1912.] NATURAL SCIENCES OF PHILADELPHIA. 175 



The fact that self-copulation had been observed by no other 

 observer than the great von Baer, the fact that a case of partheno- 

 genesis is as yet unreported in the group of the mollusks, together 

 with the fact that there is every arrangement present in the animal 

 to make self-fertilization possible — all these seem to the writer suffi- 

 cient evidence that the chance that parthenogenesis plays a part is 

 remote. This must not be taken as precluding this remote possi- 

 bility. 



Are these animals easy to handle in a breeding experiment? This 

 question may be answered, on the whole, yes. 



In 1908 the writer (Colton, 1908) published the results of a series 

 of experiments on the pond snail Lymncea columella, showing the 

 effects of various external conditions upon the growth. Among 

 other things, the writer found how easy it is to raise Lymncea from 

 the egg under ordinary laboratory conditions. They will live in as 

 little .as 500 cc. of water and require next to no care. Philadelphia 

 city water from the tap was found to be fatal to the young snails, 

 but the same water, after standing in a large aquarium for some 

 time, could then be used. In his experiments this winter even 

 this water proved fatal, so that water had to be procured from 

 the nearby pond in the Botanical Garden of the University. In 

 the previous experiments a water plant, Myriophyllum. and a little 

 soil seemed to offer the optimum conditions for growth, aeration 

 being neglected. This winter the writer has had the best results in 

 using dead leaves of trees from the bottom of the pond referred to 

 above. These were washed in running tap water as a precaution 

 against introducing young snails from the pond. The best results 

 are now procured by using about 700-1000 cc. of pond water in a 

 battery jar and placing in this a half-decayed leaf, such as a maple 

 leaf. One snail only is of course placed in the jar. 



The length of time that it takes from hatching to egg-laying 

 varies greatly. As the cause of this variation is not understood 

 at all, it will be at the present time worth while to mention only 

 some of the cases. Thirteen had the following interval from hatch- 

 ing to egg-laying: 32, 26, 35, 49, 58, 92, 50, 50, 56, 57, 63, 74, and 

 74 days. Twenty-six days was the shortest interval. This shows, 

 even if we neglect the shortest periods, how surprisingly rapid is 

 the growth. As. for ease of handling in a laboratory, Lymncea 

 columella is seen to furnish splendid material. They require little 

 care, little space, and come to maturity promptly. 



What are the characters that distinguish the various species and 



