392 



GENERAL ZOOLOGY 



Hydra attaches itself by means of the basal disk, but it can move 

 by a creeping motion of the disk. It also moves by a head-over- 



g.b. 



Fig. 221. — Hydra oligactis, semidiagrammatic longitudinal section. B, bud; 

 B.D. basal disk; E, ectoderm; En, endoderm; e.c, epitheliomuscular cells of 

 endoderm; G, gastrovascular cavity; g.b., gas bubble; g.c, gland cells; H, hypo- 

 stome; M, mesoglea (solid black line); Mo, mouth; N, nematocysts; O, ovary; 

 T, testis; Te, tentacle. (After Kepner.) 



heels movement by arching the body until the ends of the tenta- 

 cles can grasp the substratum and then, releasing the basal disk, 



