56 COMPARATIVE ANATOMY 



further, as two rims bordering a narrow groove. Immediately 

 below the point of fusion the edge of each mantle-flap is 

 thickened and pigmented, and slightly scolloped out, and below 

 this point again the thickened edges are slightly scolloped out 

 and bear a fringe of small tentacles. In this manner two slit- 

 like passages are left between the opposed edges of the mantle. 

 The lower passage, bordered by tentacles, is the inhalant, 

 the upper one is the exhalant aperture. 



M 



LP 



Fig. ii 



Anodonta cygnea. The animal is shown lying in the right valve of the shell. The 

 left valve and the left mantle-flap have been removed. A. Add, anterior 

 adductor muscle; A.R, anterior retractor muscle; Ct, gill or ctenidium ; Ct 1 , 

 posterior extension of the ctenidia forming the septum dividing the branchial 

 from the cloacal chamber ; ex, exhalant orifice ; F, foot ; H, heart seen through 

 the transparent pericardia! wall ; in, inhalant orifice ; L.P, labial palps ; M, 

 right mantle-flap; M 1 , line of attachment of left mantle-flap which has been 

 cut away ; P. Add, posterior adductor muscle ; P.R, posterior retractor muscle; 

 Pt, protractor muscle of the foot. 



The outer and middle layers of the shell are secreted by the 

 epithelium of the thickened border of the mantle. The inner 

 nacreous layer is secreted by the whole of the outer surface of 

 the mantle. On cutting away the mantle-flap of one side, the 

 general shape of the animal and some of its more important 

 organs lying in the mantle-cavity are exposed. Ventrally and 

 anteriorly is the triangular muscular foot, its anterior margin 



