CILIARY MECHANISMS OF LAMELLIBRANCHS 661 
slender siphon tubes emerge, are bent upward. The tubes of 
the siphon (es and is) are independent of each other through- 
out, and their bases lie in a special sheath-like space of the mantle. 
At the base of the incurrent, a special muscle (mm) is developed 
in the mantle and is attached to both shell valves. The open- 
ing from the cloaca into the excurrent siphon is small and is 
situated on the dorsal side of the tube. Rectum (r) and both 
siphons lie to the right-of the median plane of the body. Lying 
nearly in front of the opening of the incurrent siphon are thick 
folds of the mantle wall (mf) with serrated edges. The function 
of these was not determined. The crystalline style has a cap- 
like structure which fits into a depression of the stomach wall, 
as in Schizotherus. 
Figure 33 shows that collections of the outer surface of the 
outer demibranch (og) are passed on to the surface of the inner 
demibranch, and are borne to its margin, as in several other forms. 
The attachment of the inner lamella of this demibranch to the 
visceral mass is so near the lower margin that the organ may 
hardly be said to hang free. The ciliation of the palps is the 
usual one, and visceral mass currents, instead of being directed 
backward, carry material forward to the dorsal margin of the 
posterior or inner palp. There are narrow tracts on the folded 
regions of the palps, directed both dorsalward and ventralward, 
but from lack of time and facilities when the form was examined, 
their precise situation in reference to folds and grooves could 
not be determined. Mantle currents were of the usual sort. 
So far as these observations have gone, the Macomas are en- 
tirely unique in that the digestive tract, from esophagus to rec- 
tum, is usually packed full of sand. Not only so, but in most 
cases large quantities were found in the mantle chamber of M. 
secta and even the siphon tubes bore much of it. The form 
seems to have the earthworm habit of utilizing digestible material 
that happens to be present in large quantities of ingested soil, 
and it is of most striking interest that the outgoing tracts of 
palps and mantle are as well marked as in other forms; and, 
after cleaning the mantle chamber and using a smaller quantity 
of sand than that originally present, much of this is sent back 
