Mr. A. Hancock on the Animal of Chamostrea albida. 109 



or oval form, and are united at a point below ; at first sight they 

 appear to be composed of a single plate on each side of the 

 body ; but we shall presently see that this is not the fact. The 

 body, stretching downwards from between the gills, is of an oval 

 form, and gradually subsides into a small, compressed linguiform 

 foot (i) which projects forwards. The mouth (PL III. lig. 2 & 

 PI. IV. fig. Ij) is placed above, exactly in front of the body, and 

 is a rather large transverse slit, furnished with the usual four 

 appendages or palps {k, k'), which are well developed, and are 

 divided into a superior and an inferior pair, between which the 

 upper anterior margin of the gill-plate of each side is inserted. 

 They are flat, leaf-like, slightly tapering, and rather obtusely 

 pointed ; the external surface is smooth, as well as the margin of 

 the external border of the inner or opposing surfaces. This sur- 

 face, however, is for the greater part laminated, each palp being 

 furnished with thirty transverse laminae, inclined towards the 

 free extremity. The palps are united above and below the mouth 

 by widish membranes ; that above forming a sort of hood which 

 overhangs the oral orifice. 



To return to the gills; — The apparent single plate lying on each 

 side of the body is found, on close examination, to be divided 

 longitudinally by an obscure line (PI. III. fig. 2 h) into two por- 

 tions, the anterior {g) being much the larger; this portion is 

 formed of two laminae in the usual manner. These laminae are 

 united internally at several points, forming the interbranchial 

 space into a transverse series of tubes (/) as in most of the La- 

 mellibranchiata. This portion, then, is a complete gill-plate, and 

 is attached to the body of the animal above, from the side of the 

 mouth as far as the line defining its boundary behind ; and is 

 suspended from behind by a membrane (m), which extends the 

 entire length of the gill, its junction with the membrane being 

 indicated externally by the longitudinal line already alluded to. 

 Thus on each side of the body, along the posterior or dorsal 

 margin, there is formed a large channel (w) into which the inter- 

 branchial tubes open, and likewise the oviducts. The posterior 

 portion [g') of the supposed single gill-plate is narrow, and is 

 found to be composed of only a single lamina ; it is therefore a 

 rudimentary gill, and forms a sort of flap attached by its whole 

 length to the external lamina of the gill-plate ; its under surface, 

 dorsal and posterior margins being free. Chamostrea has con- 

 sequently only a single gill, and a rudimentary gill on each side 

 of the body. 



The branchial organ of Cochlodesma is arranged much in the 

 same manner; but in it the rudimentary portion is attached 

 throughout, there being no free margin. In Chamostrea we 

 have just seen that the rudimentary gill is a flap-like appendage. 



