MOLLUSCS, VERMES AND HYDROZOA 



Ancylus. These small snails are popularly known as "Freshwater 

 Limpets" and though belonging to the family of the Lymnaeidae greatly 

 differ from them in appearance. They are sluggish and do not swim of 

 float in the water but adhere to plants or move slowly over the bottom. 

 Their food is principally algas and aquatic fungi. 



A. rivularis. Fig. 174, inhabits slow-flowing streams and may be 

 found on aquatic plants, stones and on the bottom near the margins. 

 The y^ inch long dishlike shell terminates in a broad oval base which en- 

 tirely covers the snail so 

 that only the tentacles 

 and a part of the snout 

 protrude when the snail 

 is active. It is of pale 

 transparent horn-color, 

 with the body greyish- 

 brown marked with white, 



having a central yellowish fl*^- '74- Ancylus rivularh. Enlarged. 



longitudinal line upon the head. About 10 to iG eggs are enclosed in a 

 round capsule, which hatch in 20 to 2>(> days. It inhabits the Delaware 

 and Susquehanna rivers and tributaries, and is met with generally in the 

 Eastern and Middle States to Upper Missouri. 



A. parallelus. Fig. 175, greatly resembles A. lacustris of Europe, but 

 may be distinguished in having the apex of the shell directed to the right, 

 in opposite direction to the European species. It may be found in still 



water and sluggish streams 

 on the under sides of float- 

 ing plants, and has a very 

 thin yellowish mottled shell 

 with a sharp apex and oval 

 aperture. It grows to a 

 length of ~ inch. The 

 body is yellowish grey with 



FIG. 175. yincylus parallelus. Enlarged. the tCntaclcS of a lighter 



color. The 6 to 20 eggs are enclosed in a globular capsule and hatch in 

 16 to 30 days. 



Neither of these species of Ancylus long survive in the aquarium as 

 they fall victims to the fishes. 



Physa. This genus may be distinguished by the left-hand or sinist- 

 ral turn of the whorls of the spindle-shaped shell. It has no operculum 

 but an extended mantle and two long and slender tentacles. It is not a 

 numerous genus though widely distributed. A sub-genus is Aplexa. 



236 



