326 



APPLIED BIOLOGY 



Look for these three layers in a section of a tentacle. In the 

 ectoderm of the tentacles look for the stinging cells or nettle-cells 



(Fig. 99). Each of these contains a 

 sac filled with fluid and surrounding a 

 coiled stinging thread. If a small ani- 

 mal happens to touch a hair-like pro- 

 jection (trigger) on the outer edge of 

 one of these cells, the stinging thread 

 is suddenly ejected and spears the vic- 

 tim. It appears that some paralyzing 

 fluid is injected, for the animal caught 

 seems benumbed. Finally, the ten- 

 tacle bends over towards the mouth 

 and the captured food is taken into 

 the digestive cavity. The nettle-cells 

 may also be used for defense against 

 animals too large for food of a hydra. 



If there is a bud present on the sec- 

 tion, notice the relation of its digestive 

 cavity to that of the parent. 



Study a transverse section of a hy- 

 dra and compare with the structure 

 in the longitudinal 

 section. 



FIG. 98. Longitudinal section 

 of a hydra, t, tentacle; m, 

 mouth; e, ectoderm or outer 

 layer of cells; n, endoderm 

 or inner layer; d, digestive 

 cavity; 6, buds; s, spermary; 

 o, ovary. (From Parker.) 



283. Physiology 

 of Hydra. The 



life-activities of Hydra are slightly more 

 specialized than those of sponge-animals, 

 but they are exceedingly simple as compared 

 with those of an animal like the frog. 



Digestion. Small animals caught by the 

 tentacles and forced into the digestive cavity 

 through the mouth are softened and disin- 

 tegrated by the action of the digestive juice 

 secreted by the cells of the endoderm which 

 lines the cavity. The digestible particles set 

 free by the disintegrative action of the digestive juice are 

 taken in by endoderm cells, the free ends of which are 



FIG. 99. a, sting- 

 ing cell with 

 thread inclosed; 

 b, thread dis- 

 charged. Those 

 on the tentacles 

 of a hydra are 

 similar. (From 

 Hatschek.) 



