VIII.] HOMOLOGIES. 193 



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, the segmentation gradually 

 increases all round the central part of the spinal column. 



Mr. Spencer 1 thinks it probable that the sturgeon has 

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

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



TRILOBITE. 



states that the sluggishness of the common tope (GaUus 

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

 bodies of its vertebrae are distinct and well ossified. More- 

 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, Heptanchw, and Echinorhinus, but Muller de- 

 scribes them as possessing a persistent chorda dorsalis. 2 It 



1 "Principles of Biology," vol. ii. p. 203. 



2 Quoted by H. Stannius in his "Handbuch der Anatomic der Wirbel- 

 thiere," Zweite Auflage, Erstes Buch, 7, p. 17. 



O 



