Vol. 5, p. 5-10, pi. 2. Septembee 7, 1921. 



Occasional Papers 



OF THE 



Boston Society of Natural History. 



EGGS AND YOUNG OF THE RIVER LIMPET, 



ANCYLUS FUSCUS C. B. Adams. 



BY WILLIAM F. CLAPP. 



Inasmuch as nothing has hitherto appeared in print concern- 

 ing the eggs and early stages of American species of Ancylus, the 

 following observations seem worth recording. 



On April 25th, 1921, several specimens of Ancylus fuscus were 

 found in a small pond in Cambridge, Mass. In the few moments 

 devoted to collecting, ten specimens were obtained, all on oak 

 leaves which had fallen into the pond but were not badly decayed. 

 None was found on any other material. Seven of the specimens 

 were placed in a small jar containing about one quart of water. 

 They began copulating at once, one individual crawling partly 

 on top of another, so that the left side and posterior portion of 

 the upper specimen protruded beyond the corresponding portions 

 of the lower. Immediately upon obtaining this position, the 

 penis of the upper individual would suddenly 'bloom' from the 

 left side slightly behind its left tentacle, and bending around the 

 shell edge of the lower, came in contact with the left side of the 

 latter (Plate 2, fig. 1). In the specimens observed, this position 

 was retained for several hours. The penis waved slowly back- 

 ward and forward, in worm-like fashion, the end prodding various 

 portions of the acting female in the vicinity of the center of the 

 left side, or slightly posterior. No actual insertion of the penis 

 was seen. 



In twenty-four hours egg-cases appeared on the sides of the 

 glass jar. These were all laid at night. An hourly examination 

 of the jar during the day failed to reveal a single specimen in the 

 act of depositing eggs. Since a satisfactory observation of the 

 development of the egg through the thick wall of the jar was 

 difficult, a few egg-cases were obtained on microscope slides, by 

 placing the slides as closely as possible to the sides of the jar and 

 near the bottom. A large majority of the egg-cases were placed 

 directly on the sides of the jar, between ten and twenty-five 

 -millimeters from the bottom. Those not included in this area 



