384 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
but while this is going on as we approach the ctenidial 
axis, the same structure of the opposite demibranch 1s 
drawing very neay to it, until both demibranchs fuse by 
their inner surfaces. The cavities of the ordinary 
filaments, bounded by their chitinous skeleton, elongate 
in a direction at right angles to the plane of the axis, 
until they eventually come in contact with those of the 
opposite demibranch, and thus we have the chitinous 
skeleton of both demibranchs and the cavities of the 
filaments continuous and all opening into the efferent 
vessel of the gill axis. 
This elongation of the cavities of the filaments with 
their skeleton is probably only due to the sections passing 
through obliquely owing to the filaments of both sides 
curving over as the chitinous framework becomes 
continuous. Thus the blood, which has been forced to 
the various parts of the body with the exception of the 
mantle, and has been collected and taken to the renal 
organs (fig. 16, /to.), passes from these on each side by 
the afterent branchial vessels (fig. 45 and fig. 16, Br. aff.), 
and then from these laterally into the vessels running 
down the margins of the branchial expansions of the prin- 
cipal gill filaments. From these vessels it can pass into the 
expansions themselves, the whole of which act, therefore, 
as respiratory surfaces. ‘This brings the blood into the 
principal filaments, from which it passes into the efferent 
branchial vessel running just below the afferent. Since 
the ordinary filaments only open into the efferent vessel, 
the blood that passes through them must be partially 
aerated, and there will in all probability be no definite 
current, but a backwards and forwards motion. It seems 
certain, from the development of vessels in the mantle, 
that the great function of the gills is to produce currents 
of water for aeration, nutrition, and the carrying away 
