Animal Kingdom

sponge

Members of this phylum are commonly known as sponges. They are generally marine and mostly asymmetrical. These are primitive multicellular animals and have cellular level of organisation.

Sponges have a water transport or canal system. Water enters through minute pores (ostia) in the body wall into a central cavity, spongocoel, from where it goes out through the osculum. This pathway helps in food gathering, respiratory exchange and removal of waste.

Choanocytes or collar cells line the spongocoel and the canals. Digestion is intracellular. The body is supported by a skeleton made up of spicules or spongin fibres. Sexes are not separate (hermaphrodite). Sponges reproduce asexually by fragmentation and sexually by formation of gametes. Fertilisation is internal and development is indirect having a larval stage.

Examples: Sycon (Scypha), Spongilla (Fresh water sponge) and Euspongia (Bath sponge).

🪸 Cnidaria

They are aquatic, mostly marine, sessile or free-swimming, radially symmetrical animals. The name cnidaria is derived from cnidoblasts or cnidocytes (which contain the stinging capsules or nematocysts) present on the tentacles and the body. Cnidoblasts are used for anchorage, defense and for the capture of prey.

Cnidarians exhibit tissue level of organisation and are diploblastic. They have a central gastro-vascular cavity with a single opening, mouth on hypostome. Digestion is extracellular and intracellular. Some cnidarians, e.g., corals have a skeleton composed of calcium carbonate.

Cnidarians exhibit two basic body forms: polyp (sessile and cylindrical, like Hydra, Adamsia) and medusa (umbrella-shaped and free-swimming, like Aurelia or jelly fish). Those which exist in both forms exhibit alternation of generations (Metagenesis) — polyps produce medusae asexually and medusae form the polyps sexually (e.g., Obelia).

Examples: Physalia (Portuguese man-of-war), Adamsia (Sea anemone), Pennatula (Sea-pen), Gorgonia (Sea-fan) and Meandrina (Brain coral).

🪼 Ctenophora

Ctenophores, commonly known as sea walnuts or comb jellies are exclusively marine, radially symmetrical, diploblastic organisms with tissue level of organisation. The body bears eight external rows of ciliated comb plates, which help in locomotion. Digestion is both extracellular and intracellular. Bioluminescence (the property of a living organism to emit light) is well-marked in ctenophores. Sexes are not separate. Reproduction takes place only by sexual means. Fertilisation is external with indirect development.

Examples: Pleurobrachia and Ctenoplana.

🪱 Platyhelminthes

They have dorso-ventrally flattened body, hence are called flatworms. These are mostly endoparasites found in animals including human beings. Flatworms are bilaterally symmetrical, triploblastic and acoelomate animals with organ level of organisation.

Hooks and suckers are present in the parasitic forms. Some absorb nutrients from the host directly through their body surface. Specialised cells called flame cells help in osmoregulation and excretion. Sexes are not separate. Fertilisation is internal and development is through many larval stages. Some members like Planaria possess high regeneration capacity.

Examples: Taenia (Tapeworm), Fasciola (Liver fluke).

🐛 Aschelminthes

The body is circular in cross-section, hence the name roundworms. They may be free-living, aquatic and terrestrial or parasitic in plants and animals. Roundworms have organ-system level of body organisation. They are bilaterally symmetrical, triploblastic and pseudocoelomate animals.

Alimentary canal is complete with a well-developed muscular pharynx. An excretory tube removes body wastes through the excretory pore. Sexes are separate (dioecious). Often females are longer than males. Fertilisation is internal.

Examples: Ascaris (Roundworm), Wuchereria (Filaria worm), Ancylostoma (Hookworm).

🪱 Annelida

They may be aquatic (marine and fresh water) or terrestrial; free-living, and sometimes parasitic. They exhibit organ-system level of body organisation and bilateral symmetry. They are triploblastic, metamerically segmented and coelomate animals. Their body surface is distinctly marked out into segments or metameres and, hence, the phylum name Annelida (Latin, annulus: little ring).

