98 THE EVOLUTION OF SIAN. 



Craiiiota branched off from the Acrania, as these did from 

 the Chorda Animals. Our exhaustive study of the Compara- 

 tive Anatomy and Ontogeny of the Ascidian and the 

 Amphioxus have ah'cady afforded proof of this relation. (Cf. 

 Chapters XIII. and XIV., and Plates X. and XL with the 

 explanations.) I will only repeat, as the most imjDortant 

 fact, that the Amphioxus develops from the egg in exactly 

 the same way as the Ascidian. In both, the original Bell- 

 gastrula (Figs. 4 and 10) originates in an exactly similar 

 manner, by primordial cleavage from the simple parent-cell 

 (Figs. 1 and 7). From this originates that remarkable larva, 

 which develops a medullary tube on the dorsal side of the 

 intestinal tube, and between the two a notochord. At a 

 later period, both in the Ascidian and in the Amphioxus, the 

 intestinal tube differentiates into an anterior gill-intestine 

 and a posterior stomach-intestine. In accordance with the 

 fundamental principle of Biogeny, from these very important 

 facts we may deduce the following statement of great phylo- 

 genetic importance : the Amphioxus, the lowest Vertebrate 

 form, and the Ascidian, the most nearly allied Invertebrate 

 form, have both descended from one single extinct Worm 

 form, which must have possessed the essential structure of 

 the Chorda Animals. 



The Amphioxus, as has already been often shown, is 

 of extreme importance ; not only because it thus fills the 

 great gap between the Invertebrates and the Vertebrates, 

 but also because it represents, at the present time, the 

 typical Vertebrate in its simplest form; and because it 

 directly affords the best standpoint from which tc examine 

 the gradual historic evolution of the whole tribe. If the 

 structure and germ-history of the Amphioxus were un- 



