THE ANCESTRY OF THE OHOBDATA. lOJ 



(3) The degree of the invagination of its nervous systen 

 and the extent of the neural tube. 



(4) The extent and degree of isolation of its notochord. 



(5) The extent of the atrial folds. 



(6) The absence in B. Kowalevskii of any definite liver 

 sacculi, and the presence in B. minutus, &c., of liver saccules 

 differing from those of Amphioxus. 



The points of resemblance taken together are so consider- 

 able as to suggest that they were possessed by a common 

 ancestor of the Hemichordata and Cephalochorda. On the 

 other hand, the points of difference are nearly all differences of 

 degree, and (1), (2), (3), (4), (6) are points in which the 

 Vertebrata agree with Amphioxus. In the case of (5), how- 

 ever, the Vertebrata more nearly agree with Balanoglossus. 



Of the Vertebrata. — The common ancestor, then, of the 

 Cephalochorda and the Vertebrata may be presumed to have 

 possessed the features of mesoblastic repetition, invaginated 

 nerve-cord, and consequent extension of the neural tube, raised, 

 so to speak, to the degree in which they are found in both those 

 divisions. Also it may be believed that the prseoral lobe had 

 somewhat diminished and that the atrial folds were still small. 

 The origin of such a liver as that of Amphioxus, as a speciali- 

 sation of part of the wall of the digestive region of a young 

 B. Kowalevskii is easy to imagine, for the histology of these 

 two tissues is still almost identical. [The presence of peculiar 

 liver saccules in B. minutus, &c., presents no difiSculties, as 

 their absence in the more primitive B. Kowalevskii shows 

 that they have arisen within the limits of the group.] Animals 

 possessing those features would answer nearly to the Proto- 

 chordata of Balfour, though the structures now attributed to 

 it are somewhat different. 



The Protochordata thus constituted would then differ from 

 the Enteropneusta in the possession of a serially-repeated me- 

 soblast, in addition to serially -repeated gill-slits, and possibly 

 generative organs ; also in the complete separation of the 

 nervous system and notochord. The serial repetition of the 

 gill-slits, the small operculum, &c., they must be presumed to 



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