THE ELASMOBRANCHS 63 



an organism that 'fits' it to its 'habitat/ and it is con- 

 venient to speak of the 'urea-retention habitus' of the 

 elasmobranchs, since it is such a unique adaptation to 

 salt-water life. It is one of the most strikingly simple 

 means known for automatically maintaining an impor- 

 tant homeostatic state. This habitus is characteristic of 

 all the hving cartilaginous fishes— the sharks, rays, skates, 

 and chimaeroids— but of no other hving forms. Had this 

 fact been known when the biologist named this great 

 group of fishes it could well have supplied him with a 

 better name for the class as a whole. We shall not un- 

 dertake the responsibility of renaming the Elasmo- 

 branchii; but if and when anyone does, it may be em- 

 phasized that it is urea which is conserved, and not 

 urine. It is clear that for three hundred and fifty milHon 

 years these fishes have had a wholly adequate renal func- 

 tion and that they are not 'sick' fishes suflFering from 

 renal degeneration, as was suggested by early investiga- 

 tors in this field. 



In other fishes urea is entirely excreted by the giUs, 

 and as a step in the evolution of the mea-retention 

 habitus it was required that the elasmobranchs decrease 

 the permeability of the respiratory epitheliimi of the gills 

 to such a degree as to reduce the outward diffusion of 

 urea to a minimum. Urea is one of the most diffusible 

 substances known and, in the elasmobranchs as in all 

 other animals, it is distributed fairly imiformly through- 

 out the body water. We know almost nothing about the 

 properties of cells that determine such molecular features 

 as permeability to various substances, and all that can 

 be said is that the respiratory epithelium in the elasmo- 

 branchs holds upwards of 2.0 per cent urea in the blood 

 against a zero concentration in sea water, and this with- 

 out seriously impairing the permeability of the epithe- 

 lium to oxygen and carbon dioxide, which must be con- 

 tinuously exchanged between the blood and sea water 

 for respiratory purposes. In the mammals the renal tu- 



