HYDRA AND CCELENTERATES IN GENERAL 125 



The tentacle contains entoderm cells apparently devoid of 

 muscular fibers. Gland cells are also absent from this region. 

 The entoderm of the basal disk is provided with only a few glan- 

 dular cells. 



MESOGLEA (Fig. 54, mes.). The mesoglea in Hydra is so thin 

 as to be difficult to find, even when highly magnified; in some of 

 the other Ccelenterates this layer is very thick, constituting by 

 far the largest part of the body. 



NERVOUS SYSTEM. From recent investigations it seems well 

 established that Hydra possesses a nervous system, though com- 

 plicated 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 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 (109). 



Nutrition. FOOD. The food of Hydra consists principally 

 of small animals that live in the water. Of these may be men- 

 tioned small Crustacea such as Cyclops, Annelids, and insect 

 larvae. Hydra normally rests with its basal disk attached to some 

 object and its body and tentacles extended out into the water. 

 In this position it occupies a considerable amount of hunting 

 territory. Any small aquatic animal swimming in touch with a 

 tentacle is at once shot full of nematocysts (Fig. 55, B), which not 

 only seem to paralyze it, but also to hold it firmly. There is 

 some evidence to prove that the tentacles are able to secrete a 

 fluid which serves to paralyze the animal without the aid of 

 nematocysts (123). The viscid surface of the tentacle aids in 

 making sure that the victim does not escape. 



INGESTION. Ingestion takes place as follows: First, the 



