882 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
lamellar side, opens into the 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. 8373). The lamellae are most pleate 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, 1.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 dises 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 frontal 
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 owing 
to the fact that the development of connective tissue 
