20- HYDRA 



in consequence of the irritation. Some of the 

 nematocysts will remain in the ectoderm with 

 their threads projecting freely ; others will sepa- 

 rate completely. 

 Examine, and draw the nematocysts, showing their shape, 

 the threads with thei/r enlarged basal portions, and the barbs. 

 Sea/rch for^specimens with pa/rtially discharged threads ; 

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

 -of discharge. 

 G. 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 independent 

 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 forma- 

 tion 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 secondary 

 buds before separating from the parent animal. 

 In this way temporary colonies may be formed, 

 which, however, sooner or later break up into 

 their component units. 



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



will live and become a perfect animal. The pro- 

 cess of fission, however, is not known to occur 

 naturally. 



2. Sexual reproduction. 



Hydra is hermaphrodite, the same animal having 



