234 INVERTEBRATE PHYSIOLOGY 



Following up the work of Baumberger and Olmsted (1928) who found 

 increases at molt in the freezing point of the blood and tissues of Pachy- 

 grapsus crassipes, a crab showing hypo-osmotic regulation in sea water, 

 I have found that the total ionic concentration of Carcinus increases in 

 the premolt phase and falls again after molt. Expressed as percentage of 

 the sea-water concentrations, the total mg. ions in the plasma rise from 

 100.9 in the intermolt crabs to 107.5 in the premolt phase, falling to 102.9 

 within 24 hours of molting owing to the absorption of sea water by the 

 gut. Thereafter, a further fall to 98.2 takes place within the next fortnight. 

 Baumberger and Olmsted did not investigate the mechanism of the in- 

 crease in osmotic pressure. In Carcinus there is an increase in all the ions 

 except sulphate, but how this takes place is not clear. 



Characteristic changes in the calcium content of the plasma are found 

 in Carcinus at dififerent stages of the molt. Taking sea-water values as 

 100, intermolt crabs have a mean calcium content of 130 (range 113-160) . 

 In the premolt phase the mean is 163 (range 140-189), the increase being 

 correlated with resorption of salts from the old skeleton. A fall to 120 

 (range 103-134) takes place immediately after molt, presumably owing 

 to dilution of the blood with sea water. After two days the mean figure 

 falls to 88 (range 66-105). At this stage and for several weeks after molt, 

 calcium is being withdrawn from sea water, and it is possible that a good 

 part of this takes place by inward diffusion according to the concentration 

 gradient. In this postmolt phase calcium is probably being withdrawn by 

 the epidermis from the plasma to be deposited as calcium carbonate and, 

 to a small extent, calcium phosphate in the integument. This continuous 

 withdrawal reduces the calcium content of the plasma, enabling con- 

 tinuous absorption from the external medium to take place. 



Invertebrates in Brackish and Fresh Water, and in 

 Semiterrestrial Habitats 



Active uptake of salts in crustaceans is considered to take place through 

 the gills, but it is often difficult in experiments to eliminate the complica- 

 tions of simultaneous output of salts from antennal glands and the func- 

 tioning of the gut. 



An advance has been made by the recent finding that the isolated gills 

 of the grapsoid crab Eriocheir sinensis are still able to absorb sodium 

 chloride from a solution of 8 niM/liter, about 1/30-1/40 of the concen- 

 tration in the blood (Koch et al., 1954; Koch, 1954). The absorption 

 stops in the absence of oxygen, and is reversibly inhibited by small con- 

 centrations of carbon dioxide, potassium cyanide, sodium azide, and sul- 

 phide, and irreversibly inhibited by silver, lead, and mercury salts. These 

 gills, which are of relatively homogeneous histological composition, are 



