CILIARY MECHANISMS OF LAMELLIBRANCHS 669 
Chama exogyra Conrad 
This extremely interesting attached form was studied at 
San Diego Bay. Figure 39 represents the left side after the 
removal of the flat, lid-lke left valve and the left mantle fold. 
Like other attached species, outline and general form vary in 
conformity with the shapes of objects to which the creatures 
adhere. There has apparently been a contra-clockwise torsion 
of the body resulting in a great shifting of several organs. Usu- 
ally, in forms in which the anteroposterior axis of the visceral 
mass is moved into line with the hinge of the shell, the anterior 
adductor diminishes in size or disappears (Ostrea, ete.) the 
posterior muscle meanwhile increasing in size, since it must 
perform the function of closing the valves. In this case, how- 
ever, the anterior adductor (aa) has moved ventralward and 
backward, in reference to the axis of the visceral mass; the 
posterior (pa) dorsalward and forward, the anterior becoming 
the larger of the two. The rudimentary foot (f) has moved from 
an anterior to an extreme posterior position on the visceral mass, 
and the foot opening on the mantle edge (fo) has been shifted 
backward at the same time. Mouth and palps also have moved 
ventralward to some extent. 
Gills. The ciliation of these organs is much like that of 
Cardium. The general current is downward on the outer lamella 
of the outer demibranch (og), around the edge of the narrow in- 
ner lamella, and forward along its base to the distal oral groove. 
Material is moved on both lamellae of the inner demibranch 
to a groove on the much thickened margin, and forward to the 
lateral oral groove. The relation of this demibranch to the 
lateral oral groove is shown in figure 40. 
Palps. The palps present a condition not found in any other 
form examined, in the reversal of the direction of the streams on 
their ventral margins. The free ends of the organs project so 
far dorsalward that collections discharged from them would be 
in danger of being caught by the gill. Being reversed, the streams 
discharge on to the mantle from the anterior (fig. 40, m) and 
the visceral mass (vms) from the posterior palps. Palp folds 
