468 Cal'ses and Course of Organic Evolution 



vascular mesliwork (Siren) and again to others with "a com- 

 plicated trabecular network" (Apoda) or a finely meshed lung 

 system (Menopoma). 



The urino-genital organs afford valuable comfirmatory evi- 

 dence of the affinities now under discussion. 



The relation of the nemerteans to the cyclostomes has already 

 been dealt with. The clearly metameric character of the 

 tubules in the former is reproduced exactly in the myxinoids, 

 and in these alone the primitive or larval pronephros persists, 

 while the opening of the nephrostomes into the pericardial 

 cavity may well represent the nemertean and still earlier rhab- 

 docoel condition of distinct skin openings in which the pore 

 has now closed, and the renal products are shed internally. 

 In the Ammocoetes larva of Petromyzon the pronephros is 

 present, but in time it becomes small and is replaced by the 

 mesonephros or kidney proper. In the adult Petromyzon the 

 permanent mesonephros or functional kidney is represented 

 by a single longitudinal duct on each side, into which open 

 a number of tubules leading from closed renal capsules with 

 glomeruli. These capsules are much more numerous than the 

 segments of the body they occupy. The above conditions 

 are in marked contrast to what holds for ordinary fishes. 



In the Apoda "the pronephros is well developed in the larva," 

 which may have "as many as 12 or 13 nephrostomes" (138). 

 In the adult, however, the pronephros is replaced as in Petro- 

 myzon by a long mesonephros or kidney, composed in the 

 larva of metameric bodies which in the adult may lose meta- 

 meric number-relation, and by gradual development may at 

 length amount to hundreds. Who will say whether this is 

 a progressive or degenerate change? But so far as phylo- 

 genetic morphology goes "there is no fundamental distinction 

 between the pro- and the mesonephros; in the myxinoids and 

 Gymnophiona (Apoda) the transition from the one to the other 

 is gradual. Such differences as are found in the development 

 appear to be chiefly due to the fact that, as the mesonephros 

 arises later, the mesoblastic somites are by that time more com- 

 pletely differentiated" (1J^6: 86). 



In the Urodela the condition is fundamentally as in the 

 last, but there is a strong tendency for the posterior part of 

 the mesonephros to enlarge, and become the active functional 

 part, the narrower anterior portion being connected with the 

 reproductive organs. This state of affairs may have fore- 

 shadowed, and even stimulated to, what probably occurred 

 in the amphibio-mammalian origin of a metanephros. 



