392 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
face, though this is very doubtful. These striated fibers (smf) ramify, from their 
attachment to this membrane, in toward the center of the auricle. They are generally 
in irregular bundles, though not in the sense that the fibers are closely bound to one 
another. The fibers are very long. 
Scattered among them are many single pigment cells (pgc), which give the auricle 
its brown color. Also surrounding the fibers are many cells, apparently of an epi- 
thelial nature, which seem to be giving off vacuolated ends (v) into irregularly formed 
spaces, thus having very much the same appearance as the secreting cells of the 
nephridium. Their nature needs further study. 
The functions performed by these two portions of the adductor mentioned above 
are not well understood. Yon Jhering (No. 23) thinks that the part with the plain 
fibers is used simply as a means of keeping the valves from spreading too far apart 
on account of the action of the shell ligament. The same view is held by Lankester 
(No. 81, who does not say, however, which portion he believes to exercise this function. 
In opposing the view of Yon Jhering, Pelseneer (No. 17) says that in the Pholadidce, 
which have no ligament between the valves of the shell, the adductors are formed 
entirely of fibers which have no appearance of striation. He thinks that it is proba- 
ble that, when these two parts exist, “la partie a apparence stride des adducteurs 
sert, comme chez les autres invertebres, a produire des contractions rapides.” 
Although I had made very few observations on the subject, I had come to a dif- 
ferent conclusion from any of these. It seems to me that the fibers of the darker por- 
tion of the adductor muscles are more compact and firm, and probably supply the 
greater part of the force required in keeping the shell closed. The fibers of the lighter 
portion, not being packed so closely together, are able to contract more quickly and 
close the shell, it may be against a sudden attack. If a valve of Pecten be removed, 
the smaller darker area will be seen to preserve an extremely contracted condition, 
while the large white part, also partly contracted, now and then makes very sudden 
and violent contractions, and then immediately relaxes somewhat. These contractions 
can be made to occur by striking or cutting almost any part of the body besides the 
adductor, but more especially the mantle edge. The contractions also occur from time 
to time if the animal is undisturbed. It seems then that a sudden closing of the shell, 
so often necessary to lamellibranchs, is accomplished by the lighter portion, and that 
the darker part comes more actively into play when the shell is to be closed for some 
little time. The lighter portion in Pecten is relatively very greatly developed, and, as 
the very large size of the adductor has been brought about partly for locomotion by its 
extremely rapid contraction, the lighter part is the one which performs this function. 
Though other forms do not have this method of locomotion, the manner of contraction 
of the white and dark fibers may be the same. 
The two pairs of foot-retractor muscles are of general occurrence, except in forms 
with an aborted or absent foot, and are well seen in Venus. They are attached to the 
shell close above the adductors (Fig. 96, PI. xciv, afr and pfr), and join the anterior 
and posterior parts of the foot, respectively. The anterior ones, which pass obliquely 
backward from their attachment, are shown cut across at ar, Fig. 11, PI. lxxxi, which 
is a vertical section in the region of the anterior end of the stomach. In Fig. 12, a little 
farther back, they are cut more nearly longitudinally and show their final union with 
one another and the foot, /. The same relations are shown for the posterior retractors 
in Venus in Fig. 17, PL lxxxii, at pr. 
