450 Causes and Course of Organic Evolution 



much reduced in Petromyzon, but reaching fuller development 

 in Bdellostojna." 



The excretory system of cyclostomes is of exceptional in- 

 terest in the above relation. For, while the anterior neph- 

 ridial portion or pronephros appears in embryonic or larval 

 stages of these as of other craniates, it persists unaltered and 

 in segmental relation only in MyxinCy though the pronephric 

 tube has largely degenerated even in these. The position 

 occupied however by the pronephros in Myxine in front of 

 the gill pouches is exactly like that seen in most nemerteans. 

 In Petromyzon the pronephric tubules are absorbed, though 

 the duct more or less persists; while the solid kidney of this 

 genus is composed of numerous closely coiled tubules that 

 have lost all trace of segmental disposition, though they show 

 a similar histological structure to that already described. 



That a gradual transition occurs again to the Batrachia is 

 indicated by the following statement of the distinguished 

 worker Goodrich (146:86): '* There is no fundamental dis- 

 tinction between the pro- and the mesonephros; in the Myxi- 

 noids and Gymnophiona the transition from 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 

 completely differentiated." The carrying backward of the 

 excretory pore has probably proceeded pari passu with back- 

 ward development of the nephritic system and of its canals. 



The eggs in Turbellaria, Nemertinea, Cyclostomata, and 

 Batrachia undergo holoblastic segmentation, and the resulting 

 embryo in all is completely or partially ciliated. All stages 

 from direct and continuous development in land, fresh-water, 

 and a few marine forms to an extensive metamorphosis and 

 shedding of embryonic tissue in many marine types of nemer- 

 tean can be traced. In spite of our still Hmited knowledge 

 of immature cyclostomes it is now known for the larva or 

 Ammocoetes of Petromyzon that it undergoes a considerable 

 degree of metamorphosis in reaching the adult stage. In 

 the Apoda or csecilians a slight degree of metamorphosis occurs, 

 that becomes even less marked in urodele batrachians. 



Many other characters — alike embryonic and adult — might 

 be cited to demonstrate that striking morphological continuity 

 can be traced from the llotifera and Rhabdocoela through 



