Which kinds of forms are seen in the life cycle of coelenterats animals?
Answers
Brief History
Peyssonel (1723) and Trembley (1744) proved these to be animals.
Hence, Linnaeus (1758), Cuvier (1796) and Lamarck (1801) included these under ‘Zoophyta’, together with sponges.
Leuckart (1847) included sponges and cnidarians under his phylum Coelenterata.
Finally, Hatschek (1888) divided “Coelenterata” into three phyla–Spongiaria (= Porifera), Cnidaria and Ctenophora.
Salient Features
Phylum coelenterata has the following salient features:
(1) Coelenterates are multicellular organisms
(2) They have tissue-grade of organization
(3) The body is radially symmetrical. Radial symmetry is the symmetry of a wheelPolyps and Medusa in Hydra
(4) All the members of this phylum are aquatic
(5) They are solitary or colonial
(6) Two types of individuals occur in the life cycle. They are polyps and medusa
(7) The body wall is diploblastic. It is made up of two layers of cells, namely the ectoderm and the endoderm with a non–cellular layer called mesogloea in between.
(8) Nematocysts or stinging cells are present
(9) Coelom is absent. Hence coelenterates are acoelomate animals
(10) A gastrovascular cavity or coelenteron is present. It can be compared to the gut of higher animals.Nematocysts or stinging cells in Cnidaria
(11) Mouth is present; but anus is absent
(12) Digestion is extracellular as well as intracellular
(13) Respiratory, excretory and circulatory system are absent
(14) Nervous system is diffuse-type, formed or nerve-nets.
(15) Reproduction is by asexual and sexual methods
(16) Development is indirect as there are one or two larval forms
(17) Life history has alternation of generations or metagenesis.