why do colenterata vary in body structure than sponges?
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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.