THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 89 
irregular in shape and possess specially large nuclei. These 
cells are more irritable or sensitive than the others and 
are called nerve cells. The ectoderm cells of the base or 
foot of the Hydra are peculiarly granular, and secrete a 
sticky substance by which the Hydra holds fast to the 
stone or weed on which it is found. These cells are called 
gland cells. Imbedded in many of. the larger ectoderm 
cells, especially those of the tentacles, are small oval sacs, 
in each of which lies folded or coiled a fine long thread. 
When the tentacles touch one of the small animals which 
serve Hydra as food, these fine threads shoot out from 
their sacs and so poison or sting the prey that it is 
paralyzed. The tentacles then contract and bend inward, 
forcing the captured animal into the mouth opening 
in the center of the circle of tentacles. Through the 
mouth opening the prey enters the body cavity of Hydra 
and is digested by the cells lining this cavity. These cells 
belonging to the inner layer of the body wall or endoderm 
are mostly large, and each contains one or more contractile 
vacuoles. From the free ends—the ends which are next to 
the body cavity—of these cells project pseudopods or fine 
flagella. These projections are constantly changing: now 
two or three short, blunt pseudopods are projecting into 
the body cavity ; now they are withdrawn, and a few fine, 
long flagella are projected. In addition to these cells there 
are in the endoderm, especially abundant near the mouth 
opening and wholly lacking in the tentacles and at the 
base of the body, many long, narrow, granular cells. They 
are gland cells which secrete a digestive fluid. The food 
captured by the tentacles and taken in through the 
mouth opening disintegrates in the body cavity, or diges- 
tive cavity, as it may be called. The digestive fluid se- 
creted by the gland cells of the endoderm acts upon it, 
so that it becomes broken into small parts. These par- 
ticles are probably seized by the pseudopods of the other 
endoderm cells and are taken into the body protoplasm 
