THE HYDRA AS A SIMPLE METAZOAN 



251 



which the entire body may be shortened or bent in various direc- 

 tions. The mechanics of their concerted action can be under- 

 stood when it is recalled that they are attached to the supporting 

 lamella and hence all pull together upon this sheet-like membrane 



Fici. 122. — Ectoderm and endoderm of hydra. 



C, cnidocil; Cu, cuticle of ectoderm cell; EK, ectoderm; E\, endoderm; /, interstitial 

 cells; JZ, intercellular space; A', cnidoblast; KB, nematocyst; KN, stalk of cnidoblast; 

 M, muscle fibril of ectoderm; M', muscle fibril of endoderm; N, nerve cell; NF, process 

 of nerve cell; P, striation of supporting lamella; S, sensory cell; SF, sensory process of 

 neuro-sensory cell; S.V, neuro-sensory cell; /, supporting lamella; ZK, nucleus of epithelio- 

 muecular cell. (.'Vfter Hadzi, Arb. Zoul. Inst. Wein., Vol. 17.) 



which is continuous throughout the animal. Since they possess 

 the double function of covering the body and furnishing a mus- 

 cular system, these large ectoderm cells are epitheho-muscular in 

 character. At the base of the body they are modified as the 

 gland cells of the foot by which the animal fastens itself to the 

 substratum. The larger ectoderm cells in a typical region are 



