
MOLLUSCS, VERMES AND HYDROZOA 
Ancy.tus. ‘These small snails are popularly known as ‘Freshwater 
Limpets” and though belonging to the family of the Lymneide 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 alge 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 1% 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 FIG. 174. Ancylus rivularis. Enlarged. 
longitudinal line upon the head. About 10 to 16 eggs are enclosed in a 
round capsule, which hatch in 20 to 36 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 4. /acustris 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. Ancylus parallelus. Enlarged. the tentacles 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. 
Puysa. 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 
