OSMOTIC AND IONIC REGULATION 237 



ductivity of the external medium, when crabs with blood concentrated 

 by evaporation in air were replaced in the medium. But what part, if any, 

 is played by drinking has still to be discovered. 



Prosser et al. (1955) find that the antennal secretion of P. crassipes 

 is slightly hypo-osmotic to the blood in 50% sea water in which the animal 

 maintains hyperosmotic regulation. In 170% sea water the crabs show 

 hypo-osmotic regulation and the urine is isosmotic with the blood. At the 

 lower salinity the antennal gland secretes magnesium at a concentration 

 four times that of the blood, but in 170% sea water the secretory activity 

 becomes so great that a concentration of ten times is attained. Although 

 the blood concentration of sodium increases with increasing salinity of the 

 medium, the level of sodium in the secretion actually decreases concomit- 

 antly. The authors therefore conclude that increased active outward trans- 

 port of magnesium by the antennal glands in some way reduces the ex- 

 cretion of sodium. This reduction in sodium is probably necessary for the 

 maintenance of cation-anion balance. 



Ocypode albicans burrows in sandy beaches near or above the high tide 

 level. It seems to be very homoiosmotic, judging from the chloride con- 

 tent of the blood (Flemister and Flemister, 1951). In this species the 

 authors found very different chloride values in the antennal gland secre- 

 tion, which follow those of the environment but at a higher level, and seem 

 to suggest that the secretion may be hyperosmotic in concentrated sea 

 water. The urine-blood chloride differences would not be as large if the 

 concentrations were compared on a water-content basis, and if plasma were 

 used instead of whole blood. 



The coconut crab Birgus latro, an anomuran like the hermit crabs, is 

 even better adapted to terrestrial conditions (Gross, 1955). If given the 

 choice of both fresh and sea water, the concentration of its blood is about 

 85-92% of that of the sea water, with extremes of 64% and 123% when 

 fresh water or sea water is available alone. Water can be drunk from small 

 puddles by being passed by chelipeds and maxillipeds to the other mouth 

 parts, and the branchial chamber can also be kept moist by means of water 

 passed to it by the appendages. 



The isopod Ligia oceanica studied by Parry (1953) lives in crevices 

 about high tide level, and feeds mainly on seaweed detritus. It survives in 

 aerated sea w^ater, the freezing point of the blood being kept fairly con- 

 stant in 50-100% sea water. The blood of animals kept in atmospheres of 

 lowered humidity becomes concentrated, the integument being apparently 

 fairly permeable to water. Parry considers it to be a marine animal which 

 is able to tolerate internal osmotic changes, without any special mechanisms 

 which would enable it to become a truly terrestrial animal. Considerable 

 ionic regulation is found in the blood, where concentrations of magnesium 



