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 
(Radiata) 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 peculiar 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 meduse 
are aquatic, and move without feet, they are not the 
largest animals, either among the vertebrated or inver- 
tebrated, and, therefore, do not possess al/ 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 al/, 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 peculiar power of seizing 
their food by the mouth alone, without the assistance of 
feet or claws ; and as this power would only be necessary 
