38 ANIMAL LIFE 



tentacles. The wall of the cylinder is composed of an 

 outer and an inner layer of cells and a thin non-cellular 

 membranous layer between them. The tentacles are hol- 

 low and are simple extensions of the body wall. The cells 

 of the outer layer, or ectoderm, are not all alike. Some 

 are smaller than the others and appear to be crowded in 



FIG. 19. The fresh- water polyp, Hydra vulgaris. A, in expanded condition, and 

 in contracted condition; B, cross section of body, showing the two layers of 

 cells which make up the body wall. 



between the bases or inner ends of the larger ones. The 

 inner ends of the large cells are extended as narrow-pointed 

 prolongations directed at right angles with the rest of the 

 cell. These processes are very contractile and are called 

 muscle processes. Each one is simply a continuation of 

 the protoplasm of the cell body, which is especially con- 

 tractile. Some of the smaller ectoderm cells are very 



