28 The Molluscan Family Planorhidae 



When hatched the shells usually measure 1 mm. in height and 0.7 mm. 

 in diameter and have a trifle more than one whorl. The young scalare, 

 therefore, had more than doubled in size in less than eight weeks. On 

 August 15, after a hot summer, seven specimens remained in the aquarium. 

 Five of these measured as follows: 



Height 4.7; diameter 5.0 mm. Whorls 3%. 

 Height 4.0; diameter 4.1 mm. Whorls 3. 

 Height 5.0; diameter 5.1 mm. Whorls 3. 

 Height 6.0; diameter 5.0 mm. Whorls 3%. 

 Height 6.0; diameter 6.0 mm. Wliorls 3^2. 



These measurements indicate that the shells had almost trebled in size 

 in about twelve weeks, although but one more whorl was added. 



On June 26, during a hot period of weather, two adult snails of this 

 species died, leaving one adult, which lived over winter isolated in a small 

 aciuarium. On jMarch 5, 1933, several egg capsules were laid by this indi- 

 vidual approximately nine months after its isolation. Two empty egg cap- 

 sules were observed on the side of the aquarium and twenty-one young 

 snails were crawling about. The young shells measured 2 mm. in height and 

 2.5 mm. in diameter and had two whorls. All li\'ed from March 5 to May 

 21 when three died. On June 25 the last adult died but nine of the young 

 remained alive. 



On September 3, 1933, four of the young scalare were living. The young 

 snails at this time, about twenty-three weeks old (160 days) measured 5.0 

 mm. in height and 4.5 mm. in diameter and had three and one-half whorls. 

 Between September 3 and September 17 these young snails formed eighteen 

 egg capsules containing the following number of eggs: 6, 11, 13, 9, 10, 5, 7, 

 7, 12, 11, 2, 5, 9, 10, 10, 7, 9, 7. Embryos in but two of the capsules com- 

 pleted their development. The parent snails of this last brood, it will be 

 noted, were the offspring of an isolated adult scalar^e. The young snails 

 lived until January, 1934. 



The development of HeUso)}ia scalare is similar in many respects to 

 that described by Lankester for Lymnaea stagnalis (1874) and by Lowrance 

 (1934) for Stagnicola kingi. There are, of course, differences in the form of 

 the embryos as would be expected in two such diverse families. Lowrance 

 found that young snails of Stagnicola kingi hatched in from fourteen to 

 sixteen days (p. 10) but that with increased temperature (26 degrees 

 centigrade) this period was cut to from nine to eleven days. Liimnaea 

 stagnalis may require as much as twenty days or more for development. 



B. NIDIFICATION AND DEVELOPMENT 

 IN OTHER SPECIES OF Helisoma 



Plates 51 and 52 



Living material belonging to five additional species and races of Helisoma 

 was studied in aquaria during the years 1931, 1932, and 1933. The species 

 included were: 



Helisoma trivolvis lentum (Say) 



Helisomn trivolvis fallax (Haldeman) 



Helisoma duryi normale Pilsbry 



Helisoma duryi eudiscus Pil.sbry 



Helisoma tenue calijorniense F. C. Baker 



