COELENTERATA (se-len-tur-a'ta). The term Coelenter• ata is applied to a large group or phylum of animals of a lowly grade of organization. Only two types of animals possess a more simple structure than the Coelenterata; viz., the sponges and the Protozoa. The Protozoa are organisms whose body usually consists entirely of a single cell, which is generally of microscopic size. In certain instances a number of these cells become banded together to form a colony, but in such a case they are not arranged so as to constitute tissues, and are individually autonomous and self-supporting. In the case of the sponges we are dealing with animals consisting of a multitude of cells of different kinds which are massed together, and many of which are segregated into defi nite sheets or tissues, each sheet performing a function or func tions of its own. A sponge develops from a definite embryo which is the outcome of the cleavage of a fertilized egg, but apart from this it is very unlike the animals of the higher groups in that it constitutes in its adult condition a fixed plant-like object often of somewhat indefinite shape. The sponges probably represent a series of products of evolution which diverged from protozoan ancestors in a direction distinct from that pursued by any other animals and which led on to nothing beyond the sponges them selves. The Coelenterata, however, from beginnings similar to those of the sponges, achieved an altogether higher grade of organization, producing not only tissues but also a much more definite form and individuality than the sponges, involving the differentiation of nervous and muscular systems and consequently efficient co-ordination of parts and considerable powers of move ment and locomotion. In the sponges primitive muscular tissue exists locally, but there is no nervous system.
There is, in fact, no existing Coelenterate in which the adult animal is quite as simple as a gastrula; but there do exist crea tures in which the main differences are that the cells of the ecto derm exhibit a differentiation into more than one kind, and that there lies between ectoderm and endoderm a thin sheet of non cellular material, the mesogloea, which supports the other layers. An example of such a grade of structure may be found in Protohydra.
The majority of Coelenterata, however, develop a complexity far exceeding this. The actual form of the body varies almost infinitely, but usually a prominent feature of the organization is the presence of a number of tentacles placed in a definite manner around the mouth. These tentacles may be solid structures, con taming a core of endoderm and a covering of ectoderm, or they may be hollow, the wall in such case including both cell-layers. The symmetry of the body is usually characterized by the fact that the various parts are arranged about the radii of a circle with the mouth as its centre, and that the same structures are repeated in a regular manner in different sectors. In other words, the ani mal is radially symmetrical and can be divided by suitable radial cuts into a varying number of parts each of which is exactly equivalent to the others. This symmetry is not invariably present, but is extremely prevalent, although in one section of Coelenterata (the Anthozoa) it is accompanied by an underlying bilateral symmetry which mars its perfection. The middle layer or meso gloea varies greatly in bulk in different cases, being sometimes extremely tenuous and small in amount, sometimes very exten sive and forming the greater part of the mass of the organism.
Polyp and Medusa.—The Coelenterata exhibit two main types of shape, both founded up on the gastrula plan.