ASCIDIANS RELATED TO VERTEBEATES. 201 



Ascidians (Plate XII. Fig. A) develop the undeniable begin- 

 ning of a spinal marrow (Fig. 5 g) and of a spinal rod (Fig. 5 c), 

 and this moreover in entirely the same way as does the 

 Amphioxus. (Plate XIII. Fig. B.) It is true that in the 

 Ascidians these most important oi'gans of the Vertebrate 

 animal-body do not afterwards develop further. The 

 Ascidians take on a retrograde transformation, become 

 attached to the bottom of the sea, and develop into shape- 

 less lumps, which when looked upon externally would 

 scarcely be supposed to be animals. (Plate XIII. Fig. A.) But 

 the spinal marrow, as the beginning of the central nervous 

 system, and the spinal rod, as the first basis of the vertebral 

 column, are such important organs, so exclusively character- 

 istic of Vertebrate animals, that we may from them with 

 certitude infer the true blood relationship of Vertebrate 

 with Tunicate animals. Of course we do not mean to say 

 by this, that Vertebrate animals are derived from Tunicate 

 animals, but merely that both groups have arisen out of a 

 common root, and that the Tunicates, of all the Invertebrata, 

 are the nearest blood relations of the Vertebrates. It is 

 quite evident that genuine Vertebrate animals developed 

 progressively during the primordial period (and the skull- 

 less animals first) out of a group of worms, from which the 

 degenerate Tunicate animals arose in another and a retro- 

 grade direction. (Compare the more detailed explanation of 

 Plates XII. and XIII. in the Appendix.) 



Out of the Skull-less animals there developed, in the first 

 instance, a second low class of Vertebrate animals, which 

 stUl stands far below that of fish, and which is now repre- 

 sented only by the Hags (Myxinoida) and Lampreys 

 (Petromyzonta). This class also, on account of the absence 



