200 PRACTICAL ZOOLOGY 



the distal or free end. Around the mouth are arranged from six 

 to ten smaller tubes, closed at their outer end, called tentacles 

 (i). Both the body and tentacles vary at different times in 

 length and thickness. One or more buds (6) are often found 

 extending out from the body, and in September and October 

 reproductive organs may also appear. The male organs (testes, 

 Fig. 119, y.t) are conical elevations on the distal third of the 

 body; the female organs (ovaries, Fig. 119, y.e, m.e) are knob- 

 like projections near the basal disk. 



HABITAT. - - In spite of their simplicity Hydras are able to 

 maintain themselves in the same habitat as that of aquatic 

 insects, fish, frogs, etc., although they often fall a prey to these 

 animals. When taken out of the water, Hydra shrinks into a 

 shapeless lump, but when returned to the liquid, it soon becomes 

 extended and regains its shape. Microscopic examination will 

 show that its body is not supported by a skeleton of any kind 

 and therefore must be held up by the water. 



PROTECTION. - - The protection afforded most animals by an 

 exoskeleton is secured by Hydra with the aid of stinging organs, 

 the nematocysts (Fig. 119, n ; 120, B), that lie embedded in the 

 surface, but are discharged when properly stimulated. Not only 

 do these nematocysts protect the animal, but they also assist in 

 capturing food. If a hungry Hydra is placed in a small amount 

 of water containing small Crustacea such as Cyclops (see p. 

 141, Fig. 81), sooner or later a crustacean w r ill strike a tentacle, 

 and instead of continuing its progress will stop suddenly as 

 though shot. And shot it really is, since the contact of its body 

 with the Hydra's tentacle was all that was necessary to explode 

 the nematocysts and paralyze the Cyclops. As soon as the Cy- 

 clops is captured the other tentacles bend over and help push it 

 into the mouth. 



ACTION OF NEMATOCYSTS. - - The nematocyst acts in the 

 following manner : Within it there is an inverted coiled, thread- 

 like tube with barbs at the base. When the nematocyst ex- 

 plodes, this tube turns rapidly inside out and is able to pene- 