They possess longitudinal and circular muscles which help in locomotion. Aquatic annelids like Nereis possess lateral appendages, parapodia, which help in swimming. A closed circulatory system is present. Nephridia help in osmoregulation and excretion. Neural system consists of paired ganglia connected by lateral nerves to a double ventral nerve cord. Nereis is dioecious, but earthworms and leeches are monoecious.

Examples: Nereis, Pheretima (Earthworm) and Hirudinaria (Blood sucking leech).

🦗 Arthropoda

This is the largest phylum of Animalia which includes insects. Over two-thirds of all named species on earth are arthropods. They have organ-system level of organisation. They are bilaterally symmetrical, triploblastic, segmented and coelomate animals. The body is covered by chitinous exoskeleton. The body consists of head, thorax and abdomen. They have jointed appendages (arthros: joint, poda: appendages).

Respiratory organs are gills, book gills, book lungs or tracheal system. Circulatory system is of open type. Sensory organs like antennae, eyes (compound and simple), statocysts or balancing organs are present. Excretion takes place through malpighian tubules. They are mostly dioecious. Fertilisation is usually internal. They are mostly oviparous.

🐚 Mollusca

This is the second largest animal phylum. Molluscs are terrestrial or aquatic (marine or fresh water) having an organ-system level of organisation. They are bilaterally symmetrical, triploblastic and coelomate animals. Body is covered by a calcareous shell and is unsegmented with a distinct head, muscular foot and visceral hump.

A soft and spongy layer of skin forms a mantle over the visceral hump. The space between the hump and the mantle is called the mantle cavity in which feather-like gills are present. The mouth contains a file-like rasping organ for feeding, called radula.

Examples: Pila (Apple snail), Pinctada (Pearl oyster), Sepia (Cuttlefish), Loligo (Squid), Octopus (Devil fish), Aplysia (Sea-hare), Dentalium (Tusk shell) and Chaetopleura (Chiton).

⭐ Echinodermata

These animals have an endoskeleton of calcareous ossicles and, hence, the name Echinodermata (Spiny bodied). All are marine with organ-system level of organisation. The adult echinoderms are radially symmetrical but larvae are bilaterally symmetrical. They are triploblastic and coelomate animals.

The most distinctive feature of echinoderms is the presence of water vascular system which helps in locomotion, capture and transport of food and respiration. An excretory system is absent. Sexes are separate. Fertilisation is usually external.

Examples: Asterias (Star fish), Echinus (Sea urchin), Antedon (Sea lily), Cucumaria (Sea cucumber) and Ophiura (Brittle star).

🪱 Hemichordata

Hemichordata was earlier considered as a sub-phylum under phylum Chordata. But now it is placed as a separate phylum under non-chordata. Hemichordates have a rudimentary structure in the collar region called stomochord, a structure similar to notochord.

This phylum consists of a small group of worm-like marine animals with organ-system level of organisation. They are bilaterally symmetrical, triploblastic and coelomate animals. The body is cylindrical and is composed of an anterior proboscis, a collar and a long trunk. Circulatory system is of open type. Respiration takes place through gills. Excretory organ is proboscis gland.

Examples: Balanoglossus and Saccoglossus.

🐟 Class Cyclostomata

All living members are ectoparasites on some fishes. They have an elongated body bearing 6–15 pairs of gill slits for respiration. Cyclostomes have a sucking and circular mouth without jaws. Their body is devoid of scales and paired fins. Circulation is of closed type. Marine but migrate for spawning to fresh water. After spawning, they die. Larvae, after metamorphosis, return to the ocean.

Examples: Petromyzon (Lamprey) and Myxine (Hagfish).

🦈 Class Chondrichthyes

Marine animals with streamlined body and cartilaginous endoskeleton. Mouth is located ventrally. Notochord is persistent throughout life. Gill slits are separate and without operculum. The skin is tough, containing minute placoid scales. Teeth are modified placoid scales which are backwardly directed. Their jaws are very powerful. Due to the absence of air bladder, they have to swim constantly to avoid sinking. Heart is two-chambered. They are cold-blooded (poikilothermous). Sexes are separate. Internal fertilisation and many are viviparous.

