90 ZOOLOGY. 
by Quoy and Gaimard, who found it abundantly in very shallow brackish 
water. It has a large head ending in two lobes, is without tentacles, and 
the eyes are placed flat upon the head. The pulmonary cavity is large, in 
which it resembles that of Ampullaria. The sexes are united in the same 
individual. ‘Two species are known, A. avellana and A. fragilis. 
fram. 3. Cyclostomide. The genus Cyclostoma is monoicous ; it has a 
turbinated shell with a circular aperture, and, unlike most land snails, it has 
a paucispiral operculum. The head is proboscidiform, the tentacles two, with 
the eyes at their external base; and the foot is composed of two longitudinal 
parts which are advanced alternately. Other species have a concentric oper- 
culum, and in these the foot has the ordinary construction. Cuvier and Deshayes 
consider the mode of respiration less important than the general structure, 
and they are consequently of the opinion that the affinities of this family are 
with the Trochide. The species are numerous, amounting to nearly 200. 
felacina may be considered the type of a sub-family. It has an oblique 
aperture, and a closely fitting operculum which is not sub-spiral, but 
increases from one of the sides or angles. 
Fam. 4. Physide. This family is composed of certain genera of inoper- 
culate fragile uniformly colored shells, inhabiting quiet fresh waters, coming 
to the surface to breathe, and creeping along it with the back downwards 
by means of the vibrille which cover the foot as well as the two tentacles 
and other parts. Many eggs are deposited together in a glairy mass. The 
breathing aperture is at the external side, and is opened from time to time 
to take in a supply of air. In Physa the shell is turbinated, sinistral, the 
mantle large, with a digitated margin turned upon the shell, and the foot 
long and pointed posteriorly. The tentacles are filiform, and the eyes are 
situated upon the head near their internal base. Planorbis ( pl. 77, fig. 99) 
has a discoidal shell, with the turns visible on both sides. The mantle is 
simple, the foot rather small, and the tentacles and eyes as in Physa. In 
Limnea (L. stagnalis, pl. 75, fig. 92), the shell is dextral, turbinated, and 
generally larger than in the allied genera; the tentacles are triangular, and 
the eyes situated at their anterior base. Ancylus is a genus of little patelli- 
torm shells, the position of which in the systems has been a subject of much 
dispute, some contending that they belong here, and others to the Hypo- 
branchia. The animal is not essentially different from Limnea, and it is 
‘possible. that some species breathe air, and others water. Some authors 
have asserted that they come to the surface to breathe air, but this has 
never been observed in the American species, some of which are found 
beneath stones, in dead bivalve shells, and under other circumstances, which 
would render it difficult for them to reach the surface and descend again. 
But as free air may not be considered necessary to these animals, they 
may be placed among the Physadw, especially as a species of Physa inhab- 
iting beneath shelving rocks in rapid water is found at the mouth of the 
Nolachucky river, Tennessee, in such a position that it could not breathe 
air. It is probable that a small variety of Physa heterostropha inhabiting 
springs seldom or never breathes air. It has been ascertained that frogs 
kept under water can oxygenate the system through the skin, and the 
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