104 COMPARATIVE PHYSIOLOGY 



from the standpoint of comparative physiology, namely, the 

 osmotic pressure and coagulative power of blood. The osmotic 

 pressure of the blood is important as part of the mechanism 

 of co-ordination. Thus the low blood-pressure of Crustacea 

 makes it inconceivable that simple filtration could play any 

 role in their excretory processes (as in the glomerular function 

 of vertebrates), because of the high content of proteins whose 

 osmotic force must be overcome in some way in order to 

 effect any separation of water and diffusible salts from the 

 blood. In land vertebrates the osmotic pressure of the blood 

 is a constant quantity for any species. The following figures 

 (cf. Bayliss' " General Physiology ") for the freezing-point 

 depression (Bottazzi) bring out a fact of great bionomic interest : 



The osmotic pressure of the blood of elasmobranchs is the 

 same as that of the sea water in which they live ; and the same 

 is true of all marine invertebrates ; and, as was first shown 

 by Fredericq (1885), this is not a mere coincidence, for the 

 osmotic pressure of the blood adjusts itself to that of the 

 medium over wide changes of concentration and dilution. We 

 have already mentioned temperature optima of body proteins 

 and loading tension of respiratory pigments as possible physio- 

 logical factors in geographical distribution. The extent to 

 which an animal can rapidly adjust itself to change in osmotic 

 pressure is doubtless an important aspect of the ecology of 

 estuarine forms. Some modification in response to changed 

 conditions was shown by Dakin in Teleosts, in which we see 

 the beginnings of a fixed osmotic condition of the body fluids. 

 It may be noted in passing that the saUne constituents of 

 elasmobranch blood are not much more concentrated than 

 those of the blood of Teleostei ; the difference depends upon 

 the high concentration of urea in selachian blood. Mines 

 found that the presence of urea was necessary to ensure 

 successful perfusion of the elasmobranch heart. The hearts 

 of marine invertebrates in all cases that have been tried will 



