370 



BIOLOGY. 



[Book vi. 



pair of nerves. An enlargement wHch. may be compared witli 

 the' central ganglion of the arthropods terminates, forwards, 

 their spinal marrow. It does not perceptibly differ from the 

 others, but emits five pairs of nerves, among which are the 

 optic nerves and the auditive nerves. 



The great difference consists in the complete absence of 

 CBSophagian ring. In the vertebrate it is no longer merely 

 the cerebral enlargement, it is the whole central nervous 

 system which is above the digestive system. In saying that the 

 arthropod may be compared with a vertebrate reversed, marching 

 on the back, it has been thought that the 

 difficulty was got rid of : but it merely 

 changed its place. In effect, in supposing that 

 the arthropod is a vertebrate reversed, we cer- 

 tainly put the sub-cesophagian ganglionary chain 

 above the digestive system, but on condition 

 of making to descend below it the sub-oeso- 

 phagian ganglion, that is to say the analogue 

 of the brain. Better it is, spite of the 

 Hseckelian theories, which are still a little 

 A, brain and spinal mar- rash, to confess that the genealogy of the 



row of OHliOfiorisais ,,. •,. r i- 1-1,1 



moia. B, brain and vertebrates IS tar irom being elucidated. 



commencement of the t , , i • 1 • , ,1 , i 



marrow of Trivia adn- Let this be as it may, we cannot deny the 

 analogy of the spinal marrow of the oraniote 

 or acranian vertebrates with the ganglionary chain of the arthro- 

 pods. Rudimentary expansions continue to be found even in 

 man, and histologically and physiologically, the ganglionary chain 

 of the arthropods as well as the spinal marrow of the verte- 

 brates are cellular centres endowed with a certain sum of 

 independence. 



I In the craniote vertebrates the spinal marrow ends, forwards, 

 in an enlarged portion, in a brain properly so called. But here 

 still, the transition is graduated, and we pass with no remark- 

 able suddenness from the type acranian to the type cranian, 

 even if we compare merely the adult animals (Figs. 60 and 61). 



