250 FIRST PRINCIPLES OF NATURAL CLASSIFICATION. 



of the feet. If we look to the primary divisions of the 

 vertebrated animals, we see one of these peculiarities 

 very strongly marked in the fishes, the only class 

 wherein the feet, in all the individuals, are entirely 

 wanting, while every one is aware that no fish can exist 

 unless in its own element. On taking a wider survey of 

 the animal world, we find that the radiated mollusca 

 (^Radinta^ are pre-eminently aquatic, destitute of any 

 organs analogous to feet, but often swimming with their 

 arms, in the manner of the cuttlefish (^Cephalopoda), 

 which are, in fact, the aquatic types of the testaceous 

 Mollusca. These latter are the largest of all invertebrated 

 animals, and thus unite three of the great characteristics 

 of the aquatic natatorial type ; namely, an aquatic nature, 

 enormous size, and a large disproportionate head. It may 

 be urged, indeed, that the absence of feet cannot be looked 

 upon as a pecuHar mark of the natatorial structure, seeing 

 that these organs are wanting, not merely in one but in 

 three of the chief divisions of the animal world; but it 

 must be remembered, that a natural group rests not upon 

 any one single and exclusive character, but upon a com- 

 bination of several. Thus, although fishes and medusae 

 are aquatic, and move without feet, they are not the 

 largest animals, either among the vertebrated or inver- 

 tebrated, and, therefore, do not possess all the cha- 

 racters of the natatorial type ; but those which they do 

 possess are more developed in them than in any other 

 of the chief divisions of animals, or of the classes of the 

 vertebrata: hence, although they do not possess all, they 

 exhibit some, of the characters of their type, ■which cha- 

 racters are not found in any other of those which we term 

 primary types. As we approach the more perfect animals, 

 we begin to see the developement of another singular 

 feature ; namely, a very large, thick, and obtuse head, 

 furnished with jaws capable of great expansion, and 

 terminated by a blunt or truncated muzzle or snout. 

 This structure implies the pecuhar power of seizing 

 their food by the mouth alone, without the assistance of 

 feet or claws ; and as this power would only be necessary 



