TYPES OF CCELENTERA HYDRA. 



145 



different individuals. Abundant food favours the develop- 

 ment of female forms ; when food is scarce males are more 

 abundant. The buds have the same structure as the parent 

 body ; in origin they appear to be mainly due to multiplica- 

 tion of interstitial cells. 



Minute structure. The outer layer or ectoderm includes the 

 following different kinds of cells : 



Ect 



End 



Istc 





D E 



FIG. 69. Minute structure of Hydra. After T. J. Parker and Jickeli. 



A. Ed., ectoderm ; ing., mesogloeal plate ; st.c. stinging cell ; End., endo- 



derm with flagella and amoeboid processes. 



B. n.c., nerve cell, and st.c. t stinging cell. 



C. Stinging cell with ejected thread; ., nucleus. 



D. Mesogloeal plate (ing-.) with contractile roots resting on it. 



E. m.c.> muscular cell with contractile roots, c.r. 



(i) Large covering or epithelial cells, within or between some of 

 which lie the stinging cells. The epithelial cells are somewhat conical, 

 broader externally than internally, and in the interspaces lie interstitial 

 cells. By certain methods, a thin shred can be peeled off the external 

 surface of the ectoderm cells. This is a cuticle, i.e. a pellicle no 

 longer living, produced by the underlying cells. 



(la) Many of these large cells have contractile basal processes, or 

 roots, running parallel to the long axis of the body, and lying on a 



