CILIARY MECHANISMS OF LAMELLIBRANCHS 655 
ventral margins, and more fully in figure 23. With palps in their 
natural positions, material may be taken from the demibranchs 
by the palp folds before reaching their anterior ends. When it 
does reach the ends, however, it is taken off on to the groove 
between palps, as shown in figure 19. 
In each species close to the line where one palp fold overlies 
another, but apparently not deep in the groove, there is a dor- 
sally directed stream. Streams to the ventral margin, deep in 
the grooves, have not been observed, but very likely they are 
present. 
Visceral mass. Material collected on its walls is dropped into 
the mantle chamber from the ventral wall behind the base of the 
byssus (fig. 18 b), but not anterior to the foot. This last men- 
tioned organ is not cilated in any full grown bivalve. 
Mantle. The mantle edges are free, and there is no sharply 
defined incurrent siphon opening. There is, however, a large 
siphon membrane (fig. 18 sm). Extensive folds, lke those 
bounding the waste canals of Schizotherus and many other forms, 
are developed parallel to the mantle edge, but there is no waste 
canal, the mantle collections, instead of being carried ventral 
to and between them, are carried backward dorsal to their bases 
and, without halting to collect in masses, leave the mantle cham- 
ber directly posterior to the siphon membrane. Because the 
incurrent siphon is not an enclosed tube, probably these mantle 
folds tend to shield the waste materials from the broad incoming 
Fig. 20 Mytilimeria nuttalli. 
Fig. 21 Mytilimeria nuttallii; ventral view; fo, foot opening of mantle; so, 
supplementary mantle opening. 
Fig. 22 Mytilimeria nuttallii; view of right side; b, bay of mantle; es, ex- 
current tube of siphon; f, foot; 7g, inner demibranch of gill; 7s, incurrent tube of 
siphon; og, outer demibranch of gill; pp, posterior palp. 
Fig. 23 Mytilimeria nuttallii; mouth and palps seen from below; ap, anterior 
palps; dm, dorsal margin of palp; zg, inner demibranch of gill; mo, mouth; pg, 
proximal oral groove; pp, posterior palps; vm, ventral margin of palp. 
Fig. 24 Mytilimeria nuttallii; detail of palp ciliation; x, separation of folds 
to expose deep cilia tracts; other letters as in figure 23. 
Fig. 25 Mytilimeria nuttallii; ciliation of visceral mass, vms. 
Fig. 26 Mytilimeria nuttallii; ciliation of mantle edges; b, bay of mantle; 
g, posterior ends of gills; 7s, opening of incurrent tube of siphon; m, mantle wall; 
so, supplementary mantle opening. 
