Phylum Cnidaria: Meaning and Classification | Zoology

In this article we will discuss about the meaning and classification of phylum cnidaria.

Meaning of Phylum Cnidaria:

The phylum Cnidaria (coel=hollow; enter on—gut) includes the simplest of the cellular animals or xnetazoa. The coelenterate body is a two-layered sac, opening to the exterior by a single aperture called mouth. The mouth is surrounded by a number of tentacles and leads into the gastro vascular cavity or coelenteron. The body wall consists of two cellular layers, outer ectoderm and inner endoderm.

Between these two cellular layers, there is a non-cellular jelly-like plate called mesogloea. The cnidarians, therefore, are diploblastic double; blastos—layer). Peculiar stinging cells or nematocysts are typically present in their body wall. Most of the coelenterates are radially symmetrical, that is, their body can be divided into a number of similar parts around a median axis passing through the mouth.

The colonial cnidarians are usually polymorphic (poly=many; morphe=form). The same individual may bear different forms, some of which are tube-like feeding stage or polyps and others are saucer-shaped reproductive stage or medusae.

Alternation of generations or metagenesis is frequently observed. The polyps, reproducing asexually, bring forth the medusae, and the medusa, reproducing sexually, brings forth the polyp.

Classification of Phylum Cnidaria:

The phylum includes nearly 9,500 known species which are divided into three main classes.

Class I: Hydrozoa (Hydra = a sea-serpent):

This includes simple forms like the common Hydra and complex colonial form such as Obelia, exhibiting polymorphism as well as metagenesis.

Class II. Scyphozoa (Skyphos=cup):

Predominantly medusoid forms such as the jelly-fishes which swim about in the sea. Aurelia and Chrysaora furnish good examples.

Class III. Anthozoa (Anthos=flower):

Predominantly poly­poid forms with no medusoid stage and fixed to the bottom of the sea. The beautiful, highly coloured sea-anemones and the corals are included in this class. We shall discuss only two types of coelenterates—a simple fresh water form, the Hydra and a complex marine form, the Obelia.