348 DR N. ANNANDALE ON 



be produced occasionally, as exceptional cases ; but I have been unable to note any indi- 

 cation of the occurence of such individuals. It must also be remembered that Brauer in 

 Europe and Downing in America agree in regarding the form named Hydra dicecia by 

 the latter as dioecious, and that W. Marshall's 1 statement as to Hydra viridis being 

 protandrous have not been corroborated. It is evident that a large supply of reserve 

 material is necessary for the production of eggs, and it does not seem altogether 

 probable that a form so delicate as H. orientalis would be able to expend such 

 material after the production of a considerable number of spermaries, the bearing of 

 which obviously exhausts the organism. 



The circumstances which favour sexual reproduction are difficult to ascertain in the 

 case of an animal which cannot be kept in captivity under altogether normal conditions ; 

 but observations on freshly captured individuals, compared with those made on captive 

 specimens, lead me to the conclusion that temperature is a most important factor in 

 the induction of sexual maturity. 



I have never seen an individual living in its natural environment which showed 

 any signs of sex during the fall in temperature at the beginning of winter, or after the 

 rise in temperature had become steady in spring ; but during the winter of 1905-6 I 

 noted that any considerable rise of the thermometer was invariably followed by the ap- 

 pearance of spermaries on about half the specimens in my aquarium. As a rule, the 

 organs became visible to the naked eye on the third day after the rise in temperature 

 commenced, and they were produced, and even reached maturity, if the thermometer 

 fell again after the second day. The same phenomenon occurred, except that as much as 

 a week often elapsed before the appearance of the spermaries, among individuals living 

 free in the " tank." The majority of the captive specimens which did not produce 

 spermatozoa in these circumstances, died j but on two occasions a few neither proved 

 to be males nor died, but bore eggs. They did not do so until the males in the same 

 glasses had been discharging spermatozoa for some days. The temperature this season 

 was unusually high during the latter part of December, and the fact may have upset 

 the normal course of reproduction among the polyps, as a large proportion of them 

 perished owing to the heat. I have not been able to find a single female with ova- 

 ries or eggs in the " tanks " during the year, and I only found one or two specimens (in 

 March) in 1904-5. Judging from observations on captive specimens, it would 

 seem that an essential factor in the production of ova is a period of comparatively low 

 temperature and active nutrition, followed by a sudden, but not an excessive, rise of 

 temperature. Such conditions frequently occur in the natural environment of the 

 species, but not invariably. They are only too easily produced in an aquarium. 



I have noted above that a proportion of the polyps kept in an aquarium during win- 

 ter have invariably proved themselves males, if they survived for more than a short 

 period. Except at the very beginning of the season the spermaries have become 

 visible in a week, if there has been no considerable rise in temperature. Spermatozoa 

 were produced even when the polyps were abundantly provided with food and with 



I Die Entdeckungsgeschichte der Susswasser poly pen. Leipzig, 1885. I have been unable to consult this work in Calcutta. 



