SELF FERTILIZATION IN POND SNAILS. 49 



reflexa Say, from Chicago, Lymncea auricularia Linne, from a 

 pond near Oak Lane, Philadelphia, Physa heterostropha Say, from 

 Philadelphia, Planorbis exacutis Say, from Philadelphia, Planor- 

 bls parvus, Say, from Philadelphia, and A ncylus fluv iatilis from 

 the same place. In every case isolated snails laid fertile eggs. 1 



That self fertilization is not normal when a number of snails 

 are together in an aquarium may be inferred from the fact that 

 one so often observes two snails in copulation. The author has 

 even observed different species in copulation, Lymncea columella 

 and Lymncea humilis modicella. In the case observed the mass 

 of sperm that had been injected into the vagina of the snail 

 acting as the female was soon expelled into the water where it 

 fell to the bottom as a mass of whitish mucus. When this was 

 examined under the microscope it was seen to contain sperma- 

 tozoa. All attempts at crossing species proved failures. 



The significance of the fact that self fertilization is possible in 

 the air-breathing pond snails is that pure lines can be established 

 in this group of animals in the same sense as in plants. That 

 continued imbreeding of this closest sort does not cause the 

 strain to become weak and finally extinct is proved by the fact 

 that the author has one line of Lymncea columella which has 

 passed through thirty one self-fertilized generations. 



1 The writer wishes to thank Dr. H. A. Pilsbry for his kindness in identifying 

 the snails mentioned in this paper, and Mr. E. H. Hand for supplying material 

 from Chicago. 



