THE ASCIDIANS. 89 



activity. A support of this kind might arise by enlarge- 

 ment and concrescence of the germ-layers along the longi- 

 tudinal axis of the body; and the differentiation of an 

 independent bony cord from this axial cord gave rise to the 

 notochord. (Cf Fig. 88, 89, vol. i. pp. 300, 301.) In corre- 

 lation to the formation of this central notochord, the simple 

 nerve-ganglia, lying over the throat in the Soft-worms, 

 lengthened into a long nerve-cord, reaching from front to 

 rear, above the notochord ; in this way, the medullary tube 

 originated from the " upper throat ganglia." 



As we have already minutely considered the great 

 lignificance of the Ascidians (Fig. 188) in this respect, as 

 well as their close relations to the Amphioxus (Fig. 189). 

 we will not tarry longer over this point now. I will 

 repeat, that we must by no means regard the Ascidian 

 OS the direct parent-form of the Amphioxus and of the 

 other Vertebrates. On the contrary, we assert that, on 

 the one hand the Ascidians, and on the other the Ver- 

 tebrates, have both descended from one unknown 

 Worm form, which has long been extinct ; the nearest 

 relatives of this among existing animals are the Ascidiai- 

 larvae and the Appendicularia (Fig. 187). This unknown 

 common parent-form must have belonged to the group of 

 Chorda-animals, which we pointed out as the eighth 

 ancestral stage in the human pedigree. ^^ Although we 

 cannot form an entirely satisfactory idea as to all points 

 of external and internal structure of this Chorda-animal, 

 there is no doubt that, like its near relatives, the 

 Mantle-animals, and like the preceding ancestral stage 

 represented by the Soft- worms and Primitive Worms, it 

 must be classified in the natural system of the animal 



