SUMMARY
We have been able to broadly categorise the animal kingdom using the main fundamental characteristics, including level of organisation symmetry, cell organisation, coelom segmentation, and notochord. In addition to the essential characteristics, each phylum or class has a variety of distinctive characteristics.
Porifer is a class of multicellular organisms that lack cellular organisation and feature distinctive flagellated chanocytes. The coelenterates have cridoblasts and tentacles. They float freely or are filthy aquate. Marine mammals with comb-like plates are the Ctenophores. The bodies of platyhelminths are flat and symmetric on both sides. Specific suckers and hooks can be seen on the parasitic forms. Pecudocoelomates known as aschelminthes comprise both parasitic and non-parasitic roundworms.
Annelids are metamenically segmented animals with a true coelom. The arthropods are the most abundant group of animals characterised by the presence of jointed appendages. The molluses have a soft body surrounded by an external calcareous shell. The body is covered with external skeleton made of chitin. The echinoderms possess a spiny skin. Their most distinctive feature is the presence of water vascular system. The hemichordates are a small group of worm-like marine animals. They have a cylindrical body with proboscis, collar and trunk.
Phylum Chordata includes animals which possess a notochord either throughout or during early embryonic life Other common features observed in the chordates are the dorsal. hollow nerve cord and paired pharyngeal gill slits Some of the vertebrates do not possess jaws (Agnatha) whereas most of them possess Jaws (Gnathostomata) Agnatha is represented by the class, Cyclostomata. They are the most primluve chordates and are ectoparasites on fishes. Gnathostomata has two super classes, Pisces and Tetrapoda. Classes Chondrichthyes and Osteichthyes bear fins for locomotion and are grouped under Pisces. The Chondrichthyes are fishes with cartilaginous endoskeleton and are marine Classes Amphibia. Reptilia. Aves and Mammalia have two pairs of limbs and are thus grouped under Tetrapoda. The amphibians have adapted to live both on land and water. Reptiles are characterised by the presence of dry and cornified skin. Limbs are absent in snakes. Fishes, amphibians and reptiles are poikilothermous (cold- blooded). Aves are warm-blooded animals with feathers on their bodies and forelimbs modified into wings for flying. Hind limbs are adapted for walking. swimming, perching or clasping. The unique features of mammals are the presence of mammary glands and hairs on the skin. They commonly exhibit viviparity.
