236 INVERTEBRATE PHYSIOLOGY 



and larvae die in diluted sea water in which the adults can survive indefi- 

 nitely in the laboratory (Fox, 1941). 



Potts (1954c) has recently made some interesting calculations of the 

 thermodynamic work performed in osmotic regulation in brackish- and 

 fresh-water animals. He shows that the lowering of the blood concentra- 

 tion of animals in brackish water is the chief means of easing the strain 

 on osmoregulatory mechanisms, the production of a hypo-osmotic urine 

 giving little advantage to the animals until the medium falls well below 

 50% sea water. In fresh-water animals, however, hypo-osmotic urine 

 can reduce osmotic work by 80-90%, and be of significant value even 

 when many times more concentrated than the external medium, since 

 most of the benefit is secured in the early stages of reducing the urine con- 

 centration, not when the concentration is approaching that of the 

 medium. 



Applying his equations to data for Eriocheir in fresh water. Potts calcu- 

 lates that the osmotic work is 0.176 cal./hr. for a crab of 60 gm., about 

 1.3% of the total metabolic energy calculated from the oxygen consump- 

 tion (14 cal./hr.). If the crab had been able to produce urine as hypo- 

 osmotic as the external medium (instead of being actually isosmotic with 

 the blood), the osmotic work would have been 0.0375 cal./hr. If the crab 

 had maintained in fresh water the high concentration found in its blood 

 when in sea water, the value would have been 0.725 cal./hr. 



Similar calculations for Anodonta suggest 0.0145 cal./hr. for the total 

 osmotic work, of which 0.0131 is done at the body surface and 0.0014 

 (about 10% of the total) at the excretory organ. The total work 

 constitutes 1.2% of the total metabolism (1.2 cal./hr. for a 60-gm. 

 mussel). 



Well-marked powers of osmotic regulation are shown by crabs with a 

 semiterrestrial habitat, and Jones (1941) found that the most homoios- 

 motic of a series he studied was the burrowing fiddler crab, Uca crenulata. 

 This crab maintained a relatively low blood concentration when exposed 

 to air for 12 hours, although the water of the branchial chamber had con- 

 centrated and was about 20% above the concentration in the blood. In 

 Pachygrapsus crassipes, Gross (1955) found that in air salts were ab- 

 sorbed from the branchial tissues, but that the contribution they made to 

 the increase in salinity of the blood was relatively small. The mechanism 

 by which a crab like Pachygrapsus can maintain hypo-osmoticity in ocean 

 water or more concentrated water is obscure. The activity of the antennal 

 glands requires the absorption of water against a gradient to replace that 

 lost in the antennal secretion, and the latter is, according to Jones ( 1941 ), 

 isosmotic with the blood at all dilutions of the external medium. Such 

 active absorption has been shown by Gross (1955) by measuring the con- 



