Water 



21 



4. Highly regulating crabs which not only maintain a state of hypertonicity 

 in dilute sea water but also regulate in a medium more concentrated than 

 normal sea water by keeping their blood more dilute than the medium. They 

 may live in very dilute brackish water and many go onto land. Such regula- 

 tion in both high and low concentrations is shown by Ilea crcniilatn, llelue- 

 cius, and Palaemonetes. 



5. Animals with ability to maintain hypotonicity in concentrated and hyper- 

 tonicity in dilute media to such an extent that they can live indcfmitciy in 

 either the ocean or pure fresh water. For example, Eriocheir breeds in the 

 ocean but migrates up European rivers where it grows to maturity. 



6. Strictly fresh-water crustaceans which do not return to the ocean. Exam- 

 ples: crayfishes and fresh-water microcrustacea. 



7. Animals adapted to salinities greater than that of the ocean. E.xample: 

 the brine shrimp, Artemia. 



No Osmoregulation: Marine Crabs. Maja is a marine spider crab which 

 survives only a few hours in sea water diluted by more than about 20 per 



32 



HOURS 



Fig. 9. Percentage increase in weight in crabs transferred to dilute sea water at zero 

 hour. O. Maja in 75 per cent sea water, ▲, Carciniis in 75 per cent sea water (Schwabe 

 -"); •, Portunas in 66 per cent sea water; (§), Cancer in 66 per cent sea water (Hukada 



-). 



cent. --- Maja araneiis, Maja hnfo, and Hyas araneiis are usually taken at 

 an ocean depth of 15 to 100 fathoms. -^^' 



Maja placed in diluted (80 per cent) sea v\atcr first swelled but within 3 

 hours the weight decreased, owing to loss of salt and accompanying water 

 (Fig. 9). --- By the time the weight was back to normal the concentration of 

 the blood was the same as that of the medium (Fig. 12). The rate of water 

 intake was initially greater than the rate of salt loss. Con\ersely, when Maja 

 was put into a more concentrated medium, salt was taken up and the body 

 fluids again became adjusted to the external concentration (Fig. 11). -^^ 

 Maja rapidly comes into isotonic equilibrium with a dilute medium (Fig. 10, 

 curves C and D). 



