SEA-FISIIERIES LABORATORY. awe 
filaments make up the reflected or ascending lamellae, 
corresponding to the outer limbs of the W in the 
diagram. 
The direct ard reflected lamellae of cne demibranch 
are quite continuous, and the reflection is simply a device 
for increasing the area of the gills without occupying an 
awkwardly large space. The reflected lamella (fig. 20, 
Br.a.) does not reach the axis of the ctenidium, 
neither is 1t connected by cilia or other means with the 
mantle or with the ctenidium of the opposite side. Nor 
is there any connection between the two demibranchs of 
one side except at the axis from which they arise. 
The Ctenidial Axis (fig. 20 and 45, Lr. av.) is a plate- 
like ridge of connective tissue of considerable depth, 
depending from the body wall. On the right side where 
the adductor is inserted nearer to the hinge line than on 
the left, the axis arises not far from the labial palps and 
is attached to the adductor muscle. It continues ventrally, 
gradually increasing in depth, until it passes the point 
where the afferent branchial vessel from the renal organ 
enters it, and just where the palhal nerves from the 
visceral ganglhon enter the mantle. Jere its course is 
diverted and the ctenidial axis leaves the adductor and 
is suspended from the mantle lobe. It terminates at a 
point almost level with the end of the rectum, but its 
posterior extremity does not diminish in depth, the 
filament-bearing margin continues almost parallel to the 
attached edge, and the posterior extremity has a slightly 
bifureated appearance as shown in fig. 1. On the left 
side the ctenidial axis is attached to the adductor muscle 
along its whole length, and does not extend quite so far 
back as that of the right side. 
The axis is made up almost entirely of connective 
tissue, with a good supply of muscle fibres (fig. 45). It is 
