Nidification and E tnbryological Developinent 31 



siiles and on June 27 deposited two more capsules. These capsules were 

 laid fifteen and twenty days after the large laying period in the first 

 aquarium. Two-thirds of the eggs laid completed their development. A few 

 of the second generation remained alive for about two years but laid no 

 egg capsules. 



These examples seem to indicate that the large planorbids do not breed 

 as often as do the lynmaeids and are not as available for studies in de- 

 velopment as are members of the Lymnaeidae. Just how the smaller species 

 of the genera Menetus, Gyraulus, and Tropicorbis would react to such 

 investigations is not known.* It is probable that had Crabb kept his Heli- 

 soiita trivolvis longer they might ultimately have laid eggs. 



[b] The Development of Helisoma 



The notes that follow describe some of the results of aquarium observa- 

 tions on a few of the species of the genus Helisoma. No attempt has been 

 made to go into the finer details of embryological development. This has 

 been done by Holmes (1900) and others. 



[1] Helisoma trivolvis lentum (Say) 



SoNoral .spcfimoiis of the Ic ntiim race were received in 1930 from Dr. E. C. Faust 

 of the Tulane Medical School, New Orleans. They were collected from the canal at 

 New Orleans. This city is also Say's original locality for this species. The snails were 

 placed in aquaria and kept under observation for several years. On February 16, 1931, 

 an egg capsule was observed, apparently freshly laid, with twenty-five eggs. It 

 measured 7.5 by 6.0 mm. On February 22 four additional egg capsules were laid, one 

 4 by 3.5 mm. with nine eggs, one 7 by 4 mm. with fourteen eggs, one 9 by 4.5 mm. 

 with nineteen eggs, and one 10 by 4.5 mm. with twenty-four eggs. On March 7, two 

 other egg capsules were observed, one with fifteen eggs and the other with nineteen 

 eggs. All eggs were fertile and contained embryos in the early stages of development. 



On September 12, 1931, twenty-five young snails were counted, ranging in diameter 

 from 4.5 mm. with two and one-foui'th whorls to 16 mm. with four whorls. On No- 

 vember 28, 1931, nineteen specimens were counted ranging in size from 5.5 mm. in 

 diameter with three whorls to 14 mm. with four and one-fourth whorls. On January 

 16, 1932, the nineteen specimens were again measured, the smallest being 5.5 mm. in 

 diameter with three whorls and the largest 14 mm. with four and one-fourth whorls. 

 On March 19, 1932, the same number of specimens remained in the aquarium, the 

 smallest being 6 mm. in diameter with three and one-fourth whorls and the largest 

 14.5 mm. with foiu' and one-fourth whorls. On June 10, 1932, only fourteen specimens 

 remained in the aquarium. fi-\"c ha\ing died. The smallest was 6.5 mm. in diameter 

 and had three and one-foiu'th whorls and the largest was 12.5 mm. with four whorls. 

 August 15, 1932, only six specimens were alive, the smallest 6.5 mm. in diameter and 

 the largest 10 mm. in diameter. On October 23 and December 26 the six specimens 

 were still alive. Their size had not increased. 



On March 5, 1933, five specimens were living, the largest 10.5 mm. in diameter 

 with four whorls. On June 25, 1933, three were li^■ing, the largest being 11 mm. in 

 diameter with four and one-foin-th whorls. On September 17, 1933, two specimens 

 were living. These two specimens had li^•ed o\er two years in the aquarium. Of the 

 original 125 eggs laid in February and March, 1931, only 25 young snails survived 

 until September, 1931, and onl.y two snails were alive two and one-half years later. 

 Mortality among the young snails appears to be heavy in their earlj' postembry- 

 onic life. 



*Rearing methods for securing uninfected snails to be used in life history studies on 

 parasitic worms have been practiced in many parasitological laboratories. W. H. Krull 

 (1931) has published the jjrocedure for successful rearing of Gyrauliis parvus and 

 found that this species may be reared to full size in from four to five weeks under 

 laboratory conditions (Occas. Pap. Mus. Zool., Uni. Michigan, No. 226).— H.J.V-C. 



