Discuss the general characteristics of the phylum coelenterata or cnidaria.

Explanation

General characters of phylum coelenterata or cnidaria are as follows :
(i) Cnidarians have basically two types of bodies. These are as follows:
(a) Polyp (cup shaped): Polyp is sessile cylindrical structure with mouth and tentacles facing upwards.
(b) Medusa :It is free swimming body with mouth and tentacles facing downwards.
(ii) Symmetry of coelenterates is usually radial.
(iii) Cnidarians are diploblastic like sponges, i.e., with two germ layers, ectoderm and endoderm.
(iv) Tissue level of organisation occurs in cnidarians.
(v). Locomotory appendages are absent in cnidarians.
(vi) The specific cells in specific cnidarians include stinging cells (cnidoblasts or nematoblasts) for offence and defence, interstitial cells to replace the worn out cells and epitheliomuscular cells to provide musculature.
(vii) Digestion is mainly intracellular and intercellular
(viii) All the cells are in contact with water. Therefore, they exchange gases, directly.
(ix) In cnidarians, excretion occurs through general body surface. Cnidarians are ammonotelic
(x) Sensory cells occur in the body wall
(xi) A horny exoskeleton as well as endoskeleton is present in many cases.
(xii) Asexual reproduction occurs through fragmentation, budding and fission.
(xiii) Animals are monoecious or dioecious. Fertilization is external or internal.