8o THE BIOLOGY OF THE SEA-SHORE 



that among marine invertebrates the osmotic pressure of the 

 internal fluid and that of the external water are in equilibrium: 

 there is thus isotony or equimolecularity between these two 

 liquids, no matter what the protein content of the blood. 

 Fredericq (1889) finds that the blood of the shore crab 

 {Carcinus mcenas) has a salt content which varies with the 

 degree of salinity of the water in which the animal is living. 

 For instance, in the blood of crabs from Roscoff (sea- water) 

 he found 3*07 per cent, of salts, whereas in that of crabs 

 from the estuary of the Escaut (brackish water) he found 

 only I "48 per cent. Similarly the blood of a crab of the 

 genus Maia at Naples living in very salt water gave 3*37 

 per cent, of soluble salts, whilst at Roscoff the blood of the 

 same animal only contained 3*045 per cent, of salts. By 

 gradually altering the salt content of the medium Fredericq 

 has been able to reduce the salt content of the blood of 

 Roscoff crabs from 30*45 gr. per litre to 15*6 gr. of soluble 

 salts per litre. Again, the invertebrates of the Baltic, which 

 is of very low salinity, have only a small percentage of 

 salts in their internal fluids. A similar agreement in the 

 concentration of internal and external fluids is found also 

 among cartilaginous fishes in general. No such agreement, 

 however, is found in bony fishes, where, in consequence, 

 the osmosis must be of a selective kind. The broad features 

 of the phenomenon are, however, the same in all cases : a 

 marine animal placed in fresh water absorbs water and 

 swells, whereas with a fresh- water species placed in the sea 

 the opposite takes place. Frequently the osmotic currents 

 and consequent change in volume affect the tissues to such 

 an extent as to cause death. Where the body is protected 

 by a thick cuticle and diffusion through the skin is impossible, 

 the exchange of salts probably takes place through the thin 

 membrane of the gills, which thus acts as a dialyser. Ter- 

 restrial insects, like beetles, with thick cuticle and no gills 

 are able to resist the action of salt water almost indefinitely. 

 Without our going into unnecessary detail, the numerous 

 experiments on this subject enable us to say that where the 

 change is effected gradually the powers of adjustment of 



