EVOLUTION 129 



merits 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 Scissurella. 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 

 9 



