No. 531] GUNDLACHIA AND ANCYLUS 



183 



the external features as seen in the aquarium. In Feb- 

 ruary, 1907, probably hundreds of the unseptate ancyloid 

 form were present. There were several Vivipara in the 

 jar that winter. Subsequently they were removed, Mr. 

 Allen thinking that they might consume the food supply 

 needed by the ancyloids. Having heard that the stunted 

 growth of aquarium mollusks might be due to the pres- 

 ence of their soluble excreta in the water, he thought the 

 removal of the Vivipara might have had some influence 

 in this way. However, the removal of the large snails did 

 not stop septation. 



In the winter of 1906-07 the specimens of Planorbis 

 parvus in the jar were large and healthy. In the winter 

 of 1907-08 the individuals of this species appeared 

 dwarfed. The water in the jar was then removed and 

 replaced by distilled water. After that the Planorbis 

 (and Mr. Allen thought also the Jnacharis) took on a 

 more healthy appearance. He thought that the concen- 

 tration of saline matter due to refilling loss from evapo- 

 ration with ordinary lake water might have been influ- 

 ential injuriously, and the transfer to distilled water 

 have lessened the tendency to septation. 



In the winter of 1907-08 septate individuals of which 

 the exact number were not recorded were again found in 

 the jar. In January and February, 1908, the ancyloid 

 form was fairly plenty, though not so numerous as in the 

 previous year. In spring they became fewer and in May. 

 1908, there were none visible (although in a smaller jar 

 there were some). They reappeared in the first half of 

 June, 1908. July 3, 1908, an immature septate individ- 

 ual was taken, and another on July 20. On the theory 

 that the septum is formed during a resting stage, these 

 may have been forming during May, when nothing was 

 in sight. August 3, 1906, another specimen was taken. 

 January 11, 1909, a specimen was found which had be- 

 gun to add the third or expanding stage of the shell ex- 

 ternal to the septum. No mature Gundlachia were 

 taken from this jar during the winter of 1908-09. 



