90 ZOOLOGY. 



by Quoj and Gaimard, who found it abundantly in very shallow brackish 

 water. It has a large head ending in two lobes, is without tentacles, and 

 tlie 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. 



Fcun. 3. Cyclostomidce. The genus Cyclostoma is monoicous ; it has a 

 turbinated shell with a circular aperture, and, unlike most land snails, it has 

 a ])aucispiral 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 üeshayes 

 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 

 witli the Trochidaj. The species are numerous, amounting to nearly 200. 



Ilelicina 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. 



Fa7n. 4. Physidm. This family is composed of certain genera of inoper- 

 cu late fragile uniformly colored shells, inhabiting quiet fresh watere, coming 

 t(^ the surface to breathe, and creeping along it with the back downwards 

 by means of the vibrillfe 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 

 numtle large, with a digitated nuirgin 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. Planot'his {pi. 77, ßg. 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 (Z. stagnalis, pi. 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- 

 form 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 Physadse, 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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