EVOLUTION 129 
ments exhibit two series of plications crossing one 
another. 
In the Gastropoda the progressive atrophy and 
disappearance of the right (originally left) ctenidium 
as one ascends from lower to higher members has 
already been alluded to (ante, p. 24). The initial 
stage of this is shown in Pleurotomaria, but it is far 
more marked in Scissuvella. In the Pectinibranchia 
not only has one ctenidium disappeared, but the 
other (except in the case of Valvata) has become 
attached by its whole length to the wall of the 
pallial cavity, and as a consequence has parted with 
the whole of the row. of filaments on that side; so 
that three- -quarters of the gill-potentiality of the 
primitive mollusc is sacrificed. By way of partial 
compensation the individual gill-filaments have been 
somewhat lengthened till the familiar pectinibranch 
condition arises. In Ianthina (Plate XXX., Fig. 2) 
these gill-filaments are furthermore plicated. 
The complicated modifications that take place in 
the gills of the Pelecypoda have already been described 
(ante, pp. 28 and 29), since the classification of the 
group is based thereon, and need not be redescribed. 
Many kinds of aquatic molluscs have lost their 
gills altogether, and in these cases the respiratory 
function is taken up by the mantle. This is the 
case with the Common Limpet (Plate IX., Fig. 7); 
the circle of “ gills” surrounding the edge of the 
mantle just under the shell are outgrowths of the 
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