440 Zoological Classification, 



embryonic or immature stages of certain Radiates^ Mollusks, and 

 Articulates. These statements being admitted, it remains to en- 

 quire whether the balance of affinities inclines to one rather than 

 to the others of these provinces or sub-kingdoms. 



The great difficulty here is to find any distinct type of structure 

 in these humble creatures, and some of the naturalists best ac- 

 quainted with them hold that no such affinities are to be dis- 

 covered, while others appear to think that their affinities would 

 place them at the base of more than one province. In these cir- 

 cumstances, we are more likely to be guided aright by a conside- 

 ration of the general principles of classification, than by that 

 minute search for distinctions with which zoologists are mor^ 

 familiar. 



To any philosophical student of animals, it must be apparent 

 that our primary divisions or types are based on considerations of 

 general form, and of the arrangements of the nervous system, and 

 organs of support. On the first of these grounds alone, we must 

 of necessity divide animals into but two groups of Radiata and 

 Bilaterata; on the second, it is equally apparent that we must 

 have two groups of Vertebrata and Invertebrata. 



There has been of late a tendency among many naturalists to de- 

 ny or overlook the fact that many of the lower animals present, in 

 the words of Agassiz, " a vertical axis, around which the primary 

 elements of their structure are symmetrically arranged," while the 

 main axis of the body cannot, as in the other animals, be regard- 

 ed as a horizontal one, with corresponding parts on its two sides. 

 But nothing can be more illogical than to overlook this general 

 radiated arrangement, because some subordinate parts present 

 traces of bilateral symmetry. It is a mere forcing of nature within 

 the bonds of an arbitrary system. It would be quite as reasonable 

 to deny the prevalence of radiation from an axis or centre in 

 plants, because a is bilateral ; or to maintain that a cuttlefish has 

 no bilateral symmetry because its arms spread from a centre ; 

 or that a man is a radiated animal, because the iris of hiseye is 

 radiated. The Radiata constitute a division of animals asnatural 

 on one ground, as the Vertebrata do on another. 



Vertebrates^ again, differ from Invertebrates y in the grand dis- 

 tinguishing point of the separation of the principal masses of the 

 nervous system from the general viscera, in a distinct chamber 

 above the centres of the system of support. 



