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BULLETIN OF THE BUREAU OF FISHERIES. 
It will be noted that the blood which goes to the mantle passes through one capillary 
system before returning to the heart, that to the adductor and mantle gland passes 
through two, while that to the body proper passes through three, viz, those of the body, 
kidney, and gill. In this respect as well as in general plan the circulation seems to be 
similar in many lamellibranchs. The walls of the veins in general are not so well 
defined as those of the arteries. When a starch mass which had been strained through 
fine bolting cloth was injected into the veins, it soon spread out among the tissues, show- 
ing that the blood is not confined in a closed system of vessels. The arteries, however, 
divide into extremely small branches, so that it was impossible to force the injection 
mass to their ends. 
ADDUCTOR MUSCLES. 
There are two adductor muscles, the anterior of which is small and practically use- 
less so far as could be made out. The posterior adductor is lar>ge and powerful and is 
situated near the middle of the shell. It is composed of two distinct kinds of fibers as 
indicated by a difference in color. The ventral two-thirds is darker than the dorsal 
one-third. No attempt was made to distinguish physiological differences in these parts, 
but various opinions have been given. Von Jhering (15) experimented upon Pecten to 
determine the difference in function of the two kinds of fibers. He cut the dark portion 
and found that the remaining white portion contracted only very slowly, but it pre- 
vented the valves from opening widely under the influence of the hinge ligament. He 
next cut the white portion of another specimen and found that the remaining dark por- 
tion was capable of very rapid contraction, but it could not hold the valves closed for 
any considerable time. It also allowed the valves to gape widely. He therefore con- 
cluded that it is the function of the white portion to keep the valves from gaping widely 
and to hold them closed for a long time when occasion demands. The dark portion, 
according to his view, is the real muscle to which the contractions are due. J. I v . Kel- 
logg (7) holds the opposite opinion, that it is the white portion which is contractile, and 
that the dark part is for the purpose of keeping the valves tightly closed. 
In Atrina the mantle muscles are white and are capable of rapid contraction, while 
the anterior adductor and foot retractor muscles, which are also white, scarcely con- 
tract at all. The white muscle fibers of lamellibranchs may therefore be quick or slug- 
gish in their contractions. Von Jhering is the only investigator who has isolated the 
two kinds of fibers to test them, and his results seem conclusive, although Pelseneer (n) 
and Kellogg have opposed his view. 
RETRACTOR MUSCLES OF THE FOOT. 
There are two pairs of foot retractors, one posterior and the other anterior. The 
latter is so situated that the foot would be extended by its contraction rather than 
retracted, but it is customary to call this pair of muscles retractors. As a matter of 
fact none of these muscles has any considerable power of contraction. Their function 
seems rather to be to support the body. They suspend it in the manner of a hammock. 
