VERTEBRATED TYPE OF NERVOUS SYSTEM. 



77 



mouth, as in the Radiata nor disposed in a long row, as the 

 Articulata we should be right in ranking 

 it as a Mollusk. The Vertebrata have 

 their principal nervous centres arranged 

 in one continuous mass, which is enclosed 

 within their bony skeleton : this mass 

 consists of the Brain, which occupies the 

 skull ; and of the Spinal Cord, which is 

 contained within the back-bone; and 

 from it are supplied not only the organs 

 of locomotion and sensation, but those 

 concerned in the reception of food, and in 

 the respiratory actions, which have dis- 

 tinct ganglia of their own in most Inver- 

 tebrata. 



54. A case in which the place of a 

 doubtful group, the Barnacle tribe, might 

 have been decided by the conformation of 

 its nervous system, has been already 

 adverted to ( 32) : here the division of 

 the body into segments is very indistinct, 

 and its external form has a general resem- 

 blance to that of the Mollusca ; but the 

 double nervous cord, studded with ganglia 

 at intervals, which runs from one extremity 

 to the other, assigns its place to be among 

 the Articulata, a decision which has 



r> , , ,. ,. . , A FIG. 3?. BRAIN AND SPINAL 



been confirmed by the discovery of its CORD OF MAN. 

 curious metamorphosis. 



55. We have hitherto spoken chiefly of the principles, upon 

 which a Natural System or Classification should be constructed; 

 a few observations may be added, on the manner in which it may 

 be advantageously applied. It has been shown that, in first ar- 

 ranging the different species into groups, and combining these 

 groups into others, we must be guided by the whole collection 

 of characters, which each animal presents. But if, whenever 

 we meet with a form of animal life that is new to us, we were 



