MORPHOLOGY OF CYCLOSTOMAÏA. 411 



to the posterior end of the phaiynx, so that, in hiter sta^^es, the 

 coincidence of the number of the slits with that of the myotomes 

 is lost ; and that this addition of the slits continues throughout 

 life. The nephrotomes in these new slits have been, I think, 

 originally coincident, in each segment, with the m3^otomes, from 

 "which the}' were cut off in early stages and have remained 

 undeveloped until the new appearance of the added slits. It 

 seems, therefore, probable that the branchial region of AinphloxiDi 

 once extended over the largest portion of the enteric canal, 

 while a very small section in the posterior part of the canal 

 performed the nutritious function, as is seen now in the Ascidian''. 

 The " Nierencanälchen " in this hinder part may represent the 

 pronephric tubules in the post-hepatic section of the segmental 

 duct of Cyclostomata'-'. 



The pronephric system of Fctrcniiyzon comes to have the same 

 relations with the epiblast as the " Nierencanälchen " oî Aniphioxus 

 at three different points : the free ends of the two anterior 

 pronephric tubules and the hind end of the segmental duct 

 (probably the hindmost pronephric tubule). Whilst in the greatest 

 section of the system the comnnmication with the exterior has 

 been lost, these three points might have preserved it to a 

 considerably later phylogenetic stage : the two anterior tubules 

 playing the same ph^^siological part as the " Xierencanälchen " 

 of Amphioxus, and the posterior being employed as the only 



excretory pore of the system secondarily established by the union 

 of the tubules. 



In main points (with exception of the presence of the 

 tubules in the branchial region, of the contact or connection of 



])BalJbiu" says: "In Ascidiaus the respiratory sack is homologous with tlie respiratory 

 tract of Amphioxus"' {loco cit., p. 758.) 

 2) See p. 108. 



