19 



also shown that in Gardium " oblong um" C. echinatu?n, 

 and G. rusticum a rudimentary by ssus gland, corresponding 

 in all respects to the structure above described, is present. 

 Finally, Gwyn Jeffreys* mentions a case of G. edule itself 

 in possession of a functional byssus. 



Of the apertures of the body, the mouth (ilf., fig. 3) is a 

 wide slit lying between the viscero-pedal mass and the 

 anterior adductor; the anus {An.) lies on the dorsal surface 

 of the posterior adductor. The apertures of the gonad and 

 renal organ are rather difficult to observe. They may be 

 seen by folding back the internal branchia (see fig. 3). The 

 ureter (Ben 1 ., fig. 4) is a mere slit in the wall of the 

 renal vestibule, and lies on the lateral wall of the renal 

 organ just dorsal to the origin of the posterior retractor 

 muscles of the foot. The external opening of the gonad 

 lies in nearly the same position, but on the lateral and 

 posterior wall of the viscero-pedal mass ; it is slightly 

 elongated and sometimes has tumid lips. 



The Musculature. 



The muscles may be conveniently arranged into four 

 groups : (1) the adductor muscles of the shell ; (2) the 

 extrinsic muscles of the foot ; (3) the intrinsic muscles of 

 the foot ; and (4) the pallial muscles. The extrinsic and 

 intrinsic pedal musculature form really one system. The 

 pallial muscles may be divided into the pallial muscles 

 proper and the muscles of the siphons. 



(1) There are two adductor muscles of the shell (Add.a. 

 and Add.p., fig. 3), anterior and posterior. Each is a 

 strong bundle inserted on the dorsal oblique surface of the 

 shell near the margin, and running across in an exact trans- 

 verse direction from valve to valve. Owing to the curve 

 of the shell the scars of attachment (Add.a'. and Add.p'., 



* British Conehology, vol. II., p. 208. 



