in these animals are made of cartilage rather than bone. They have a closed circulatory system. Cyclostomes mainly live in marine environments but migrate to freshwater to spawn. After spawning, they die within a few days. Their larvae undergo metamorphosis and then return to the ocean. Examples of this class are Petromyzon (Lamprey) and Myxine (Hagfish).

Class Chondrichthyes

Class Chondrichthyes consists of marine animals with a streamlined body and a cartilaginous endoskeleton, meaning their internal skeleton is made of cartilage instead of bone. Their mouth is located on the ventral (lower) side. The notochord remains throughout their life. They have several separate gill slits but no operculum (gill cover). Their skin is tough and covered with tiny placoid scales. These scales are modified into sharp, backward-pointing teeth that make their jaws very powerful. Chondrichthyes are predators and because they lack an air bladder to help with buoyancy, they must keep swimming constantly to avoid sinking. Their heart has two chambers (one auricle and one ventricle). Some species have special electric organs (like the Torpedo fish), while others have poisonous stings (like the Trygon or stingray). These animals are cold-blooded (poikilothermic), meaning they cannot regulate their body temperature. The sexes are separate, and males have claspers on their pelvic fins used during reproduction. Fertilization is internal, and many species are viviparous, giving birth to live young. Examples include Scoliodon (Dog fish), Pristis (Saw fish), Carcharodon (Great white shark), and Trygon (Sting ray).

Class Osteichthyes

Class Osteichthyes includes both marine and freshwater fishes that have a bony endoskeleton. Their bodies are streamlined for easy movement in water. The mouth is mostly terminal, meaning it is located at the front of the head. They have four pairs of gills, each covered by a protective flap called the operculum. Their skin is covered with cycloid or ctenoid scales which provide protection. Osteichthyes have an air bladder (swim bladder) that helps in regulating buoyancy, allowing them to stay afloat at different water depths. Their heart has two chambers—one auricle and one ventricle. These fishes are cold-blooded, so they cannot regulate their body temperature internally. The sexes are separate, and fertilization is mostly external. Most of them are oviparous (egg-laying), and their development is direct, meaning the young hatch as smaller versions of adults. Examples include marine fishes like Exocoetus (Flying fish) and Hippocampus (Sea horse); freshwater fishes like Labeo (Rohu), Catla (Katla), and Clarias (Magur); and popular aquarium fishes like Betta (Fighting fish) and Pterophyllum (Angel fish).

Class Amphibia

Class Amphibia includes animals that can live both in aquatic (water) and terrestrial (land) habitats, which is why their name means “dual life.” Most amphibians have two pairs of limbs. Their body is divided into a head and a trunk, and some species have a tail. Their skin is moist and scaleless, which helps in respiration through the skin. They have eyes with eyelids and a hearing structure called the tympanum (eardrum). Their alimentary canal, urinary, and reproductive systems open into a common chamber called the cloaca, which opens to the outside. Amphibians breathe through gills, lungs, and their skin. Their heart has three chambers—two auricles and one ventricle. They are cold-blooded animals, meaning they cannot regulate their body temperature internally. The sexes are separate, fertilization is usually external, and they are oviparous (egg-laying). Their development is indirect, involving a larval stage before becoming adults. Examples include Bufo (Toad), Rana (Frog), Hyla (Tree frog), Salamandra (Salamander), and Ichthyophis (Limbless amphibian).

Class Reptilia

Class Reptilia includes animals known for their creeping or crawling movement, which is why their name comes from the Latin word meaning “to creep.” Most reptiles are terrestrial and have bodies covered with dry, cornified skin made up of epidermal scales or scutes. They do not have external ear openings, but instead have a tympanum that functions as the ear. When limbs are present, reptiles have two pairs. Their heart usually has three chambers, except for crocodiles, which have a four-chambered heart. Reptiles are poikilotherms (cold-blooded), meaning they cannot regulate their internal body temperature. Snakes and lizards shed their skin periodically. The sexes are separate, fertilization is internal, and reptiles are mostly oviparous (egg-laying) with direct development. Examples include Chelone (Turtle), Testudo (Tortoise), Chameleon (Tree lizard), Calotes (Garden lizard), Crocodilus (Crocodile), Alligator (Alligator), Hemidactylus (Wall lizard), and poisonous snakes like Naja (Cobra), Bangarus (Krait), and Vipera (Viper).

