THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 39 



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 the body cavity ; now they are withdrawn, and a few fine, 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 



