ASCIDIANS RELATED TO VERTEBRATES, 20! 
Ascidians (Plate XII. Fig. A) develop the undeniable begin- 
ning of a spinal marrow (Fig. 5 g) and of a spinal rod (Fig. 5 ¢), 
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 organs 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. 4.) 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 
still 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 
