60 



lamellar side, opens into tlie true cavity of the principal 

 filament at one edge and into the longitudinal vessel 

 running down the free margin of the expansion at 

 the other. This brings us to the changes at the ctenidial 

 axis, bringing about the attachment of all the filaments 

 (text-fig. 2, p. 41). The lamellae are most plicate at this 

 level, and the filaments closely crowded together. The 

 first change is a permanent fusion of the ordinary 

 filaments at the apex of the plicae, i.e. at the part of the 

 fold most remote from the interlamellar side. This 

 fusion is due to a development of connective tissue on 

 the sides of the filaments near their interlamellar 

 margins, that is, in the position where ciliated discs are 

 found. The epithelium of the interlamellar margins of 

 the filaments thus becomes continuous and cut off from 

 that of the frontal surface. By further fusion of the 

 filaments they eventually all become continuous ; the 

 principal filament also takes part in this fusion, so that 

 there results a plicate lamella having the shape of the 

 former plicae, but made up of organically connected 

 filaments, traces of which are still seen owing to the 

 epithelium on the frontal surface dipping into every former 

 interfilamental gap. The epithelial walls of the principal 

 filaments have become separated by a larger interval from 

 the chitinous endoskeleton, that of the frontal surface 

 becomes continuous with the epithelium of the froutal 

 surfaces of the fused ordinary filaments, and that of the 

 other side with the epithelium of their interlamellar 

 surfaces. 



The next series of sections, taken still nearer to the 

 ctenidial axis, show that the grooves between the 

 principal filaments, that is, the deep grooves which open 

 to the interlamellar side, become reduced in depth oM'ing 

 to the fact that the development of connective tissue 



