STUDIES ON THE PKOTOCHORDATA. 345 



correct, the post-anal gut being typically in many of the 

 higher Vertebrates — e. g. Selachians — that portion of the 

 enteric cavity into which the neurenteric canal opens^ just as 

 it is in the Ascidiaus. 



In Araphioxus the neurenteric canal, according to Hatschek 

 (15), persists much longer than in the Ascidians, but the post- 

 anal tract of the body does not grow out until after its closure. 

 In Ascidians the neurenteric canal, as shown by Kowalevsky, 

 and as I have seen by sagittal sections through embryos of 

 Cynthia papillos a, becomes obliterated at a very early stage, 

 namely, as soon as the tail commences to grow out, this being 

 also the time at which the curvature of the embryo begins. 



The tail, therefore, of the Ascidian tadpole is a solid out- 

 growth from the trunk involving the nerve-cord, notochord, 

 lateral mesodermic bands, and a solid one-layered cord of endo- 

 derm-cells. Tlie latter, as Kowalevsky (21) discovered, become 

 converted into blood-corpuscles, and not, as Balfour says (p. 

 634, loc. cit.), probably by inadvertence, into blood-vessels. 



If reference be made to DavidofF^s (8) pi. xxi, fig. 54, the 

 contrast between the tail and the trunk in the embryo of 

 Distaplia magnilarva will be found to be particularly 

 striking. 



With reference to the tail of the larva of Phallusia mam- 

 millata, Kowalevsky says (21, p. 112), "Er wachst als 

 eine einfache Verlangerung des Hinterendes, immer in der 

 Langsrichtung zum Embryo; also ganz in derselbeu 

 Weise und Richtung wie der auswachsende Schwauz 

 der Wirbelthiere." 



The extreme probability from physiological considerations 

 that the intestine of Ascidians is homologous with that of 

 Amphioxus, together with the fact which I believe to have 

 proved above, contrary to the opinion of van Beneden and 

 Julin, that the pharynx or branchial sac of the Ascidians is 

 essentially homologous with that of Amphioxus, would appear 

 to render any opposition to the view that the tail of the 

 Ascidian tadpole is a new tail and not an old trunk, gratuitous. 

 In making use of the expression " new tail,'' I refer to the 



