Osmotic regulation is the maintenance of a difference between the osmotic pressure of the body fluid of an aquatic organism and the osmotic pressure of the surrounding water. Since not all the species considered in this paper possess such regulation, it would perhaps be preferable to speak of the osmotic "behavior" of these animals, meaning by this the relation, whether regulation or not, between the water and blood osmotic pressures. "Regulation" has been used in the title, however, because of its frequency in the literature. According to whether the osmotic pressure of the body fluid is greater or smaller than that of the water, regulation can be characterized as "hyperosmotic" or "hypoosmotic" Osmotic regulation in aquatic animals in general has been reviewed by Schlieper ( '30, '35) and by Krogh ( '39). The crabs (Brachyura) have been the subjects of a large share of the osmotic behavior investigation among the invertebrates. Their excellence for this purpose is due, in large measure, to the variety of habitats which they frequent. On the Pacific Coast of North America, however, only one species of crab, Pachygrapsus crassipes, has been made the subject of a previous study on osmotic regulation (Baumberger and Olriisted, '28).
The present paper deals with the osmotic behavior of nine species, included in five families of Brachyura. These species, briefly characterized, are : (1) Uca crenulata (Lockington) of the family Ocypodidae, a fiddler-crab which burrows near the high tide mark in the muddy shores of bays ; (2) Pachygrapsus crassipes Randall, of the Grapsidae, an abundant and conspicuous inhabitant of the rocky ocean coast; ( 3 ) Hemigrapsus nudus (Dana), also of the Grapsidae, and likewise found in high locations on the rocky coast, although less active and conspicuous than Pachygrapsus ; (4) Heniigrapsus oregonensis (Dana), characteristic of muddy bays in intertidal and lower situations ; (5) Rhithropanopeus harrisii (Gould) of the Xanthidae, native to the Atlantic 'Now a t the Uiiiversity of Oregon.