BLOOD OP FRESH-WATER MUSSELS 
523 
When foot strips were mounted in glass containers through which well aerated 
unionid ringers flowed continuously, they maintained the typical rhythmical con- 
tractions characteristic of the foot muscle when in place in the body of the mussel. 
These contractions continued for hours and gave ample opportunity to observe the 
action of the various blood salts of the mussel blood. (See figs. 5, 6, 7, and 8.) 
A summary of these tests will suffice here. The rhythmic activity of the foot 
strip stopped very quickly when the strip was transferred to distilled water, to tap 
water, or even river water; that is, the small quantities of salts present in the mussel 
blood are essential to the activity of the foot tissue. Activity of the foot strips also 
failed rapidly and finally ceased if the strips were placed in a solution of sodium 
chloride of the same strength as the sodium chloride in the blood or the unionid 
ringers, but without the small quantities of potassium, calcium, and magnesium salts 
found in the mussel blood. It was evident that these other salts, even though present 
in very small amounts, exercise a regulatory action over the activities of the foot 
muscle and that sodium chloride alone will not maintain life activities in the mussel. 
This is similar to the results obtained in experimental studies of other animals. 
Potassium, calcium, and magnesium salts alone, and in concentrations found the 
mussel blood, also failed to maintain foot-strip activity. If the balance between 
calcium and potassium were disturbed — that is, if more or less potassium or calcium 
were used in proportion to the opposing salt — the activity of the foot strip was 
quickly disturbed. Excess or unbalanced potassium caused cessation of activity, 
the strip passing into a condition of rigor; and excess or unbalanced calcium causing 
cessation of activity accompanied by great loss of tone and relaxation. The limits 
between which the potassium content of the fluid could be varied were much nar- 
rower than those through which calcium variation was tolerated. This is in accord 
both with the analyses, and with the findings connected with the use of calcium as a 
buffer by the mussel. 
Taken collectively, these tests with the foot strips show that the mussel is 
dependent upon the salts in the blood for the same types of activity regulation as 
those found in the higher animals and that although these salts are in very low con- 
centrations in the mussel blood, those concentrations and the balances between the 
several salts can not be greatly changed without the cessation of activity and other 
serious consequences. These activity experiments, therefore, validate the analyses 
of the blood by giving physiological proof that salts are required in the concentrations 
and proportions determined. As a supplementary check to these tests, the entire 
heart of the mussel was carefully removed in several cases and mounted in unionid 
ringers, in which fluid it continued to beat regularly for several hours. Both heart 
and foot strips were very sensitive to oxygen want and activity ceased almost imme- 
diately, no matter what the concentration of the surrounding fluid, if the oxygen 
supply were shut off. 
CHANGES IN BLOOD OF LIVING MUSSELS INDUCED BY 
ENVIRONMENTAL FACTORS 
It was noted repeatedly while collecting the data for the normal values of the 
mussel blood that the physical factors of the environment influenced to some extent 
the values obtained. It was necessary, therefore, to select as normals only those 
animals which had just been removed from the water and which seemed to be in good 
condition. Nevertheless, even with these precautions, the constituents and char- 
15392—31 3 
