88 ZOOLOGY FOR MEDICAL STUDENTS chap. 



coverslip, and to cut other specimens into fine slices or sections for 

 examination under the microscope. 



The Hydra as already indicated is tubular, its cavity (Fig. 32, A, c) 

 being known as the coelenteron. The coelenteron communicates with 

 the exterior at the mouth : it extends up to the tip of each tentacle 

 where it ends blindly. The coelenteron is surrounded by the body wall 

 consisting of two layers of cells, an outer ectoderm (Fig. 32, A, ect) and 



ic 



Fig. 33. 



m.t. 



The structure of Hydra as seen in transverse sections, x 150. A, Normal structure ; B, eariy 

 stage of ovary ; C, testis, ect^ Ectoderm ; E, egg ; end, endoderm ; i.c, interstitial cells of ectoderm ; 

 w, mesogloea ; m.t, muscular tail of ectoderm cell, lying in close contact with mesogloea and seen 

 here in transverse section ; ov, ovary ; t, testis. 



an inner endoderm {end), separated by an apparently structureless 

 membrane — the mesogloea (Fig. 33, A, w). The ectoderm (Fig. 33, A, ect) 

 is in great part made up of tall columnar-shaped cells which taper off 

 towards their inner ends and there pass into two tail-like prolongations 

 which being in line with one another resemble the crosspiece of a letter J 

 (Fig. 34, A). This crosspiece is in great part composed of protoplasm 

 which, like the myoneme of a Protozoon, is specialized in the direction 

 of extreme contractility. 



