THE HYDRA AS A SIMPLE METAZOAN 249 



individuals as well as in different species. The body consists of 

 but two layers of cells surrounding a central digestive cavity, the 

 enteron. and this same organization extends to the tentacles, the 

 cavities of which communicate with that of the body. Between 

 the outer and inner laj'ers of cells, which are known respectively 

 as the ectoderm and endoderm, is the supporting lamella, which sep- 

 arates the two layers and may be regarded as a secretion formed 

 by the basal ends of the cells. Functionally, this lamella is com- 

 parable with an elastic skeleton since it serves as a place of attach- 

 ment for the cells and thus gives support and continuity to the 

 entire organism. In contrast with most familiar animals, which 

 are bilaterally symmetrical, the hydra is radially symmetrical, since 

 its body is radially arranged about an axis of symmetry. This 

 is one of the most obvious characteristics of the Phylum Cocloi- 

 terata whether attached or free-living. 



Fig. 120. — Locomotion in h^dia. 



(Redrawn froni Borradaile, "The Animal and Its Environment," copyrignt, 1923, 

 by Oxford University Press, printed by permission.) 



Cellular Structures and Functions. — General Organization of 

 the Ectoderm. — The cellular structure of the hydra is such that each 

 layer of the body is largely composed of cells of one particular type 

 with lesser numbers of cells of one or more other types. The 

 ectoderm is made up primarily of large cells of the epithelial type 

 (Fig. 122), having muscidar processes extending laterally from the 

 bases of the cells and attached to the supporting lamella. The 

 muscular processes are further differentiated by the presence of 

 a fibril which is presumably the essential contractile portion. 

 Since they run lengthwise, these muscular outgrowths of the ecto- 

 derm cells are equivalent to a longitudinal muscular layer by 



