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 
¥ia. 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 
