CILIARY MECHANISMS OF LAMELLIBRANCHS 693 
several organs bearing ciliated surfaces. In figure 67 the 
reference letters ap are placed on the anterior or outer palp, 
the edge of the posterior, which it overlies, appearing beyond 
its back margin. Arising from about the middle of the dorsal 
margin of the outer palp, is a long appendage (pa) which may be 
extended beyond the shell margin for a distance greater than the 
length of the shell. Above and behind the palps hang the gills 
(g) that are suspended from above by a membrane (gm). It is 
with the relative positions of gills and palps that we are here con- 
cerned. The two palps, on either side of the body, are united 
along their dorsal edges, upward from the point where the ap- 
pendage arises from the anterior or outer, and along this line of 
union is a deep groove, an extension of the lateral oral groove, 
which lies hidden between the palps below the region of the vis- 
ceral mass (vms). This extension of the groove lies beneath the 
reference letters lg in figure 67 and is shown in figure 70, at lg. 
It is often placed in such a position that it touches the lower 
surface of the anterior end of the gill, though in the first figure, 
the gill is raised somewhat above it, and in the second (fig. 70) 
this entire dorsal region of the palps is pulled downward, their 
free ends being spread apart in order to expose all of their cili- 
ated surfaces. In its normal position, then, this extension of 
the oral groove (lg) lies against, or close to, the ventral surface 
of the gill, the reason being, as will be shown, that, from time to 
time it receives the gill collections. 
The anatomy of the gill, first described in detail by the writer, 
is well known, as it has been studied by several observers, but 
one important structural modification has been overlooked. 
I know of but one brief reference to it—that found in Drew’s 
paper on Yoldia (’99) the reference being :—‘‘ With the exception 
Fig. 70 Yoldia limatula; detail of palp ciliation; dm, dm, dorsal margins of 
palps; g, gill; gm, suspending membrane of gill; 1g, extension of lateral oral groove; 
paa, palp appendage; vm, ventral margin of palp; vms, visceral mass; x, notch in 
groove passing to mouth. 
Fig. 71 Yoldia limatula; ciliation of outer surface of palp appendage. 
Fig. 72 Yoldia limatula; ciliation of mouth region; ap, anterior palp; lg, 
lateral oral groove; m, mantle; mo, mouth; pg, proximal oral groove; pp, posterior 
palp; vm, vm, ventral margins of palp; vms, visceral mass. 
