EEPEODUCTION 21 



both testes and ovaries. Contrary to the rule among 

 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 up 

 to twenty. They are usually situated near the 

 oral or distal end of the body, nrfb 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 become directly con- 

 verted into spermatozoa by modification 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 rup- 

 ture of the capsule at its most prominent point. 



Select a Hydra in which the testes are of large size ; 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 

 ■^ inch in length. Each consists of a small 

 oval highly refractive head, containing the nu- 

 cleus, and a long actively moving thread-like 

 tail. 



b. The ovaries usually develop a little later than the 



testes. In Hydra virid/is 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 thebasal end of the animal. 



