HYDROZOA. 



39 



the circle of tentacles is an area terineil the hypostuim.- (Fitr. In, /<y//. j. 

 The animal is a simple sac, the wall of whi<;h is formed of two biyei-s 

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

 inner or cndoderm layer, one cell deep (Fij(. In, ed. end.), the lavere 

 being separated by a thin strnetureless lamella (Fiir. 1, nis. i/l.). 



I'l.;. 1. 



A. Hydra on pond weed, slightly enlarged. (I'roin ualuri'.) ii. Section of 

 Hydra, highly magnified, mth. moutli ; hyp, hypostnme : ent. cac, 8toriiat>li ; 

 end, endodtmi ; ect, ectoderm; msgl, strnotun-less liinii'lln; l>il 1, •_'. Inula: 

 OB)/, ovary : .s'^jf/, spermary ; «/c, thread cell. (.Mtcr I'urker and Ilnswell.) 

 c. Thread cell, very highly magnitied. (After 1' Iv Schulze.) 



The endoderm, which lines tlie whole inner cavity and i he interior 

 of the tentacles, is concerned in tlie di;^'estion of food. In t.hc 

 ectoderm are certain peculiar cells, each contaiiiinir a cyst with u 

 barbed thread coiled up inside (Fig- '^) > '^ point<.'d procc-^s pro- 

 jecting from the outer surface of the cell acts :is ;i triirjfcr, which. 



