34 RESEARCHES UPON THE HYDROBIIX.^ 



it is by exaggeration represented a little raised from the ground, 

 in order better to indicate its movement. At the same moment 

 the snout is thrust forward, and its disk-like extremity alfixed to 

 the ground as far ahead as possible. Theu comes the second 

 motion (Fig. 26) ; in which, the snout and the posterior part of 



the foot being firmly affixed 

 ^^S" ^^* and supporting the body, 



the anterior part of the foot 

 becomes free, and is thrust 

 forward to the disk of the 

 rostrum where it is again 

 planted. Tiie operations 

 of the first motion are tlien 

 repeated. Thus the animal moves by regular steps, upon three 

 points of support, of which alternately two and one are used. Dur- 

 ing the movement the lateral folds of the body and foot are seen 

 sliding upon each other, showing how their arrangement contri- 

 butes to the facility with -vv^hioh this kind of progression is effected. 

 The surface of the animal is constantly lubricated with mucus 

 apparently much greater in amount than is seen in the Hclicidae 

 and other Pulmonates. The foot is capable of adhering with con- 

 siderable tenacity. While these animals were under my observa- 

 tion, many of them escaped over the edge of the plate in which they 

 were placed, and crept without much difficulty upon its under side. 

 The females in Pomatiopsis lapidaria are considerably more 

 numerous than the males, and are more elongated, having a more 

 cylindrical shell. The outer whorl of the male is proportionally 

 larger in order to afford space for the great verge. The ovary of 

 the female lies further up in the spire, giving the shell its less 

 conical form. 



The eggs of Pomatiopsis have not yet been observed. Most 

 probably they are deposited in the water. 



It will not be out of place here to mention a cercarian parasite 

 with which the P. lapidaria was thickly infested at the time of 

 observation (May 6th). When the mollusk is extracted from its 

 shell and placed in water, numbers of little white worms scarcely 

 visible to the naked eye are washed out from the branchial cavity, 

 which prove, on microscopic examination, to be the cercaria- 

 nurses of a species of Histrionella. They were filled with young, 

 which were found when extruded to be normally of a tad-pole 



