370 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
THE GILLS. 
The Gills or Ctenidia (PI. II., fig. 1) are conspicuous 
in the palhal cavity when the valves are opened, and 
extend like orange tinted curtains on each side of the 
visceral mass, with their free edges reaching from the 
labial palps to a point opposite the end of the Reetum. 
At first sight there appear to be two gills on each side of 
the body, but the morphological identity of the lamelli- 
branch g@ills with the ctenidia of other mollusca is now 
fully established, and there is but one ctenidium on each 
side. These two apparent gills are two plates (double 
for the greater part of their area), formed by a series of 
filaments loosely attached to one another. The two 
opposite and innermost lamellae meet, fuse, and become 
continuous with a supporting ridge at their proximal 
edges, but their lower or distal edges are reflected so that 
in section each gill has the appearance of a W, the two 
outside limbs being the reflected portions, and only two- 
thirds the height of the middle ones by which the 
ctenidium is attached to the body. The transverse 
section is diagrammatically represented in fig. 20. Mach 
of these double plates is known as a demibranch so that 
there are two demibranchs on each side, an inner and an 
outer, which together make up a ctenidium. 
Kach Ctenidium consists of a supporting axis or 
ridge (fig. 20, Br. av.) from which depend two regular 
series of long delicate filaments. These two series form 
the two direct or descending lamellae, this part of the 
filaments being known as the descending filament (fig. 20, 
Br. d.). The lower ends of the filaments are reflected as 
previously seen (fig. 20), on the outer side of the external 
demibranch and the inner side of the internal dem1- 
branch, and the ascending or reflected portions of the 
