ANIMAL KINGDOM

Animals are multicellular, eukaryotic and heterotrophic organisms. They do not have cell walls, usually ingest food, and differ widely in body design. NCERT classifies them using level of organisation, symmetry, germ layers, coelom, segmentation and the presence or absence of a notochord.

Organisation, symmetry and germ layers

Poriferans show cellular level organisation; cnidarians and ctenophores show tissue level; platyhelminthes show organ level; and higher animals show organ-system level. Sponges are generally asymmetrical. Cnidarians and adult echinoderms are radially symmetrical, while most animals are bilaterally symmetrical. Cnidaria and Ctenophora are diploblastic; all higher animals are triploblastic.

Body cavity and digestive plan

In acoelomates such as platyhelminthes, the space between body wall and gut is filled with cells. Aschelminthes have a pseudocoel. In coelomates, the cavity is lined by mesoderm. A blind-sac digestive system has one opening; a complete digestive system has separate mouth and anus.

Porifera - sponges

Poriferans are mostly marine, asymmetrical and cellular in organisation. Water enters through ostia, passes through a canal system and leaves through the osculum. Collar cells (choanocytes) line the canals; the skeleton has spicules or spongin fibres. They are generally hermaphrodites, reproduce asexually by fragmentation and sexually by gamete formation, and have internal fertilisation with indirect development. Examples: Sycon, Spongilla, Euspongia.

Cnidaria - coelenterates

These aquatic, mostly marine, radially symmetrical diploblastic animals possess cnidoblasts with nematocysts for defence, anchorage and prey capture. They have a central gastrovascular cavity and are either polyp or medusa. Examples: Hydra, Obelia, Physalia, Aurelia, Adamsia, Pennatula.

Ctenophora - comb jellies

Ctenophores are marine, diploblastic and radially symmetrical. Eight external rows of ciliated comb plates help in locomotion; colloblasts capture prey. They show bioluminescence and usually reproduce sexually with external fertilisation and indirect development. Examples: Pleurobrachia and Ctenoplana.

Platyhelminthes - flatworms

Flatworms are bilaterally symmetrical, triploblastic, acoelomate and dorsoventrally flattened. They have organ-level organisation and may be free-living or parasitic. Flame cells provide osmoregulation and excretion. Many are hermaphrodites. Examples: Planaria, Fasciola (liver fluke) and Taenia (tapeworm).

Aschelminthes - roundworms

These animals are bilaterally symmetrical, triploblastic and pseudocoelomate, with organ-system level organisation. They have a complete digestive tract and a muscular pharynx. Sexes are separate and sexual dimorphism is often distinct. Examples: Ascaris, Wuchereria and Ancylostoma.

Annelida

Annelids are bilaterally symmetrical, triploblastic, coelomate animals with organ-system organisation and metameric segmentation. They have a closed circulatory system and nephridia for osmoregulation and excretion. Examples: Nereis, Pheretima (earthworm) and Hirudinaria (leech).

Arthropoda

Arthropoda is the largest phylum. Its members have a segmented body, jointed appendages and chitinous exoskeleton. The body cavity is haemocoel; circulation is open. Excretion occurs through Malpighian tubules in insects and many arachnids, green glands in crustaceans, or coxal glands in arachnids. Examples: cockroach, honey bee, prawn, scorpion and spider.

Mollusca

Molluscs have a soft unsegmented body, generally divided into head, muscular foot and visceral hump. A mantle usually secretes a calcareous shell; the radula is a rasping feeding organ in most molluscs. They generally have open circulation and kidney-like organs for excretion. Examples: Chiton, Dentalium, Pila, Unio, Octopus and Sepia.

Echinodermata

These are marine, triploblastic coelomates with organ-system level organisation. Adults show radial symmetry while larvae are bilateral. Their endoskeleton consists of calcareous ossicles, and the water vascular system helps in locomotion, capture and transport of food, and respiration. Sexes are separate; fertilisation is external and development indirect. Examples: Asterias, Echinus, Ophiura, Cucumaria and Antedon.

Hemichordata

Hemichordates are worm-like marine animals whose body is divided into proboscis, collar and trunk. They possess gill slits, but a true notochord is absent; therefore Hemichordata is a separate non-chordate phylum. Examples: Balanoglossus and Saccoglossus.

Chordates possess a notochord, dorsal hollow nerve cord, paired pharyngeal gill slits and a post-anal tail at least at some stage of life. They are bilaterally symmetrical, triploblastic, coelomate animals with organ-system level organisation and a closed circulatory system.

Amphibia

Amphibians live both on land and in water. Skin is moist and glandular; larvae have gills while adults use lungs and skin. The heart is three-chambered and fertilisation is external. Examples: Bufo (toad), Rana (frog), Hyla and Salamandra.

Reptilia

Reptiles are mostly terrestrial with dry, cornified skin, internal fertilisation and amniotic eggs. Their heart is usually three-chambered; crocodiles have four chambers. Examples: turtles, Calotes, Chameleon, Ophiosaurus, Hydrophis and Crocodylus.

Aves and Mammalia

Birds have feathers, forelimbs modified into wings, pneumatic bones, a four-chambered heart and oviparous reproduction with hard-shelled eggs. Mammals have hair, pinnae and mammary glands; they are mostly viviparous and have a four-chambered heart. Monotremes lay eggs, while marsupials have poorly developed young. Examples: Corvus, Columba, Ornithorhynchus, kangaroo, bat, whale and human.

1. List the six major bases used to classify animals.
2. Differentiate acoelomates, pseudocoelomates and coelomates with examples.
3. Compare Porifera, Cnidaria and Ctenophora.
4. State the distinguishing characters of the major non-chordate phyla.
5. Why is Arthropoda considered the largest phylum?
6. Compare Chondrichthyes and Osteichthyes.
7. State the chordate features and identify key vertebrate exceptions.