26 HYDRA. 



phrodite animals, the reproductive organs are 

 extremely simple in structure, and the animals 

 are capable of fertilising their own ova. 



a. The testes are conical or spherical swellings of the 

 body-wall, varying in number from one or two 

 to twenty. They are usually situated near the 

 oral or distal end of the body, not far below the 

 tentacles ; but when numerous they may extend 

 along the greater part of the length of the body. 

 Each testis is formed by local proliferation of 

 the interstitial cells of the ectoderm, giving rise 

 to small conical heaps, which project externally 

 and are covered on their outer surfaces by 

 capsules formed from the large ectoderm cells. 



The interstitial cells, after repeated divisions, 

 become converted into spermatozoa by modifi- 

 cation of their nuclei to form the heads, and 

 elongation of their protoplasm to form the long 

 swimming tails. When fully formed they are 

 discharged by rupture of the capsule at its most 

 prominent point. 

 Select a Hydra in which the testes are large ; put it into a 

 drop of water on a slide ; cover it ; and, by gentle pressure 

 on the cover-glass, rupture one of the testes, and so discharge the 

 spermatozoa. Examine with a high power. 



The spermatozoa are filiform bodies about 

 3~i^- inch in length. Each consists of a small 

 ovoid highly refractive head, containing the 

 nucleus, and a long active thread-like tail. 



b. The ovaries usually develop a little later than the 

 testes. In Hydra viridis there is seldom more 

 than a single ovary present at a time, but in the 

 brown species there may be as many as eight. 

 They form, when ripe, spherical projections of 

 the body-wall considerably larger than the testes, 

 and situated as a rule nearer the basal end of 

 the animal. In each ovary only a single ovum 



