Tlie phylum Cnidaria or Coelenterata, includes the faliiiliar hydras, jelly fishes, sea anemones and corals. You have olreacly studied about Cnidarialis in Units 4 and 7 of LSE-09 course. These are often brilliantly colourcd. They exhibit radial symnietry, The Cnidarians possess two basic metazoall structural features. Onc, there is an intenla1 space for digestion, called in Cnidarinns as gastrovascular cavity. This cavity lies along the polar axis of the animal atid opens to tlie outside at one end to for111 noutli, Tlie presence of a mouth a~id digestive cavity permits the use of a mucll greater rnlige of food sizes than is possible in tlie protozoarls and sponges. Two, in Clidariails a circle of tentacles, representing exte~lsiolls of the body wall, surrounds tlie ~lioiltli to aid in tlie capture and ingestion of food. Tlie Cnidarian body wall consists of three basic layers: an outer lnycr of cpidennis, nn inner layer of elidoder~nal cells lining Ilie gastrovnscilar cavity, and between these two a layer called mesoglea. The rnesoglea ranges from n tliin, non cellilar membranc to a thick, fibrous, jelly-like, coid lnaterial with or without wanderillg a~noebocytes, A considerable number of different cell types compose tlie epidermis and gastrader~nis, but there is only a limited degree of organ development.