510 
BULLETIN OF THE BUREAU OF FISHERIES 
which is extruded between the valves of the shell, may be expanded during activity 
to many times its retracted volume by an inflowing of blood which can be held in the 
foot. A considerable volume of blood is required for this procedure and provision 
must be made for this fluid when the foot is retracted into the shell. Consequently 
the volume of blood in proportion to the size of the animal is large, and there are 
numerous sinuses — -that is, reservoirs for blood — in various parts of the body. (See 
figs. 1, 2, 6, 7, and 8.) 
Although no data are available for the exact total volume of the blood of fresh- 
water mussels, Weinland (1919) gives some figures on the relative weights of the shell, 
the soft parts, and the body fluid of the European fresh-water mussel, Anodonta 
cygnea, which are suggestive in this connection. He describes the fluid as that which 
could be drained or easily pressed from the soft parts of the animal, so his figures do 
not apply to blood alone, nor do they necessarily represent all of the blood; but the 
major portion of this fluid was blood. He found this body fluid as described to con- 
stitute from 46 to 57 per cent of the total weight of the animal, including the shell, or 
almost two and one-half times the weight of the drained soft parts. 
In view of the large volume of blood in a fresh-water mussel and the importance 
of this fluid not only in the general metabolism but in the locomotion of the animal, 
the blood has been used as a starting point for the physiological studies of fresh-water 
mussels, as the blood is known to reflect the condition of the individual in both health 
and disease in the higher animals. 
NORMAL BLOOD OF FRESH-WATER MUSSELS 
METHOD OF TAKING SAMPLES 
As the first task in these studies was the determination of the normal values for 
fresh-water mussel blood from which deviation as produced by various factors could 
be observed, only vigorous individuals taken directly from the water were used unless 
otherwise stated. All animals failing to give prompt and strong contractions of the 
pedal muscle and mantle margin, and in which the heart was not beating regularly 
when the valves were opened, were rejected from these groups of normals. 
In taking blood for analyses care was used to avoid dilution with the water con- 
tained in the gill cavities of the animal. The two shells were opened gently with 
mussel tongs (Coker, Shira, Clark, and Howard, 1921), and the water allowed to 
drain out of the gill and mantle cavities. The two large muscles holding the valves 
together were then cut and one valve turned back, the exposed gill and mantle on that 
side being cut away. This procedure left the pericardial cavity intact but easily 
accessible. The animal was again drained free of any water or blood which might 
have accumulated in the mantle cavity during the operation. The pericardial cavity 
was then opened with a pair of small iridectomy scissors and the blood taken directly 
from the pulsating heart. It was found in actual practice that in an animal opened 
in this manner blood rapidly accumulated between the uninjured mantle and the 
shell, and several cubic centimeters of blood could be extracted by making a small 
opening near the center of the mantle on the uninjured side. Tests of the blood 
accumulating in this mantle pocket showed that, if taken immediately, this blood 
did not differ in composition from blood drawn directly from the heart, and con- 
sequently this blood could be used when large samples were desired. However, as 
most of the determinations required only small samples of blood, the blood was usually 
taken directly from the heart as described above. 
