HYDROZOA. 
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, ect. end .), the layers 
being separated by a thin structureless lamella (Fig. 1, ms. gl.). 
Fig. 1. 
a. Hydra on pond weed, slightly enlarged. (From nature.) b. Section of 
Hydra , highly magnified, mth, mouth ; hyp, hypostome ; ent. cav., stomach ; 
end , endoderm ; ect , ectoderm ; msgl , structureless lamella ; bd 1, 2, buds ; 
ovy , ovary ; spy, spermary ; ntc, thread cell. (After Parker and Has well.) 
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. lc) ; a pointed process pro- 
jecting from the outer surface of the cell acts as a trigger, which. 
