CILIARY MECHANISMS OF LAMELLIBRANCHS 683 
over the distance of a foot—as some of it must be carried—in the 
midst of the rushing stream of the incurrent siphon, would be 
attended by so much uncertainty in an open groove, that com- 
pletely covered passages have been developed on the edges of the 
demibranchs, which assure the delivery of the gill collections to 
the palps. When material from the lamella reaches the closed 
groove, its walls part, admit the collection, and close over it, 
if its volume is not too great. 
Palps. These organs are relatively large, and bear large folds. 
The anterior palp is not shown in figure 54. In figure 55, the 
mantle has been cut ventrally, the right side being lifted to show 
the attachment of the anterior palp to it, as well as the short line 
of union between posterior palp (p) and visceral mass below the 
line of gill attachment (gb). The ciliation of folds and margins 
is beautifully clear, and is found to be essentially like that of the 
Schizotherus palp, except that no dorsally directed currents were 
observed anywhere on the crests of the folds. 
Visceral mass. Posteriorly the visceral mass is extended into 
a point, just below which its collections are cast into the mantle 
chamber, as shown in figure 55. 
Mantle. No case has been observed among lamellibranchs, 
in which the inner walls of the siphon tubes were ciliated. There 
is in Zirfaea a downwardly directed ciliation on the mantle walls 
opposite the visceral mass, that trends posteriorly on its ventral 
surface. There is no bay beneath the siphon base, as in other 
forms, and apparently no point where a collection is made. 
Material simply gathers on the ventral mantle wall, and is no 
doubt discharged through the ventral siphon from time to time, 
as a result of the contraction of the adductor muscles. The 
mantle streams, in the forms examined, were very feeble. 
Barnea costata Lin 
This species, studied on the coast of Louisiana near the mouth 
of the Calcasieu river, is remarkable in having developed an 
organ, the function of which is to extend and perfect the ciliary 
mechanism for freeing the body of undesirable foreign matter. 
