HYDRA 



137 



networks (Fig. 57); and of the contractile fibers of the epithehomuscular 

 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. 

 These fibers are not found in the tentacles, which are extended by water 

 forced into them from the enteron. The circular contractile fibers also 



Cnjdocil 



cyst 



Fig. 56. — Sketches illustrating nematocysts and their action. A, a cnidoblast contain- 

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

 cyst discharged. (A and B from Dahlgren and Kepner, '"Pririciples 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.) 



tend to be concentrated around the mouth and at the base of each ten- 

 tacle, where they act like sphincters. 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. 



