Phylum : Coelenterata (Cnidaria), its classification and characteristics

March 13, 2018

multicellular with tissue grade of body organization.
aquatic, mostly marine, few are found in fresh water.
sedentary or free swimming.
mostly radially symmetrical whereas few are asymmetrical.
diploblastic i.e. outer layer is called ectoderm and inner layer is called endoderm. There is a jelly like layer present in between these 2 layers called mesogloea.

Internal hollow body cavity is present that is known as coelenteron or gastrovascular cavity.
blind sac body plan present. Coelenteron opens out through a mouth which helps in both ingestion and egestion.
mouth is surrounded by tentacles which contain stinging cells nematocytes. Tentacles help in capturing the prey, locomotion, offence and defence.
two types of individual are present. These are asexual polyps and sexual medusae. Some are noted for varieties of forms i.e. polymorphism.
nutrition is holozoic i.e. feeding on solid food particles in the manner of most animals.
digestion is both intracellular and extracellular.
primitive type of nervous system present in mesogloea in the form of nerve network.
respiratory, circulatory and excretory systems are absent.
muscular system consists of epithelio and endothelio muscle cells.
some of them are noted for alternation of generation (metagenesis) in between asexual polyp and sexual medusae.
reproduce asexually by budding and sexually by fusion of gametes.
development is indirect. They have free swimming planula larva.

Classification

They are divided into three classes on the basis of presence of polyp and medusa. They are :

Class 1: Hydrozoa

mostly marine, few are found in fresh water.
solitary as well as colonial, sessile and free swimming.
only polyps or both polyps and medusa stage are present. But polyp stage is dominant.
mesogloea is simple and not cellular.
gonads are usually ectodermal in origin.
exhibit tetramerous or polymerous radial symmetry.
examples: Hydra, Obelia (sea fur)