30 The Molluscan Family Planorhidae 



Crabb states (1927, p. 91) that self-fertilization is the normal method 

 of rei^roduction in Lynmaea and other fresli-water pulmonates and that 

 cross-fertilization seldom or never occurs. With this general statement the 

 writer can not fully concur. It is doubtless true in many cases of reproduc- 

 tion during isolation but that it is the chief method among free snails when 

 in aggregation in their native environment or even in the laboratory is 

 scarcely possible. The writer has observed many species of Lynniaea, 

 Physa, and the planorbids in coitu. If the method of reproduction men- 

 tioned by Crabb is the chief or possibly the only method, why should the 

 male portion of the genitalia be so elaborately developed with prostate and 

 the penial complex provided with such diverse accessories in its makeup? 

 The spermatheca has been found to contain a spermatophore containing 

 sperm which was obviously deposited during copulation. Self-copulation 

 probably occurs when a snail is isolated, at least in those species having 

 a penial complex of sufficient length to reach the female opening, which is 

 the case in many species of the Planorbidae. 



Crabb states that he was unable to raise progeny from isolated Pla- 

 norbis trivolvis iHelisoma) , four individuals being carefully reared in iso- 

 lation for 377 days without the laying of a single egg capsule. Ten to 

 fifteen small empty egg masses were deposited. Even the control culture, 

 consisting of two snails, formed no capsules. Lymnaca stagnalis, on the 

 contrary, was very prolific. 



My isolated laboratory stock lays about e\-ery third day for a period of about twenty- 

 five days, rests a few days, then begins another cycle of laying. This is continued 

 throughout the year. Of fifty-four L.s.appressa isolated from the same egg mass, 

 seven laid their first eggs fifty-eight days after hatching. However, they do not reach 

 their maximum laving capacity until they are about one hundred days old (Crabb, 

 p. 73). 



Colton's experiments on Lynmaea {Pseudosuccinea) columella indicated 

 that the interval between hatching and egg laying was widely variable 

 (1912, ]i. 175). Thirteen isolated individuals had the following interval 

 between hatching and egg laying: 32, 26, 35. 49, 58, 92, 50, 50, 56, 57, 63, 

 74, and 74 days. Twenty-six days was the shortest interval and ninety-two 

 days the longest interval. 



In the family Planorbidae the intervals are much longer, not only of 

 the specimens placed in isolation, but for those in aggregation. Helisoma 

 scalare laid thirteen egg capsules in early March, 1932. In early April, 

 six additional capsules were produced. An isolated individual which lived 

 over winter (nine months) without egg laying, deposited two fertile egg 

 capsules on March 5, 1933. Between September 3 and September 17 the 

 young snails, 160 days old, laid eighteen egg capsules containing from six 

 to thirteen eggs in each capsule. 



Specimens of Helisoma trivolvis lentum were placed in aquaria in 1930. 

 On February 16, 1931, a capsule was laid, on February 22 four capsules 

 were deposited and on March 7 two capsules were laid. No other eggs were 

 laid by these specimens which lived until Se]it ember 17, 1933, or a period 

 of 943 days. A young specimen from the parent lentum. was isolated Janu- 

 ary 16, 1932. It was about 325 days old. Between May 27 and June 9, 

 1932, this individual, after inhabiting the aquarium for 132 days, laid 

 twenty egg capsules aggregating 325 eggs. On June 20, 1932, the parent 

 snail was removed to another aquarium and on June 22 laid two egg cap- 



