Til 



MOLLUSCA— RESPIRATORY ORGANS 



85 



bilateral symmetry of the body, such as the Lamellihranchia, the 

 Cephalopoda, and — a point of great importance — also in the primitive 



Fig. 81.— Ctenidia of various Molluscs (after Ray Lankester). A, Chiton ; B, Sepia ; C, 

 Fissurella ; D, Nucula ; E, Paludina. ft. Longitudinal branchial muscle ; a&y, afferent branchial 

 vessel ; ehv, efferent branchial vessel (branchial vein) ; fjl, paired lamellas (leaflets) of the feathered 

 gill ; in D : d, position of the axis ; a, inner ; 6 and c, outer rows of branchial lamellas ; in E : i, 

 rectum ; &r, branchial Jilaments ; o, anus. 



Gastropoda, the Zeugohranchia. In the latter, however, the left ctenidium 

 was originally the right and vice versd, but this will be dealt with more 

 in detail later. 



