248 FROM FISH TO PHILOSOPHER 



latter classification seems preferable because the modem 

 Chimaera collei, C. monstrosa and Callorhynchus millii 

 share with the sharks, rays, and skates the urea-retention 

 habitus and the dependent mode of reproduction by in- 

 ternal fertilization {69}; these characters are so distinc- 

 tive as to override the presence or absence of an oper- 

 culum and other features that have hitherto been the 

 basis of taxonomic separation. If fusion into one class is 

 to be made, the term Elasmobranchii, anatomically de- 

 scriptive of the HolocephaH as well as of the other 

 cartilaginous fishes, is to be preferred because the carti- 

 laginous state (on which the name Chondrichthyes is 

 based) is now accepted to be secondary, whereas the gill 

 structure (elasmos = plate; branchm = gill) is primitive 



{49}. 



The chief metaboUc waste-product requiring excre- 

 tion in all animals is the nitrogenous end-product formed 

 in the combustion of protein. All the available evidence 

 warrants the conclusion that throughout the history of 

 the vertebrates up to the evolution of the reptiles this 

 end-product has been urea, the most diffusible and non- 

 toxic nitrogenous substance known. However, another 

 nitrogenous metaboHte, trimethylamine oxide, is present 

 in substantial amounts in the body fluids and urine of 

 the elasmobranchs and some marine teleosts; in the dog- 

 fish it is actively reabsorbed by the renal tubules and 

 contributes significantly to the osmotic pressure of the 

 blood {63a, 63b, 63c, 69}. 



The role of urea in the elasmobranch fishes is dis- 

 cussed by Smith {59, 68, 69, 70}. 



The studies of J. W. Burger on the secretion of the 

 rectal gland of the dogfish, Squalus acanihias, have not 

 been published at the time of writing. The urine in 

 freshly and carefully caught dogfish is distinctly more 

 hypotonic to the plasma than is suggested by the pub- 

 Hshed figures, which are based on animals generally 

 caught on line trawls and hence restrained on the hook 



