270 



GENERAL ZOOLOGY 



found both on the tentacles and on the body. Each cell 

 (Fig. 141) consists of a minute microscopic sac within 

 which is coiled a hollow thread. A small hair-like projec- 

 tion from the surface of the body near 

 each nettling cell may control the action 

 of the cell. When this hair is touched 

 the coiled thread within the sac is shot 

 out with force sufficient to pierce the 

 shell of a small crustacean. The thread 

 carries with it a fluid which paralyzes 

 the prey. 



Reproduction may take place by the 

 sexual method, or by budding. Eggs 

 (Fig. 140) are developed in the ecto- 

 derm near the aboral end. In the same 

 individual spermatozoa (Fig. 140) are 

 formed in the ectoderm nearer the oral 

 end, and these on escaping fertilize the 

 eggs, which remain in the ectoderm for 

 some time as embryos. Then the em- 

 bryos separate from the parent, and 

 after swimming about for a variable 

 period develop into organisms like the 

 parent. Nonsexual reproduction is the 

 more frequent method, however. Buds 

 involving both ectoderm and endoderm 

 form on the side of the body ; four ten- 

 tacles appear, the mouth forms, and 

 the base of the bud constricts, setting 

 the young hydra free to begin its own 

 career. This asexual method is so effec- 

 tive that hydras become extremely numerous in some lakes. 

 In fish hatcheries hydras sometimes become so abundant as 

 to destroy the very young fishes (Fig. 139). 



Fig. 141. Nettle cells 

 of a hydra. (Much en- 

 larged) 



A, nettle cell, with un- 

 discharged nettling cap- 

 sule ; B, nettle cell, with 

 discharged nettling cap- 

 sule ; 1 , nettling capsule; 



2, nucleus of nettle cell ; 



3, bristle of nettle cell; 

 U, hollow filament of 

 nettling capsule. (After 



Schneider) 



