112 



COLLEGE ZOOLOGY 



From recent investigations it seems well established that 

 Hydra possesses a nervous system, though complicated staining 

 methods are necessary to make it visible. In the ectoderm there 

 is a sort of plexus of nerve-cells connected by nerve-fibers with 

 centers in the region of the mouth and foot. Sensory cells in 

 the surface layer of cells serve as external organs of stimulation, 

 and are in direct continuity with fibers from the nerve cells. 

 Some of the nerve-cells send processes to the muscle fibers of 



Fig. 67. — Nematocysts of Hydra 

 and their action. A, portion of a ten- 

 tacle showing the batteries of nemato- 

 cysts ; cl, cnidocils. B, insect larva 

 covered with nematocysts as a result of 

 capture by Hydra. (From Jennings.) 



Fig. 68. — The action of 

 nematocysts. A, a nematocyst 

 piercing the chitinous covering 

 of an insect. B, nematocysts 

 holding a small animal by coil- 

 ing about its spines. (After 

 Toppe in Zool. Anz.) 



the epitheliomuscular cells, and are therefore motor in function. 

 No processes from the nerve-cells to the nematocysts have yet 

 been discovered, though they probably occur. The entoderm 

 of the body also contains nerve-cells, but not so many as are 

 present in the ectoderm. 



Physiology. — Nutrition. — Hydra lives on minute aquatic 

 animals which come within reach of its tentacles. The nemato- 

 cysts, and probably a secretion from the tentacles, paralyze 

 the prey, while the viscid surface of the tentacle prevents it 



