HYDRA 



139 



derm; of the nerve cells, which with their processes form conducting 

 networks (Fig. 60); and of the contractile fibers of the epitheliomuscular 

 cells. The sensory cells receive stimuli and the contractile fibers cause a 

 modification in the form of the body. The contractile fibers connected 

 with the ectoderm run longitudinally and those of the entoderm trans- 

 versely, the two sets thus acting like longitudinal and circular muscles. 

 The circular contractile fibers also tend to be concentrated around the 

 mouth and at the base of each tentacle, where they act like sphincters. 



Cnjolocil 



Nemafo^ 

 cyst 



A Nuc/euSy 



r^5: 



Fig. 59. — Sketches illustrating nematocysts and their action. A, a cnidoblast contain- 

 ing an undischarged nematocyst and possessing a cnidocil. B, the same with the nemato- 

 cyst discharged. {A and B from Dahlgren and Kepner, "Principles of Animal Histology," 

 after Schneider.) C, portion of a tentacle, showing the batteries of nematocysts. D, an 

 insect larva covered with nematocysts as a result of capture by a hydra. (C and D from 

 Jennings, " Behavior of the Lower Organisms," by the courtesy of Columbia University Press.) 

 E, last segment of the leg of a small aquatic animal, with nematocysts of a barbless type 

 shown coiled about its spines; this impedes the movements of the animal. (From Hegner, 

 "College Zoology" after Toppe.) (A, B, and E are by the courtesy of The Macmillan 

 Company.) 



The nerve cells seem to be most numerous around the basal disc and 

 on the hypostome, which indicates a certain degree of localization of 

 nervous activity. Owing to the more complete network formed by its 

 nerve cells, the ectoderm is more active and its movements are more 

 definitely coordinated than are those of the entoderm. 



The nematocysts seem to be stimulated directly by chemicals in the 

 water, such as the secretions from the body of the animals which serve 

 as prey, and not by the nervous mechanism. 



