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 



v the form of defi- 

 nite layers, but the 

 middle one is rep- 

 resented only by 

 a thin gelatinous 

 sheet. 



31. Organs of 

 defense. These 



FIG. 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 SCHNEIDER) ; D, section through l asso or nettle-cells 



(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 



