58 



NOTES ON LIMNiEA PEREGER m. SINISTRORSUM. 



By W. HARRISON HUTTON. 



(Read before the Society, September Sth, 1915). 



I HAVE always found it difficult to keep Littmcea pereger alive for any 

 length of time in confinement, never over five or six months, except- 

 ing in the case of young specimens, which seem to become more 

 accustomed to their surroundings and in some cases have lived 

 longer. This refers to the type, but m. sinistrorsum seems more 

 delicate, and adult specimens that I have placed in an aquarium, 

 with one or two exceptions, have only averaged about a month or 

 two of life. 



Another sign that they are not so robust is that their shells are more 

 fragile than shells of the type taken out of the same habitats. 



On April 26th I isolated two specimens of L. pereger m. sinistror- 

 sum that ha'd been in copulation with typical pereger (dextral). In 

 all cases that have come under my observation, about a dozen, the 

 pairing is always one of the type with a sinistrorsum, never two sinis- 

 trorsum, which I do not think would be possible. 



On May 5 th I observed the pereger extruding egg-sacs, and by 

 May 13th they had deposited four egg-sacs in each of which I 

 counted from 250 up to 300 embryos. On May 14th I noticed 

 that one of the egg-sacs had become of a light amber tint, though 

 they were all transparent and colourless when deposited ; it was also 

 surrounded by a green alga. This never came to anything, and the 

 embryos rotted away. The sacs had all by this time become much 

 larger, in some cases measuring 34 mm. in length and 8 mm. in dia- 

 meter. One of the pereger died during the extrusion of an egg-sac. 



On June 30th as the young fry had now left the egg-mass, which 

 had broken up and had become larger, I counted them with a lens, 

 and found them to number 50 per cent, sinistrorsum and 50 percent, 

 type. A week afterwards, as they were dying off very rapidly, I 

 removed them to a pond, where there was no sign of pereger and the 

 habitat looked very suitable ; but although I visited it once or twice 

 for two or three years I never saw anything more of them. 



The parents, as in all cases that have come under my observation, 

 died shortly after depositing the egg-sacs. 



After eight years of observation of the habitats of the L. pereger m. 

 sinistrorsum, I come to the conclusion that their appearance and 

 almost total disappearance at different periods is the effect of the law 

 of periodicity, and that every third year they are most numerous, 



