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 
aes (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- 
