CILIARY MECHANISMS OF LAMELLIBRANCHS 675 
occurs In about a second of time. The whole demibranch, also, 
may present a wavy surface, and sway, fanwise, toward the man- 
tle and inward—apparently an adaptive movement, for material 
is sometimes directly transported from the outer demibranch 
to the mantle surface, and in any case, tends to be shaken off 
into the mantle chamber, so rapid is the swaying movement. 
Extensive movements of the gill of Yoldia have been described 
by Drew (’99) and the writer (90) in which organs there are 
well developed muscles; but in the Pecten gill and others, 
also capable of extensive movements, such muscles are absent. 
Large quantities of material on the palps also cause the mouth 
to close tightly. 
Suspending membrane. Currents on the suspending membrane 
in P. tenuicostatus are shown in figure 42. On the opposite 
side of the membrane the currents are practically the same, 
both carrying material to the free edge above, where it is cast 
off into the mantle chamber. Between the points x and y 
material is received, on this outer face of the membrane, from 
the under surface of the adductor muscle (a). Curiously enough, 
the muscle wall received this material from the mantle (between 
points x and y in figures 46 and 47). 
Palps. Figure 44 represents the mouth and the highly 
modified palps of P. irradians, the anterior palp (ap) being folded 
forward. The anterior ends of both demibranchs of the left 
gill (g) lie well in between the palps, and the manner in which the 
various gill streams are continued into the oral groove is shown. 
Many times, very small quantities of material were seen to pass 
from gills to oral groove and into the mouth, while larger masses 
were drawn out of the groove and carried diagonally across the 
folds, being conducted to the mantle by the anterior, and to the 
visceral mass by the posterior palps. The margins (on which 
the reference letters are placed) are partially united to the 
mantle, in the case of the anterior, and to the visceral mass in 
the posterior palps, and material on mantle and visceral mass 
near the palps is transferred to them across their margins, and 
back to them again after crossing the folds. The almost uni 
JOURNAL OF MORPHOLOGY, VOL. 26, NO. 4. 
