

REPRODUCTION. 25 



of the nematocysts will remain in the ectoderm 

 with their threads projecting freely ; others will 

 separate completely. 

 Examine, and draw the nematocysts, showing their shape, 

 the threads with their enlarged basal portions, and the barbs. 



Search for specimens with partially discharged threads ; 

 and note that the threads are turned inside out in the process 

 of discharge. 



0. Reproduction. 



Hydra reproduces asexually and sexually. 



1. Asexual reproduction. 



a. Gemmation or budding consists in the formation 

 of a hollow outgrowth from the side of the body, 

 which acquires a mouth and tentacles at its 

 distal end, and ultimately constricts at its base, 

 separates from the parent, and becomes an in- 

 dependent animal. 



The rate of budding depends largely on the 

 supply of food and on temperature. If a rapidly 

 budding Hydra be transferred to water in which 

 there is little or no food to be obtained, the 

 formation of the buds will be stopped, and buds 

 already formed may even be absorbed. 



A single Hydra may give rise to more than 

 one bud at once, and these may develop second- 

 ary buds before separating from the parent 

 animal. In this way temporary colonies may 

 be formed, which, however, sooner or later 

 break down into their component units. 



b. Fission. A Hydra may be cut in two, and each part 

 will live and become a perfect animal. The 

 process of fission is, however, very rare. 



2. Sexual reproduction. 

 Hydra may be of separate sex but is often 



hermaphrodite, the same animal having both testes 

 and ovaries. Contrary to the rule among herma- 



