C LASSIFICATION OF A NIMALS The broad classification of Animalia based on common fundamental features as mentioned in the preceding sections is given in Figure . . The important characteristic features of the different phyla are described. .
. Phylum – Porifera Members of this phylum are commonly known as sponges. They are generally marine and mostly asymmetrical animals (Figure . ).
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 of water transport is helpful 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 ), i.e., eggs and sperms are produced by the same individual.
Sponges reproduce asexually by fragmentation and sexually by formation of gametes. Fertilisation is internal and development is indirect having a larval stage which is morphologically distinct from the adult. *Echinodermata exhibits radial or bilateral symmetry depending on the stage. Figure .
Broad classification of Kingdom Animalia based on common fundamental features (c) Figure . Examples of Porifera : (a) Sycon (b) Euspongia (c) Spongilla 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 (Figure . ).
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 of the cnidarians, e.g., corals have a skeleton composed of calcium carbonate.
Cnidarians exhibit two basic body forms called polyp and medusa (Figure . ). The former is a sessile and cylindrical form like Hydra, Adamsia , etc. whereas, the latter is umbrella-shaped and free-swimming like Aurelia or jelly fish.
Those cnidarians which exist in both forms exhibit