6o COMPARATIVE ANATOMY 



length of its upper edge to the mantle just below the line of 

 attachment of the latter. The inner lamella of the outer gill- 

 plate is attached throughout the whole length of its upper edge 

 to the corresponding edge of the outer lamella of the inner gill- 

 plate. The fused edges are somewhat thickened, and in the 

 region of the foot are firmly attached to the body-wall. The 

 inner lamella of the inner gill-plate is attached in its anterior 

 portion for a distance of about three-quarters of an inch to the 

 body-wall. Its upper edge then becomes free, leaving a slit- 



Fig. 13 



Diagrammatic transverse sections through Anodonta cygnca to show the at- 

 tachment of the gill-lamella3 to one another, to the mantle, and the foot. A^ 

 section through the anterior part of the foot ; /•', through the hinder part 

 of the foot ; C, through the region posterior to the foot, a^ outer lamella of 

 outer gill-plate; /', inner lamella of outer gill-plate; c, outer lamella of 

 inner gill-plate ; d, inner lamella of inner gill-plate \f, foot ; in, mantle. 



like space between itself and the posterior moiety of the foot, 

 and behind the foot its upper edge is firmly fused with the cor- 

 responding edge of the corresponding lamella of the other side 

 of the body. Thus, in the anterior part of the gills there are 

 four supra-branchial passages, two on each side of the body, 

 each enclosed between the outer and inner lamella of a gill- 

 plate. In the posterior region of the foot the four passages are 

 still present, but the inner passage of either side communicates 

 by a slit-like aperture with the branchial chamber, owing to the 

 detachment of the inner lamellEe of the inner gill-plates from 

 the body-wall. Behind the foot, where the gills project freely 

 backwards into the mantle-cavity, the four branchial passages 

 open into one another above as is shown in fig. 13, C, and pass 



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