The Evolution of the Kidney 47 



problem of water regulation. And it may safely be assumed 

 that in the Cambrian or Ordovician prochordate ancestor of 

 the vertebrates the kidney was little, if at all, concerned with 

 the excretion of water, but wholly with the excretion of 

 nitrogenous waste. 



Judging from the evidence of comparative anatomy, the 

 marine ancestor of the chordates had in each of the middle 

 segments of the body a pair of open tubules which connected 

 the primitive body cavity, or coelome, with the exterior; 

 these segmental tubules were probably originally gonaducts 

 serving to carry the eggs and sperm out of the coelomic cav- 

 ity. Possibly before the chordate stage the coelomic mem- 

 brane had come to play a part in excretion, and the segmental 

 gonaducts which connected it with the exterior, and which 

 were themselves formed by an exvagination of the coelomic 

 membrane, perhaps participated in the regulation of the 

 composition of the blood by reabsorbing valuable substances 

 from the coelomic fluid, or by secreting waste products into 

 this fluid as it passed out of the body. 



With this rather meagre equipment of a coelomic mem- 

 brane and a number of segmental ducts the first chordates 

 essayed to enter the fresh waters of the paleozoic continents. 

 In migrating from the sea to brackish estuary and thence up 

 the rivers to the inland lakes these chordates were probably 

 following a protoplasmic impulse to search for peace, but 

 they were destined never to have that impulse satisfied. They 

 encountered trouble, as is obviously revealed by the defensive 

 armor which they soon evolved. The first vertebrates to ap- 

 pear in abundance in the fossil record, the Silurian and De- 

 vonian ostracoderms, the arthrodires, antiarchs and the earli- 

 est shark-like forms bearing jaws, the acanthodians, and even 

 the later advanced fishes, were typically encased from snout 



