HYDRA AND CCELENTERATES IN GENERAL 123 



tne nematocyst assumes its final form. When nearly ripe a nema- 

 tocyst, still contained in its mother cell or cnidoblast, migrates into 

 the inside of an epitheliomuscular cell and approaches the surface. 

 The external end of the cnidoblast is produced to form a cnidocil 

 which perforates the cuticle. . . ." (101, p. 259.) Since the 

 tube of the nematocyst cannot be returned to the capsule, nor 

 another one be developed by the cnidoblast, new capsules must 

 be formed from interstitial ceils to replace those already exploded. 



The second kind of nematocyst is cylindrical (Fig. 55, A) and 

 contains a thread which lacks the barbs so characteristic of its 

 larger neighbor. The third variety is almost spherical and smaller 

 than the others, measuring only .005 mm. in diameter. The 

 thread contained in this nematocyst likewise bears no barbs, but 

 when discharged resembles a corkscrew (Fig. 57 B). It aids in 

 the capture of prey by coiling around the spines or other struc- 

 tures that may be present (121). 



The interstitial cells also develop at a certain period of the year 

 (September and October) into germ cells. The origin and his- 

 tory of these cells will be found fully described on pages 134-136. 



The BASAL DISK (54, b. d) differs somewhat in function from the 

 rest of the body. It is the point by which Hydra attaches itself 

 to solid objects, and for this purpose secretes a sticky substance. 

 It is also said to effect the movement of the animal from place to 

 place by a sort of gliding motion, not yet fully explained, but 

 possibly brought about by pseudopodia-like processes thrust out 

 from some of the cells. Epitheliomuscular cells and a few inter- 

 stitial cells are present, but no nematocysts are to be found here. 

 The columnar epitheliomuscular cells are not only provided with 

 contractile fibers at their bases, but, being secretory, also contain 

 a large number of small refringent granules, as shown in Figure 58, 

 ad. sez. 



The TENTACLES (Fig. 54, /) are provided with an ectoderm 

 consisting of large flat cells, thin at the edges and thick in the 

 center. The thicker portions give the surface of the tentacle 

 a lumpy appearance. In the center of each thickening is a 



