THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 121 



shown that among annulose animals, the divisibility into 

 homologous parts is most clearly demonstrable in the lowest 

 types. Though in decapodous Crustaceans, in Insects, in 

 Arachnids, there is difficulty in identifying some or many of 

 the component somites; and though, when identified, they 

 display only partial correspondences; yet on descending to 

 Annelids, the composition of the entire body out of such 

 somites becomes conspicuous, and the homology between each 

 somite and its neighbours is shown by the repetition of one 

 another's structural details, as well as by their common 

 gemmiparous origin: indeed, in some cases we have the 

 homology directly demonstrated by seeing a somite of the 

 body transformed into a head. If, then, a vertebrate animal 

 had a segmental composition of kindred nature, we ought to 

 find it most clearly marked in the lowest Vertebrata and 

 most disguised in the highest Vertebrata. But here, as before, 

 the fact is just the reverse. Among the Vertebrata of 

 developed type, such segmentation as really exists remains 

 conspicuous is but little obscured even in parts of the spinal 

 column formed out of integrated vertebras. Whereas in the 

 undeveloped vertebrate type, segmentation is scarcely at all 

 traceable.* The Amphioxus, Fig. 191, is not only without 



tff* 



ossified vertebrae; not only is it without cartilaginous repre- 

 sentatives of them; but it is even without anything like 

 distinct membranous divisions. The spinal column exists 

 as a continuous notochord: the only signs of incipient seg- 

 mentation being given by its membranous sheath, in the 

 upper part of which " quadrate masses of somewhat denser 

 * See note at the end of the chapter. 



