524 
BULLETIN OF THE BUREAU OF FISHERIES 
acteristics of the mussel blood varied over wider ranges than those for dog or human 
blood. The fresh-water mussel, however, in a physical sense at least, is in more 
intimate contact with its environment than many other animals, and the oppor- 
tunities for the modification of the blood of the mussel are perhaps correspondingly 
greater. 
When the shell of the mussel is open and the foot extruded, the water in which 
the animal is living has free access to a relatively large surface of soft tissue, and even 
if the foot be retracted and the shell closed a considerable volume of this same water 
is retained between the valves within the shell, where this water still bathes the soft 
parts of the animal. In addition, the mussel pumps through its gill system many 
times its own volume of water in the course of an active day, and although there is 
some opportunity to reject suspended objects of unsuitable size or quality at the 
siphon, because of the innervations of the siphon margins, substances in solution 
and fine material in suspension have full contact with the large surface of the soft 
parts and with the delicate structures of the gill system, as long as the animal pumps 
the water required for respiration and from which it takes its food. The mussel 
may avoid polluted water temporarily by closing its shell, and preliminary experi- 
ments completed by the writers show that these fresh-water mussels can remain 
closed, at 25° C., for 48 hours or more at a time, if the water included in the shell at 
the time of closure were well aerated. However, during the period that the mussel 
is closed it can not move, and therefore it is not able to leave the region of polluted 
water. Even if the polluted water can be tolerated for a time by the open mussel, 
the locomotion of the fresh-water mussel is so laborious and slow that the mussel 
has a much smaller chance of escape than a fish which can swim rapidly to other 
water. The fresh-water mussel therefore both because of the large amount of soft 
tissue in contact with the water, and because of its limited locomotion, is particularly 
exposed to the action of substances in the water. 
From the normals obtained for the blood of the fresh-water mussel, the adapta- 
tion of these animals to the low osmotic pressure of the fresh-water in which they live 
is evident by comparison with the blood values of the marine clams — the nearest 
related forms. In the marine bivalves, the salt balance and adjustments of the blood 
are in accord with the salt content and the osmotic pressure of the sea water, but 
when these marine animals are placed in water of higher or lower salt content than 
that of the sea water in which they normally live (see Kokubo, 1929, on the Japanese 
oyster), changes in the blood follow shortly, and these changes tend to move the pH, 
specific gravity, and salt balance toward the level of the new environment, thus 
tending to equalize the osmotic balance between the animal and its environment. 
In view of these observations on marine forms and the known activities of the 
fresh-water mussels, correlations have been made between the environment, both 
natural and modified, and the condition of the blood of the fresh-water mussels. 
EFFECTS OF CHANGES IN SALT CONTENT OF WATER 
To test the effect of changes in the salt content of the water in which the fresh- 
water mussels were living, both as regards the changes in osmotic pressure and the 
specific action of the salts themselves, on the blood of the mussels, these animals were 
placed in glass jars of about 8 liters capacity, containing solutions of various inorganic 
salts. Fresh-water mussels usually close for a considerable time after being disturbed, 
particularly if transferred to a new environment, and when they open again the 
