47 



termed the respiratory expansion, for it is probably here 

 that most of the respiration is performed in the gills. 

 It (see figs. 23, 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 betweeu 

 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 tAvo walls 

 diverge to form a tube, circular in transverse section 

 (fig. 23, 25, 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 shoTild 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 



