CHAPTER V 



THE PHYLUM COELENTERATA 



Metazoa, either sedentary or free-swimming, with primarily radial 

 structure ; the body wall composed of two layers of cells, the ectoderm 

 and endoderm, and between these a layer secreted by them which is 

 originally a structureless lamella {mesogloea) but usually contains 

 cells derived from the primary layers ; within the body wall a single 

 cavity, the enteron, corresponding to the archenteron of the gastrula, 

 having a single opening for ingestion and egestion, and often compli- 

 cated by the presence of partitions or by the formation of diverticula 

 or canals; digestion partly intracellular; the nervous system a net- 

 work of cells with anastomosing processes ; commonly with the power 

 of budding, by which either free individuals or colonial zooids may 

 be formed; and whose sexual reproduction typically produces an 

 ovoidal, uniformly ciliated larva, known as the planula, which has at 

 first a solid core of endoderm. 



Thus defined, this phylum contains the whole of the diploblastic 

 animals, that is, those in which the space (blastocoele) between ecto- 

 derm and endoderm is either devoid of cells, or contains only cells 

 derived late in development by immigration from ectoderm or en- 

 doderm. Of such animals there are two very distinct stocks — the 

 Cnidaria, characterized by muscular movements, which possess 

 nematocysts (p. 127), and are reducible either to the polyp or to the 

 medusa type (p. 128); and the Ctenophora, which retain the ciliary 

 locomotion of the planula, are without nematocysts, and are not to be 

 assigned either to the polyp or to the medusa type. 



In the Coelenterata the Metazoa are at the beginning of their 

 evolution and we have a primitive type with great potentialities, 

 though these animals have also already acquired strange specialized 

 features. The tissues consist of two single layers of cells, the ectoderm 

 and endoderm, which constitute a thin body wall surrounding the 

 central cavity (Fig. 106): the only increase in thickness and com- 

 plexity of the body wall that is possible is by development of a 

 gelatinous intermediate layer. Thus, while the typical polyps like 

 Hydra have a very thin layer of this kind, it has become thicker, very 

 much folded and penetrated by cells in the actinozoan polyps and 

 exceedingly thick in the large jellyfish, forming not only a kind of 

 internal skeleton but even a reservoir of food. 



The principal type of cell found in the tissues, both ectoderm and 

 endoderm, of the primitive coelenterate is the musculo-epithelial cell 



