118 FOUNDATIONS OF BIOLOGY 



vey of certain morphological principles as exhibited among 

 the Invertebrates selecting as types the Hydra, Earth- 

 worm, and Crayfish in order to afford a background for 

 the consideration of Vertebrate structure and function. 



B. HYDRA 



In discussing the development of animals, it was pointed 

 out that the dividing egg typically forms a blastula which, in 

 turn, becomes transformed by the invagination of its wall at 

 one pole into the gastrula stage. This early gastrula is essen- 

 tially a sac composed of two layers of cells : an outer or ecto- 

 derm and an inner or endoderm layer. Although no adult 

 animal retains this simple gastrula form, the animals com- 

 posing the group known as the COELENTERATES are to all 

 intents and purposes permanent gastrulae since their bodies 

 are built on the plan of a two-layered sac. This is well ex- 

 hibited in Hydra, an almost microscopic, fresh-water Coelen- 

 terate which is commonly found attached to submerged 

 vegetation or stones in brooks and ponds. (See p. 414.) 



The body of Hydra somewhat resembles a tube closed at one 

 end, constituting the FOOT, and open at the other, forming the 

 MOUTH. Surrounding the mouth is a circle of outpocketings 

 of the body wall termed TENTACLES. The main axis of the 

 body extends from foot to mouth, and every plane passing 

 through this axis divides the body into symmetrical halves. 

 In other words, the parts of the body are symmetrically dis- 

 posed about, or radiate from, the main axis, and so Hydra 

 affords an example of RADIAL SYMMETRY. (Fig. 64.) 



The tubular body wall of Hydra is composed of two dis- 

 tinct cell layers, ectoderm and endoderm, separated by a thin 

 non-cellular supporting layer of jelly-like material (MESO- 

 GLOEA) secreted by the cells of both ectoderm and endoderm. 

 Hydra thus illustrates a simple type of Metazoan structure 



