56 Physiology of the Kidney 



surrounding sea water and causes water to move from the 

 sea into the body, through the gills; and thus, pure water, 

 free from salt, moves continuously inward at a sufficient rate 

 to afford a vehicle for the urinary excretion of waste pro- 

 ducts and such excess salt as is present in the food.^"'^^ Where 

 the bony fishes must continuously drink sea water in a steady 

 stream, in the elasmobranchs this fluid serves only to wet the 

 gills. 



A unique tubular segment is present in the elasmobranch 

 kidney, just distal to the glomerulus, which is thought to be 

 the site of the active reabsorption of urea from the glomeru- 

 lar filtrate. None of the elasmobranch fishes, in spite of their 

 long residence in the sea, is aglomerular; having always had 

 abundant water available for filtration, there has been no 

 need to abandon their glomeruli. 



It is especially interesting that the method of reproduc- 

 tion in this subclass is highly specialized, the majority of the 

 Elasmobranchii being viviparous, the rest producing an Qgg 

 inclosed in a relatively impermeable egg case. The latter is 

 apparently the more primitive mode of reproduction. Both 

 the viviparous forms and those that have a cleidoic or "closed" 

 egg utilize internal fertilization, for which purpose there 

 exist claspers in the male and accessory reproductive glands 

 in the female. One supposes that this specialized mode of re- 

 production is concerned with the conservation of urea in the 

 young embryo until such time as its kidneys and its respira- 

 tory and integumentary membranes are organized. The 

 Cladoselachii of the Devonian apparently lacked claspers, but 

 these were present in the Carboniferous and Permian hybo- 

 donts and pleuracanths and in all the Jurassic sharks. Fur- 

 ther paleontological research may, in the above view, be able 



