SEA-FISHERIES LABORATORY. 379 
termed the respiratory expansion, for it is probably here 
that most of the respiration is performed in the gills. 
Tiiceeties. 20, 24, 25) is formed of two bounding 
layers, a single cell thick, of epithelial cells, continuations 
of the sides of the principal filament. The space between 
them is bridged across at numerous intervals by one or 
two cells, serving to keep the two walls a definite distance 
apart. At the free margin of the expansion the two walls 
diverge to form a tube, circular in transverse section 
(fie. 23, 20, Br.v.), which is the channel for the blood. 
The free margin of the expansion is attached some 
distance up the side of the ctenidial axis, and the vessel at 
its edge becomes directly connected with the afferent 
vessel of the gills (fig. 45, Br. v. and Br. aff.). 
The structure of this expansion is more peculiar than 
it appears from transverse sections, and should be 
examined in surface view, the whole filament being 
stained and mounted in Canada Balsam. It then has a 
folded appearance, as if there were a series of pockets on 
the sides of a plane surface (fig. 23). A section parallel 
with the plane of the ctenidial axis shows that this 
appearance of pockets is simply due to an extensive 
folding having taken place, as if the greater part of the 
expansion had, through increased growth, doubled and 
re-doubled on itself whilst the free margin remained 
straight (fig. 24). Cilia are to be found on the free outer 
edge of the expansion, that is on the vessel itself, but do 
not appear elsewhere. 
It will be best now to indicate the changes in detail 
in the various parts of the lamellae, and the mode of 
attachment of the filaments to the gill axis. 
Commencing with sections through the ventral edge 
of the filaments, where they are reflected, there is a row 
of ciliated discs extending along the whole length of the 
