VIII.] HOMOLOGIES. 19a 



effected by repeated and -vigorous lateral flexions of the 

 body, we ought to find the segmentation much more com- 

 plete laterally than on the dorsal and ventral aspects of 

 the spinal column. And yet, in those species which, 

 taken together, constitute a series of more and more 

 distinctly segmented forms, tlie segmentation gradually 

 increases all round the central part of the spinal column. 



Mr. Spencer^ thinks it probable that the sturgeon has 

 retained the notochordal (that is, the primitive, unseg- 

 niented) structure because it is sluggish. But Dr. Glinther 



TRILOBITE. 



states that the sluggishness of the common tope {Galcus 

 vulgaris) is much like that of the sturgeon, and yet the 

 bodies of its vertebrae are distinct and well ossified. ]\Iore- 

 over, the great salamander of Japan is much more inert 

 and sluggish than either, and yet it has a well-developed 

 bony spine. 



The author can learn nothing of the habits of the sharks 

 Hexanchus, ffeptanchiis, and Echinorliinus, but IMiiller de- 

 scribes them as possessing a persistent clwrda dorsalis.^ It 



1 "Principles of Biologj^" vol. ii. p. 203. 



2 Quoted by H. Stannius in his "Hanclbucli der Anatomic der Wii'bel- 

 thiere," Zweite Aiiflage, Erstes Buch, § 7, p. 17. 



