THE INVERTEBRATE BODY 



99 



forming the mouth. Surrounding the mouth is a circle of outpocket- 

 ings 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 disposed about, or radiate 

 from the main axis, and so Hydra affords an example of radial 

 symmetry. (Figs. 38, 57, 155.) 



The body wall of Hydra is composed of two distinct cell layers, 

 ectoderm and endoderm, separated by a thin non-cellular support- 



Tentacle 



Mouth-. 



Older bud 



Ovary 

 B 



fgjEEajg jgSi 



Fig. 57. — Hydra. A, two specimens with buds, one contracted; B, dia- 

 grammatic longitudinal section. (From Newman, after Pfurtscheller, and 

 Parker.) 



ing layer of jelly-like material (mesogloea) secreted by the cells 

 of both ectoderm and endoderm. Hydra thus illustrates a simple 

 type of Metazoan structure in which but two primary tissues 

 exist; such specializations as are necessary for the performance of 

 the essential life functions being confined to the various cells that 

 compose these layers. The majority of the cells of the endoderm 

 which line the enteron, enclosing the einteric cavity, are con- 

 cerned with the digestion of solid food taken in through the mouth, 



