THE COMMON HYDRA OF BENGAL. 349 



oxygen from growing plants, which also gave them shade. The development of the 

 gonads was slower when they were kept in enamelled-iron or earthen vessels than in 

 glass jars. If the glass jars were placed in a dark corner, the appearance of the sper- 

 maries was also retarded ; but in this case the plants were less vigorous and the water 

 was consequently less well supplied with oxygen, which may be an important factor 

 in reproduction. It is not easy, especially under tropical conditions, to distinguish 

 between the direct effects of light and those of heat, and there are practical difficulties 

 in shutting off light from an aquarium in Calcutta, as this appears to encourage the 

 growth of a bacterium which turns the water foul. I am inclined to think, however, 

 that light is quite a secondary factor in the induction of sexual maturity, if it is of any 

 direct importance at all. 



Spermaries — 



I have not investigated the early stages in the development of either the sperma- 

 ries or the ovaries, my observations having been confined hitherto to living " material." 

 Superficially the spermaries resemble those of Hydra viridis. The first to appear are 

 arranged in two vertical rows situated in opposite quadrants of an ideal cross-section 

 of the body and near the tentacles. As a rule, the uppermost organs attain maturity 

 first, and others appear below them in the same vertical line. This process goes on 

 until the two rows reach the upper limit of the budding zone, on which they do not tres- 

 pass. Spermaries also appear in the two remaining quadrants, showing a tendency 

 to be arranged in vertical rows but not being quite constant in this respect. From 

 eighteen to twenty-four testes are produced by a vigorous individual ; but it usually 

 dies before the whole of them are mature. 



In their earlier stages the individual spermaries are mound-shaped and solid ; but 

 on approaching maturity they become mamilliform and a large cavity is developed 

 within the organ. As the spermatozoa are formed from the sperm mother cells they 

 escape into this cavity and take up a position with their ' ' heads ' ' facing outwards. 

 Their motile ' ' tails' ' are already active, and, as a mass of them accumulates in the cavity, 

 they are gradually thrust outwards by the movements of the ' ' tails, ' ' until a passage 

 is bored through the pap-like projection at the free extremity of the organ. At least 

 two hours are occupied in this process, but when the passage has once been forced it re- 

 mains open as a duct for the exit of freshly formed sperms. The spermary gradually 

 collapses as its contents are thus discharged and soon resembles an unripe testis in 

 appearance. I have not known one to be altogether re-absorbed after becoming 

 functionless. 



At least two, sometimes as many as five, days elapse between the first appearance 

 of a spermary and the escape of its sperms. 



The spermatozoa resemble those of Hydra fusca, except that the ' ' heads ' ' are 

 perhaps a little broader and shorter relatively. 



There are several interesting points in this account of the male organs. In the 

 first place, their position on the body is a further argument in favour of the idea that 

 something approaching four-rayed symmetry exists in Hydra orientalis. The fact that 