Examples: Scoliodon (Dog fish), Pristis (Saw fish), Carcharodon (Great white shark), Trygon (Sting ray).

🐟 Class Osteichthyes

Both marine and fresh water fishes with bony endoskeleton. Body is streamlined. Mouth is mostly terminal. They have four pairs of gills which are covered by an operculum on each side. Skin is covered with cycloid/ctenoid scales. Air bladder is present which regulates buoyancy. Heart is two-chambered. Cold-blooded. Sexes are separate. Fertilisation is usually external. Mostly oviparous.

Examples: Marine — Exocoetus (Flying fish), Hippocampus (Sea horse); Freshwater — Labeo (Rohu), Catla (Katla), Clarias (Magur); Aquarium — Betta (Fighting fish), Pterophyllum (Angel fish).

🐸 Class Amphibia

As the name indicates (Gr., Amphi: dual, bios: life), amphibians can live in aquatic as well as terrestrial habitats. Most have two pairs of limbs. Body is divisible into head and trunk. The amphibian skin is moist (without scales). The eyes have eyelids. A tympanum represents the ear. Alimentary canal, urinary and reproductive tracts open into a common chamber called cloaca. Respiration is by gills, lungs and through skin. Heart is three-chambered. Cold-blooded. Sexes are separate. External fertilisation. Oviparous and development is indirect.

Examples: Bufo (Toad), Rana (Frog), Hyla (Tree frog), Salamandra (Salamander), Ichthyophis (Limbless amphibia).

🦎 Class Reptilia

The class name refers to their creeping or crawling mode of locomotion. They are mostly terrestrial animals and their body is covered by dry and cornified skin, epidermal scales or scutes. They do not have external ear openings. Tympanum represents ear. Limbs, when present, are two pairs. Heart is usually three-chambered, but four-chambered in crocodiles. Reptiles are poikilotherms. Snakes and lizards shed their scales as skin cast. Sexes are separate. Internal fertilisation. Oviparous and development is direct.

Examples: Chelone (Turtle), Testudo (Tortoise), Chameleon (Tree lizard), Calotes (Garden lizard), Crocodilus (Crocodile), Alligator (Alligator). Poisonous snakes — Naja (Cobra), Bangarus (Krait), Vipera (Viper).

🦅 Class Aves

The characteristic features of Aves are the presence of feathers and most of them can fly except flightless birds (e.g., Ostrich). They possess beak. The forelimbs are modified into wings. The hind limbs generally have scales and are modified for walking, swimming or clasping tree branches. Skin is dry without glands except the oil gland at the base of the tail. Endoskeleton is fully ossified and long bones are hollow with air cavities (pneumatic). The digestive tract has additional chambers, the crop and gizzard. Heart is completely four-chambered. They are warm-blooded (homoiothermous). Respiration is by lungs. Air sacs supplement respiration. Internal fertilisation. Oviparous.

Examples: Corvus (Crow), Columba (Pigeon), Psittacula (Parrot), Struthio (Ostrich), Pavo (Peacock), Aptenodytes (Penguin), Neophron (Vulture).

🦁 Class Mammalia

They are found in a variety of habitats — polar ice caps, deserts, mountains, forests, grasslands and dark caves. Some have adapted to fly or live in water. The most unique mammalian characteristic is the presence of milk-producing glands (mammary glands) by which the young ones are nourished. They have two pairs of limbs, adapted for walking, running, climbing, burrowing, swimming or flying. The skin of mammals is unique in possessing hair. External ears or pinnae are present. Different types of teeth are present in the jaw. Heart is four-chambered. Homoiothermous. Respiration is by lungs. Sexes are separate and fertilisation is internal. They are viviparous with few exceptions and development is direct.

Examples: Oviparous — Ornithorhynchus (Platypus); Viviparous — 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), Panthera leo (Lion).