VII 



MOLLUSC A -RESPIRATORY ORGANS 



85 



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

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



FIG. SI. Ctenidia of various Molluscs (after Ray Lankester). A, Chiton; B, Sepia ; C, 

 Fissurella; D, Nucula; E, Paludina. ft, Longitudinal branchial muscle; abv, afferent branchial 

 vessel ; ebv, efferent branchial vessel (branchial vein) ; gl, paired lamellss (leaflets) of the feathered 

 gill ; in D : d, position of the axis ; a, inner ; 5 and c, outer rows of branchial lamellse ; in E : i, 

 rectum ; 6r, branchial filaments ; , anus. 



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

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

 in detail later. 



