150 BRITISH MOLLUSKS. 



large size, and nine of them are of a form (Ameria) found in no 

 other part of the world, in which the upper, or hinder, portion of 

 the shell is sharply angled. As many as fifty species of Physa inhabit 

 the continent and islands lying between Bahia and the Northern 

 United States, three of which, P. Maugerce, influviata, and aurantia, 

 natives respectively of Jamaica, Guatemala, and Mazatlan, are con- 

 spicuous for their size and dark shining surface. In Europe there 

 are but four species of Physa, and only two of these pass into 

 Britain. P. fontinalis is plentifully distributed in our ponds and 

 ditches and running brooks. P. hypnorvm is more partially diffused 

 and scarcer, and on the Continent it is one of the few mollusks 

 whose range of habitation is limited to the north side of the 

 Pyrenees. 



Our two Physce are very distinct from each other. P. fontinalis 

 is of a rather inflated oval form, having the mantle reflected over 

 the shell on either side in a digitate fringe-like lobe. Sometimes it is 

 lobed pretty evenly along each side, sometimes, as in the variety re- 

 presented in our vignette, more in the direction of the columella and 

 spire, the lobe on the columellar side being the larger. The por- 

 tion of the animal seen through the transparent shell, is delicately 

 ocellated. P. hypnorum is of a fusiformly oblong shape, and has a 

 more attenuately convoluted shell with no lobed overlapping of the 

 mantle. For this reason it has been set apart as a separate genus 

 {Aplexa). In both species, the foot of the mollusk is lanceolately 

 pointed behind, the respiratory and other important organs are on 

 the left side of the animal, the shell is therefore convoluted to the 

 left, and the tentacles are filiform, or bristle-like, as in Planorbis, 

 which genus it resembles also in having the eyes at their inner 

 base. The difference in the structure of the shell in Planorbis 

 and Physa, one producing a discoidly convoluted, the other an in- 

 flated oval one, contrasts remarkably with their similarity in other 

 respects. 



The Physce, especially P. hypnorum, are active in habit, whether 

 swimming, foot uppermost, on the surface of the water, holding 

 themselves stationary at different depths in the water, or gliding 

 through it in sudden jerks by an hydrauHc action of the foot. By 

 bringing the lateral margins of this organ into contact, the animal 

 constructs a tube for inhaling and suddenly expelling the water 

 either upwards or downwards. Montagu stated, and the statement 

 has been repeated by Jeffreys, that the animal spins a mucous 

 thread for letting itself down in the water and rising again for re- 



