TYPES OF C@LENTERA—AHVDRA. 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 :— 
that 
b 
Fic. 69.—Minute structure of Hydra.—After T. J. Parker and Jickeli. 
A. Ect., ectoderm ; #g., mesogleeal plate 5 s¢.c. stinging cell; Zxd., endo- 
derm with flagella and amceboid processes. 
B. 2.c., nerve cell, and st.c., stinging cell. 
C. Stinging cell with ejected thread; ., nucleus. 
D. Mesoglceal plate (#g.) with contractile roots resting on it. 
E. m.c., muscular cell with contractile roots, ¢c.7. 
(1) 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 cztzcle, z.e. a pellicle no 
longer living, produced by the underlying cells. 
(Ia) Many of these large cells have contractile basal processes, or 
roots, running parallel to the long axis of the body, and lying on a 
10 
