COELENTERATA 



55 



a basal disk for attachment, and a ?nouth surrounded hy Jive to ten 

 tentacles at the other. 



Reproduction. — Buds appear laterally, and during the repro- 

 ductive season in October the spermaries (testes) may be seen on 

 the anterior third of the body, while the ovaries are seen on the 

 posterior or basal end. Hydra eggs resemble an ameba in ap- 

 pearance. Buds are formed 

 by the outgrowth of the endo- 

 derm and the ectoderm, and at 

 first include an enteron connec- 

 tion with that of the parent. 



The outside of the Hydra, 

 except the basal disc, is cov- 

 ered by a thin cuticle. Hydra 

 has two distinct cellular lay- 

 ers, an outer ectoderm, which 

 is thin and colorless, and an 

 inner layer, the endoderm, 

 which is more than twice as 

 thick as the outer layer and 

 has in it brown or green color- 

 ing matter depending on the 

 species. Between the ecto- 

 derm and endoderm there is 

 a jelly-like substance called 



the mesozlea. The body and t- it j • -j- l • 



"^ ■' ^IG. 21. hydra vtriats, showing testes 



the tentacles are hollow, the above and ovaries below. (After Hertwig- 

 space being called the gastro- Kingsley, Manual of Zoology. Courtesy of 

 vascular cavity or cloaca. Henry Holt & Co.) 

 (Figure 22, A, B, C.) 



The ectoderm, protective and sensory, consists of (a) epithelio- 

 muscular cells, inverted cones with contractile fibrils; (b) interstitial 

 cells, which produce three kinds of nematocysts, (i) barbed with 

 hypnotoxin; (2) cylindrical, with a coiled thread and no barbs, and 

 (3) spherical, with a barbless thread in coils; (c) glandular cells at 

 the basal disk, which aid in attachment. The sex cells of both 

 ovaries and spermaries are derived from ectodermal interstitial 

 cells. The mesoglea is thin, jelly-like and non-cellular. The 

 endoderm has large digestive cells with muscle fibrils at the base, 

 and with flagellae or pseudopodia projecting into the cloaca; 



