26 The MoUuscan Family Planorhidae 



March 10. 18 in gastrula stage ; 4 trochophore ; 3 veliger ; 3 embryos with shelL 



11. 18 in gastrula stage ; 4 vehger ; 3 postvehger; 3 embryos with shelL 



12. 18 in gastrula stage; 3 vehger; 4 postvehger; 3 embryos with shelh 



13. 18 in gastrula stage; 3 veliger; 3 postvehger; 3 embryos with shell ; 1 hatched. 



14. 18 in gastrula stage; 3 veliger; 3 postvehger; 2 embryos with shell; 1 hatched. 



15. 18 in gastrula stage ; 2 veliger ; 2 postvehger ; 2 embryos with shell ; 2 hatched. 



16. 18 in gastrula stage ; 2 postvehger ; 3 embryos with shell ; 1 hatched. 



17. All hatched or dead, many of the gastrula stage filled with Protozoa. 



In the above table it will be observed that only ten embryos completed 

 their development. Eighteen embryos did not continue their growth after 

 reaching the gastrula stage, for what reason is not apparent. In the stage 

 shown in fig. 9 on plate 51 the snail is very active, moving about in the 

 egg and grazing like the adult snail. The radula can be seen working with 

 its peculiar lapping motion, the heart is beating regularly, and the pul- 

 monary siphon is functioning, for it can be seen to emit bubbles. One 

 young snail four days out of the egg was transferred to an aquarium. This 

 specimen was very active, crawling about the glass sides of the jar. The 

 eyes were black and very conspicuous. 



Several of the embryos that completed their development showed the 

 following history. 



EGG NUMBER 1 



March 2-3. Segmentation and gastrula. March 10. Embrvo stage. Like fig. 10. 

 Like fig. 2, 3. 11. Embryo stage. Like fig. 10. 



4. Early trochophore stage. 12. Embryo stage. Like fig. 11. 



5. Trochoiihore stage. 13. Young snail. Like fig. 12. 



7. Posttrochophore stage. 14. Young snail. Like fig. 12. 



Like figs. 4, 5. 15. Young snail. Like fig. 12. 



8. Veliger stage. Like figs. 6, 7. 16. Hatched from egg. Like fig. 13. 



9. Veliger stage. Like fig. 9. 



EGG NUMBER 8 



March 2-4. Early segmentation and ga.s- March 10. Veliger stage. Like fig. 9. 



trula stages. Like figs. 2, 3. 11. Embryo stage. Like fig. 10. 



5. Po-sttrochophore stage. 12. Embryo stage. Like fig. 10. 



Like figs. 4, 5. 13. Young snail. Like fig. 12. 



7. Veliger stage. Like fig. 6. 14. Young snail. Like fig. 13. 



8. Veliger stage. Like fig. 6. 15. Young snail. Like fig. 13. 



9. Veliger stage. Like figs. 7, 8. 16. Hatched. Like figs. 13, 14. 



Both of the above embryos completed their development in fifteen days. 

 Probably some early cell divisions took place the day before observations 

 began, which would increase the development period to sixteen days. The 

 temperature of the room was 70-75 degrees Fahrenheit. The other eggs in 

 this capsule showed approximately the same development. The embryo in 

 egg number 5 migrated to egg number 6 at the late embryo stage and 

 both embryos occupied the same chamber until they were hatched, three 

 days later. 



Several adult Helisoma scalare received from Dr. Andrews, collected in 

 Lake Butler, Florida, were placed in an aquarium where they bred and 

 laid eggs which successfully completed their development. Eggs were first 

 observed on Alarch 13 (see plate 52, fig. 1). The egg capsules were de- 

 posited on the glass sides of the aquarium and were numbered so that 

 development could be easily and accurately observed. Thirteen egg cap- 

 sules were deposited early in Alarch: 



