142 



THE RISE OF ANIMAL LIFE 



CIRCULAR MUSCLE FIBERS LONGITUDINAL MUSCLE FIBERS 



INTERSTITIAL CELLS 



NEMATOCYST 



EPIDERMIS 



MESOGLOEA 







(-1 



:4 





if'i 





■'■■■■! 

 ■■•<-' 



•-1 :..; 

 1 { ; : 



r\ 



':! 





Fig. 8-6. Hydra often "hangs" from the water surface. A portion of the body wall is magnified here in order to 

 distinguish its cellular structure. Note the variety of cells in both the epidermis and gastrodermis. 



gether, there is a need for some cells to 

 protect the entire group from the outside 

 world, others to locate food, and still others 

 to detect the possibility of danger. This is 

 observed in hydra. The outer layer, the 

 ectoderm, sometimes called the epidermis, 

 is made up of cells which form a protective 

 covering. Some of these cells have an inner 

 contractile portion which enables them to 

 serve as muscle cells. Finally, scattered 

 among the cells of the epidennis are spe- 



cialized sensory cells. The inner layer of 

 cells, the endoderm or gastrodermis, pro- 

 vides all other cells with nourishment. 

 These tall, glandular cells secrete the en- 

 zymes which digest food that is brought into 

 the coelenteron (Fig. 8-6). These cells also 

 possess muscle fibers at their bases which 

 run at right angles to those in the epidermis. 

 It is by the combined action of these muscle 

 cells with those of the ectoderm that hydra 

 is able to contract and extend itself. Lying 



