218 PROCEEDINGS OF THE ACADEMY OP [1883. 



Turton ^ observed. Moquin-Tandon^ states that he had never 

 observed the animal creeping or swimming on the surface of the 

 water. 



The shell of the animal is fastened to the body b}' a muscle, 

 which, as already said, passes perpendicularly from the shell and 

 enters the foot obliquely, and with which it coalesces, forming with 

 the foot the sides and floor of the visceral cavity respectively. 

 In the figure (PI. X, fig. 1) we have a cross-section of the animal 

 about the middle, drawn with a camera lucida, and to which I 

 have added the lines s, which represent a cross-section of the shell. 

 The letters m c represent the musculus cochlearis, which enters 

 the sides of the foot ; q m are the transverse fibres. The longi- 

 tudinal fibres are not represented, as they are transversely cut 

 and onl}' appear as points. 



In the musculus cochlearis of the left side in A. fluviatilis and 

 on the right of A. lacustris a cavity is found in which the heart 

 is situated. The walls of this cavity form the pericardium. 



The gill. — In the space between the foot and the mantle in A. 

 Jluviatilis on the left side is found a broad, leaf-like fold of the 

 integument, the gill. This fold or gill reaches down as far as the 

 lower border of the mantle. In the figure (PL X, fig. 1) the gill 

 (k) is represented on the right side of the section, although really 

 on the left side of the animal, and we must imagine that we are 

 looking at the animal from the front. The gill is one-third as 

 long as the whole animal and lies in the middle third of the 

 body. In the living animal it is of a lighter color than the sur- 

 rounding tissues and the surface of it is smooth. Although the 

 gill of ^4. lacustris is on the right side of the animal, its relative 

 position is the same as in A. Jluviatilis. The space between the 

 foot and the mantle, into which the gill hangs, ma}^ be called the 

 branchial chamber. 



I believe that the organ which Moquin-Tandon ^ speaks of as 

 the lobe auriforrae is what I prefer to call the gill. It is physio- 

 -logicall}'^ one, as we will presentl}' see. 



The whole surface of the gill is covered with ciliated epithelium, 

 and the interaal part is formed of cutis, consisting of loose 

 connective-tissue fibres which run in all directions and between 



' Manual of Shells, ed. ii, 1840. 



^ Recher. anat. physiol. s. I'Ancyle, etc., p.'3o. 



^ Recher. anat. physiol. s. I'Ancyle, etc., p. 12. 



