4 ON CLASSIFICATION. 



been allowed to give place in importance to other characters of 

 secondary weight, obviously embraces creatures of very dissimilar 

 and incongruous formation. 



The VERTEBRATA are distinguished by the possession of an 

 internal nervous centre or axis, composed of the brain and spinal 

 cord, which is enclosed in an osseous or cartilaginous case, and placed 

 in the median plane of the body, giving off symmetrical nerves, 

 which are distributed to all parts of the system. This general 

 definition indicates a large division of the animal world, which, 

 by secondary characters drawn from the structure of their organs 

 of respiration and circulation, is separable into mammals, birds, 

 reptiles, amphibia, and fishes. 



The MOLLUSCA have a nervous system constructed upon a very 

 different type, and do not possess any vertebral column or articu- 

 lated skeleton. The nervous centres consist of several detached 

 masses placed in different parts of the body, without regularity 

 of distribution or symmetrical arrangement ; and the entire group 

 is obviously natural, although Cuvier has ranged in it some creatures 

 which, in the structure of their nervous system, differ essentially 

 from those comprised in his own definition. 



The class of ARTICULATED ANIMALS is likewise well cha- 

 racterized by the nervous system, which, in all the members of it, 

 is composed of a double series of ganglia or masses of neurine, 

 arranged in two parallel lines along the abdominal surface of the 

 body, united by communicating cords, and from which nerves are 

 given off to the different segments of which the body consists. 



But the fourth division of Cuvier, namely, that of ZOOPHYTES 

 or RADIATED ANIMALS, is confessedly made up of the most hete- 

 rogeneous materials, comprising animals differing in too many 

 important points to admit of their being associated in the same 

 group ; and the efforts of subsequent Zoologists have been mainly 

 directed to the establishment of something like order in this 

 chaotic assemblage. 



(6.) The evident relation which the perfection of the nervous 

 system bears to that of animal structure, and the success of Cuvier 

 in selecting this as the great point of distinction in the establish- 

 ment of the higher divisions of the animal kingdom, necessarily 

 led succeeding naturalists still to have recourse to this important 

 part of the economy in making a further subdivision of the 

 Radiata of Cuvier. In some of the radiated forms, indeed, 

 nervous filaments are distinctly visible, and such are among the 



