III. 



NIDIFICATION AND EMBRYOLOGICAL 

 DEVELOPMENT 



THE EARLY STAGES of cell growth and development in the phi- 

 norbids {Hclisonia trivolvis) have been well described and figured bv 

 Holmes (1900, pp. 369-458). Crabb (1927, pp. 67-108) has described 

 and figured the early stages of Lijmnaca stagnalis appressa ( =jugi(laris) 

 and Lankester (1874) has given us the full development of the European 

 Lymnaea stagnalis. More recently (1934) Lowrance has very fully de- 

 scribed the early development of Stagjiicola kingi. Little work has been 

 done, however, on the embryonic development of the different species of 

 the family Planorbidae and especially on their nidification, period of 

 development, and the time element in the growth of the voung animal in 

 its shell.* 



A dozen or more aciuaria were operated during the years 1930 to 1934 

 and careful data were gathered concerning the period of egg deposition, 

 number of capsules laid, changes taking place within the egg capsule, and 

 growth of the young snails within the egg and after hatching. 



A. DEVELOPMENT OF Helisoma scalare (JAY) 



In 1932, Dr. E. A. Andrews, of the Johns Hopkins University, sent the 

 writer a piece of pond lily leaf upon which was a large egg capsvde of 

 Helisoma (Seminolina) scalare (Jay), containing twenty-eight eggs in vari- 

 ous stages of development. This capsule was deposited in an aquarium but 

 the parent came from Lake Butler, Florida. This egg capsule was observed 

 as opi^iortunity afforded until all of the embryos were hatched or had died. 

 From this capsule a fairly good idea of the development of this planorbid 

 was obtained. Observations began when the specimen was received on 

 March 2 and the last specimen was hatched on March 17. The different 

 stages are figured on plate 51 and are described in the explanation of 

 that plate. 



Development was observed to be sporadic in many of the eggs. There 

 were resting stages between periods of great activity of rotation about the 

 interior of the egg, the rotation beginning in this species the latter j^art of 

 the gastrula stage, the second day of development, and becoming marked 

 from the early trochophore stage. This rotation is caused by the vibration 

 of the numerous cilia attached to the foot of the embryo (CL in figures). 



Observations on the egg capsule began on March 2, when twenty-eight 

 fertile eggs were counted. The observations, made several times a day, 

 showed the following degrees of development : 



March 2-3. 24 eg^cs in segmentation stages. 4 in early trochophore stage. 



4. Same condition as on March 3. 



5. 20 eggs in gastrula stage ; 4 in trochophore stage ; 4 in preveliger stage. 



7. 18 in gastrula stage ; 5 in trochophore stage ; 3 in veliger stage ; 2 postveliger. 



8. 18 in gastrula stage ; 4 trochophore ; 3 ^'eliger ; 3 embryos with shell. 



*After this manuscript was in press, a very significant experimental study, b.v 

 H. B. Roney, on the development of Helisoma eggs, under controlled conditions, 

 appeared in Ecology, v. 24, pp. 218-243.— H.J. V-C. 



25 



