These are also known as the sting cells. Their stinging nature was discovered by Aristotle. They provide the link between animals and plants. Peyssonel and Trembley grouped them as animals. Coelenterates were put together with the sponges by Leuckart. They were further divided into different phyla by Hatschek.
All coelenterate reside in water. They are mostly marine and rarely occur in fresh water. They may occur alone or in colonies. They are attached to the substratum or may be free living. They have different size and shapes. They form a trimorphic colony with the different kind of zooids. Zooids are colonial cnidaria.The colonial form of cnidaria is obelia. It has polyps, blastostyles and medusa. Polyps are nutritive which provide food. Blastostyles are budding type of coelenterate which produces medusa. Medusa are sexual organs which have testes or ovaries to produce gametes which help in fertilization. There occurs a phenomenon of polymorphism in which different type of individual perform different functions. Polymorphism means many forms. They have a radial symmetry and are derived from two germ layers which are known as ectoderm and endoderm respectively. So they are diploblastic animals which are multi cellular. Their level of organization is tissue. The body wall consists of epidermis and gastrodermis. Epidermis is the outer later and gastrodermis is the inner layer. A third layer known as mesogloea which occurs between epidermis and gastrodermis. The epidermis consists of many cells which are as follows: epithelio muscular cells which provide musculature, cnidoblasts they have given name to the phylum cnidaria. The cnidoblasts have nematocysts which are stinging organ. It consists of capsule, shaft and thread tube. The stinging organ provides defence and offence. Interstitial and nerve cells are present which acts as a reserve cells and acts as a primitive nervous system respectively. There are sensory cells which are sensory in function and are composed of gland cells, nerve cells, interstitial cells and the epithelio musculo nutritive cells. Gland cells secrete the digestive enzymes for digestion. Epithelio musculo nutritive cells show intra cellular digestion and provide support by musculature. There is a body cavity known as gastro vascular cavity which has single opening for ingestion and egestion. This is known as incomplete digestion. They show both the intra and extra cellular digestion. They respire and excrete through the process of diffusion. It occurs on the body surface with ammonia as their chief waste. Their mode of reproduction is both sexual as well as asexual. Asexual reproduction occurs by budding. They have higher power of regeneration. Their skeleton is made up of calcareous structures and their cleavage is holoblastic. Their development may be direct or indirect which leads to the formation of larvae in some cases. They show alternation of generation in which asexual generation alternates with the sexual generation.
Cnidaria are better than the sponges in their digestive tract, nerve cells, and their level of organization.
