22 HYDEA. 



hermaphrodite 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 cap- 

 sules formed from the large ectoderm cells. 



The interstitial cells, after repeated divisions, 

 become converted into spermatozoa by modifica- 

 tion of their nuclei to form the heads, and elonga- 

 tion of their protoplasm to form the long swim- 

 ming tails. When fully formed they are dis- 

 charged by rupture of the capsule at its most 

 prominent point. 



Select a Hydra in which ike testes are large; put it 

 in 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 

 ,r,,',,T inch in length.^ Each consists of a small 

 ovoid highly refractive head, containing the nu- 

 cleus, and a long active thread-like tail. 

 b. The ovaries usually develop a Uttle 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 is 



