HYDEOZOA. 



39 



the circle of tentacles is an area termed the hypostome (Fig. 1b, hyp.). 

 The animal is a simple sac, the wall of which is formed of two layers 

 of cells, an outer or ectoderm layer, one or more cells deep, and an 

 inner or endoderm layer, one cell deep (Fig. 1b, ed. end.), the layers 

 being separated by a thin structureless lamella (Fig. 1, w.s. gl.). 



Fig. 1. 



A. Hydra on pond weed, slightly enlarged. (From nature.) b. Section of 

 Hydra, highly magnilied. mth, moutli ; hyp, hypostome ; erit. cav., stomach ; 

 end, endoderiii ; ed, ectoderm ; msgl, structureless lamella ; bd 1, 2, buds ; 

 ovy, ovary ; spy, spermary ; ntc, thread cell. (After Parker and Haswell.) 

 c. Thread cell, very highly magnified. (After F. E. Schulze.) 



The endoderm, which lines the whole inner cavity and the interior 

 of the tentacles, is concerned in the digestion of food. In the 

 ectoderm are certain peculiar cells, each containing a cyst with a 

 barbed thread coiled up inside (Fig. lo) ; a pointed process pro- 

 jecting from the outer surface of the cell acts as a trigger, which, 



