30 ANIMAL FORMS 
means of a “ measuring-worm ” movement travel to another 
place. 
Examined under a hand lens, the free end of the body 
will be found to support six to eight prolongations known 
as tentacles, which 
serve to convey 
food to the mouth, 
centrally located 
in their midst. 
This opening, un- 
like that of the 
sponges, is the 
only one leading 
directly into the 
large central gas- 
tric cavity which 
occupies nearly 
the entire animal 
(Fig. 16, D). As 
in the sponge, the 
cells of the body 
are arranged in 
the form of defi- 
nite layers, butthe 
middle one is rep- 
resented only by 
a thin gelatinous 
sheet. 
31. Organs of 
defense. — These 
Fic. 16.—The fresh-water Hydra. A, entire animal, de- 
veloping a new individual (enlarged 25 times). B,C, are the so-called 
nettle-cells (after ScuNEIDER); D, section through lasso or nettle-cells 
the body. 
(Fig. 16, C). Some 
of the cells of the outer layer possess, in addition to the 
elements of the typical cell, a relatively large spherical sac 
filled with a fluid, and also a spirally wound hollow thread 
