630 JAMES L. KELLOGG 
food to the mouth, and the removal of material when it has 
become too abundant. Nowhere, perhaps, are cilia tracts to 
be seen more clearly than in the immense Washington or gaper 
clam, Schizotherus, of Puget Sound. Fullgrown individuals of 
this species average about two pounds in weight, and certain 
ones have been found that considerably exceeded this weight. 
Schizotherus is particularly favorable for a demonstration be- 
cause the palps, bearing the most important and complicated 
of the ciliary mechanisms, are relatively and actually of im- 
mense size. 
Visceral fold. The creature is peculiar in the possession of an 
immense vascular fold which arises from the visceral mass (vf, 
fig. 1), a structure not previously described, so far as I know. Its 
base, or line of origin, is very broad, and extends from a point 
far forward on the side of the body, around posteriorly to a simi- 
lar point on the other side. As shown in figure 1, it is connected 
for a great distance with the dorsal margin (dm) of the posterior 
or inner palp (pp). That it is a posterior extension of the palps 
themselves, cannot be determined without embryological evi- 
dence. In some species, as Zirfaea (fig. 55), Pholadidea (fig. 53), 
Barnea (fig. 59), and others, there is a more or less extensive 
attachment between the upper margin of the palp and the vis- 
ceral mass, and it is conceivable that membranes formed by such 
an attachment might be extended entirely around the visceral 
mass, giving rise to an organ like this found in Schizotherus. 
But there is no justification for an opinion in the comparison, 
and it may be that this curious organ has had an independent 
origin, and has been secondarily united with the palps. 
As to function, the organ, as shown in figure 1, has developed 
cilia tracts used in collecting material which, anteriorly, is passed 
on to the palp surfaces, ultimately, if very small in amount, 
reaching the mouth. Posteriorly, collections are discharged from 
its free margin into the water filling the mantle chamber, and are 
subsequently removed from the body. Its inner surface seems 
not to be ciliated. The organ is thin walled and contains a very 
large quantity of blood, and being immersed in water, this must 
become oxygenated. But it seems very doubtful if ciliary or 
