THE CCELENTERATES 



199 



Hydra. — Hydras are easily seen with the naked eye, being 

 from 2 to 20 mm. in length. They may be likened to a short, 

 thick thread unraveled at the unattached distal end. 



Fig. 119. — Diagram showing the structure of hydra. 



b, bud; b.d, basal disk; bl, blastula ; ec, ectoderm; en, entoderm; g, 

 gastrula ; gv.c, gastro-vascular cavity; hy T hypostome ; m, mouth; m.e, 

 mature egg ; m.t, mature testis ; n, nematocysts ; p. b, polar bodies ; t, tentacle ; 

 y.e, young egg ; y.t, young testis. All the structures shown do not occur on a 

 single animal at one time. 



The body is really a tube (Fig. 119) usually attached by a 

 basal disk (b.d) at one end, and with a mouth opening (m) at 



