128 THE LIFE OF THE MOLLUSCA 



as the right (originally left) ctenidium becomes 

 aborted in the higher Rhipidoglossa, and disappears 

 in the rest, as the result of the general torsion of the 

 body, so the corresponding auricle diminishes and 

 disappears also. The simplification of the heart in 

 this case, therefore, is not due to any progressive 

 development from a less to a more perfect condition. 

 The respiratory system supplies some very inter- 

 esting points. There is every indication that the 

 primitive gill of the Mollusca must have consisted 

 of at least a pair of very simple plume-like structures, 

 and that as increased facilities for respiration were 

 required, which, of course, implied increase of gill- 

 surface, it could only be obtained in one of two 

 ways : the flattening out into a leaf-like expansion of 

 the individual gill-filaments (aspidobranch), or their 

 prolongation (pectinibranch). The former modifica- 

 tion is the one that appears in all the archaic 

 members of the different classes, and may be recog- 

 nized in the Polyplacophora, the rhipidoglossate 

 Gastropoda, the protobranchiate Pelecypoda, and in 

 the Cephalopoda. This structure, nevertheless, is 

 limited by the confined space of the pallial cavity, 

 and further increase of surface can only be gained 

 by the corrugation of the gill-filament. A beginning 

 of such plication is observable in the case of Pleuro- 

 tomaria, and doubtless it exists in other Aspido- 

 branchs as well ; but it is carried to a much greater 

 degree in the Cephalopoda, in which the gill-fila- 



