GENERAL ZOOLOGY 



The Coelenterate Body Plan 



In the simplest coelenterates the body is Hke a sac with a wall composed 

 of two layers, the epidermis and the gastrodermis; hence the animals are 

 called diploblastic. The single opening into the coelenteron, termed the 

 mouth although it functions as both mouth and anus, is surrounded by 

 tentacles. The radial symmetry characteristic of the phylum is obvious in 

 most species. In the more specialized coelenterates there is a middle region, 

 between epidermis and gastrodermis, which may have several kinds of cells 

 and may then be termed a mesenchymal layer, of mesodermal origin. Such 

 coelenterates are more properly called triploblastic than diploblastic animals. 

 Compared with sponges, the coelenterates are evidently animals of more com- 

 plex type; compared with vertebrates, however, they are very primitive. 

 They have, nevertheless, important features in common with all the higher 

 phyla. If comparisons are made between cell layers, it will be seen that 

 the epidermis of the coelenterate is comparable with the outer layer of the 

 skin of a vertebrate and the gastrodermis with the lining of the digestive 

 tract. The parts of the body derived from mesoderm in such forms as verte- 

 brates are absent, except insofar as the middle region in many coelenterates 

 may be cellular and thus comparable with the mesenchyme of higher forms. 



Their relatively simple, primitively two-layered construction, with what 

 may be called tissues but without well-developed organs, together with their 

 basic radial symmetry and their coelenteron with only one opening, constitute 

 consistent features which make it possible to set the coelenterates apart from 

 the more advanced Metazoa. 



The Phylum Ctenophora 



The Ctenophora, commonly known as sea walnuts or comb jellies, are 

 animals with biradial symmetry, epidermal and gastrodermal layers like those 

 of coelenterates, definite muscular elements and a mesenchymal middle layer 

 (collenchyme) that are both derived from mesoderm, and eight meridional 

 rows of swimming plates or combs formed of fused cilia. Ctenophores are all 

 small marine animals which float and swim near the surface, although a few 

 aberrant species that have become adapted to a creeping existence upon the 

 bottom are known. The free-swimming forms are sometimes present in such 

 tremendous numbers that they constitute an important element in the floating 

 life of the ocean. Their food consists of whatever small animals they may 

 capture with the aid of the tentacles which some ctenophores possess, or with 

 the parts related to the mouth. Abundant mucous secretions of the epidermis 

 aid in trapping small animals which are then driven toward the mouth by 

 ciliary currents. In contrast to the coelenterates, ctenophores lack nema- 

 tocysts, but some possess adhesive cells on the tentacles which hold fast to 



310 