Class Aves (birds)

Class Aves (birds) are mainly characterized by the presence of feathers, and most of them have the ability to fly, except for some flightless birds like the Ostrich. They have a beak instead of teeth. Their forelimbs are modified into wings used for flying, while their hind limbs usually have scales and are adapted for walking, swimming, or grasping tree branches. Their skin is dry and lacks glands except for an oil gland at the base of the tail, which helps in maintaining feather condition. Birds have a fully ossified (bony) endoskeleton, with hollow long bones containing air cavities called pneumatic bones to reduce weight for flight. The digestive system includes special chambers called the crop and gizzard for food storage and grinding. Their heart is completely four-chambered, allowing efficient separation of oxygenated and deoxygenated blood. Birds are warm-blooded (homoiothermic), meaning they maintain a constant body temperature. Respiration takes place through lungs supplemented by air sacs connected to the lungs, improving breathing efficiency. The sexes are separate, fertilization is internal, and birds are oviparous (egg-laying) with direct development. Examples include Corvus (Crow), Columba (Pigeon), Psittacula (Parrot), Struthio (Ostrich), Pavo (Peacock), Aptenodytes (Penguin), and Neophron (Vulture).

Class Mammalia

Class Mammalia includes animals found in a wide range of habitats such as polar ice caps, deserts, mountains, forests, grasslands, and dark caves. Some mammals have adapted to fly or live in water. The most distinctive feature of mammals is the presence of mammary glands, which produce milk to nourish their young ones. Mammals have two pairs of limbs adapted for various activities like walking, running, climbing, burrowing, swimming, or flying. Their skin is unique because it has hair. They have external ears (pinnae), and their jaws contain different types of teeth for various functions. The mammalian heart is four-chambered, supporting efficient blood circulation. Mammals are homoiothermic (warm-blooded), meaning they maintain a constant body temperature. They breathe through lungs. The sexes are separate, fertilization is internal, and most mammals are viviparous (give birth to live young), although there are exceptions. Development is direct, meaning the young are born in a form similar to adults. Examples include the oviparous Platypus (Ornithorhynchus); and viviparous animals like Macropus (Kangaroo), Pteropus (Flying fox), Camelus (Camel), Macaca (Monkey), Rattus (Rat), Canis (Dog), Felis (Cat), Elephas (Elephant), Equus (Horse), Delphinus (Common dolphin), Balaenoptera (Blue whale), Panthera tigris (Tiger), and Panthera leo (Lion).

2.2 Summury

The animal kingdom is classified broadly based on fundamental features like level of organization, symmetry, cell organization, coelom, segmentation, and presence of a notochord. Each phylum or class also has unique characteristics. Porifera are multicellular animals with a cellular level of organization and special cells called choanocytes. Coelenterates have tentacles with stinging cells called cnidoblasts and are mostly aquatic, either fixed or free-floating. Ctenophores are marine animals with comb plates for movement. Platyhelminths have a flat body with bilateral symmetry, and their parasitic forms have suckers and hooks. Aschelminthes are pseudocoelomates and include both parasitic and non-parasitic roundworms. Annelids have bodies divided into segments (metameric segmentation) with a true coelom. Arthropods are the largest group with jointed appendages and an external skeleton made of chitin. Molluscs have a soft body covered by a calcareous shell. Echinoderms have spiny skin and a unique water vascular system. Hemichordates are worm-like marine animals with a body divided into proboscis, collar, and trunk.

Chordates possess a notochord during some part of their life, a dorsal hollow nerve cord, and paired pharyngeal gill slits. Among chordates, Agnatha (jawless fishes) are represented by the class Cyclostomata, which are primitive ectoparasites on fishes. Gnathostomata (jawed vertebrates) include two superclasses: Pisces and Tetrapoda. Chondrichthyes and Osteichthyes are fish with fins and belong to Pisces; Chondrichthyes have cartilaginous skeletons and live in the sea. Tetrapods, which have two pairs of limbs, include Amphibia, Reptilia, Aves, and Mammalia. Amphibians live both on land and water, while reptiles have dry, cornified skin and no limbs in snakes. Fish, amphibians, and reptiles are cold-blooded (poikilotherms). Birds (Aves) are warm-blooded with feathers, forelimbs modified into wings, and hind limbs adapted for walking, swimming, or perching. Mammals are unique due to their mammary glands and hair, and most give birth to live young (viviparous).

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