Metazoas are further classified into two main groups, viz:
(1) Invertebrates – They do not possess a vertebral column and are classified into a various phyla:
(a)Porifera – are fixed multicellular organisms, aquatic and have a porus and cylicrical body, e.g. Ascon Sycon (sponges).
(b)Coelenterata – are sac-like multicellular aquatic creatures with tentacles stinging cells, possess tissues and a digestive cavity, e.g. Hydra, Obelia, Aurelia (jelly fish).
(c)Platyhelminthes – are both parasitic and free-living organisms that have a flat body, thin, soft and appear leaf or ribbon-like, e.g. liver fluke, pork tapeworm, blood fluke, etc.
(d)Nemathelminthes – are both parasitic and free-living organisms, with a round body and possess a mouth and anus. E.g. roundworms, filaria, hooke worms, etc.
(e)Annelida – these are mostly free-living wormlike organisms having soft, round and segmented bodies (marked with external rings). E.g. earthworms.
(f)Anthropoda – is a large group of organisms having joined feet, segmented body and an external skeleton (exoskeleton) and are classified into four large classes:
(i)Crustaceans – arthropods with limbs attached to most segments, e.g. Palaemon (prawn), Dolphia (water flea), Branchipus (shrimp).
(ii)Insects- Arthropods with three pair of legs and body divided into three parts, e.g. Periplanata (cockroach), Papilio (butterfly), Musca (housefly), Culex (mosquito).
(iii)Arachnids – arthropods with four pair of legs and body divided into two parts, e.g. Buthus (scorpion), Lycosa (wolf spider).
(iv)Myriapods – arthropods with body divided into a number of similar segments with one or two pairs of legs to most segments, e.g. Scolopendra (centipede), Julus (millipede).
(v)Mollusca – are shell bearing organisms having an unsegmented body without appendages. They possess ventral muscular organ called a foot for locomotion, e.g. Pila (apple or pond snail), Limax (slug), Sepia (cuttlefish).
(vi)Exhinodermata – include spiny-skinned animals built on a five-radial plan. They move by numerous soft tube-like structures called tube feet, and are marine by habitation, e.g. Astropecten (starfish), Antedon (sea-jelly), Exchinus (sea urchin), etc.
Largest known invertebrates – The Giant Squid belonging to Phyla Mollusca, the body of which measures about 6m and tentacles may measure upto 10 m.
(2) Vertebrates – The remaining animals with a vertebral column are grouped under this main class also known as Phylum Chordata. There are however, a few forms that lack backbones and are considered primitive protochordates. Phylum Chordata has three unique characteristics, viz. (i) a dorsal, hollow tubular nerve cord, (ii) a notochord which is ventral to the nerve cord and is replaced by cartilage or bone to form a ventral column in higher vertebrates, and (iii) gill slits in the pharynx. Chordata is further divided into the following classes:
(a)Pisces (fishes) – are aquatic, cold-blooded, jawed vertebrates. Their bodies are covered with scales; they respire through gills, move with the help of fins (no legs) and have a two-chambered heart, e.g. Labeo (rohu), hippocampus (sea horse), Torpedo (electric fish).
(b)Amphibia (frogs) – are cod-blooded vertebrates with four legs and scaleless skin and adapted to live both in water and on land. Respiration is done by gills or the skin or lungs. Fertilisation is external, i.e. takes place outside the body, generally in the water. In most amphibians there are two stages of life. In one, i.e. larval form, they live only in water and the other, i.e. adult stage; they live in water as well as on land. e.g. Rana (frog), Bufo (toad).
(c)Reptiles (creeping creatures) – are cold-blooded vertebrates, adapted to life in dry places. Respiration is by means of lungs. The heart is three-chambered, except crocodiles, which have a four-chambered heart. There is no larval stage in their development. Fertilisation is external, e.g. Testudo (tortoise), Hemidactylus (house lizard), Naja (cobra).
(d)Aves (birds) – are warm-blooded animals with a body-covering of feathers. Their four legs are adopted as wings and are usually used to fly. There are,however,a few birds which cannot or hardly fly, e.g. an ostrich, kiwi, etc. The heart is four-chambered and the digestive tract has two additional organs, viz. crop which stores food and gizzard which grinds the food. Fertilisation is internal, i.e. takes place inside the body, e.g. Passer (sparrow), Columba (pigeon), pavo (peacock).
(e)Mammalian – are warm-blooded vertebrates and usually nourish their young ones with their milk produced in special glands called mammary glands. The heart is four chambered and respiration takes place through the lungs. There is a muscular partition (diaphragm) that separates the thoracic from the abdominal cavity. The embryo receives nourishment and oxygen through the placenta in the female’s womb and disposes of wastes through the maternal blood circulation, e.g. Macropus (kangroo), Felis (cat), Canis (dog), Balaena (whale), Homo (man),