THE NATATORIAL FORM. 951 
to such animals as lived upon others, we accordingly 
find that nearly all natatorial types are carnivorous : the 
meduse, the cuttlefish, the whales, and innumerable 
other groups demonstrate this, but none more perfectly 
than the order Natatores among birds, the owls among 
the Raptores, and the Fissirostres in the circle of the 
perchers (Insessores). Sub-typical forms, as we have 
already seen, are pre-eminently carnivorous, but they 
differ from the natatorial (which always follow them) 
in this, that the food is captured by the aid of the claws, 
whereas in the types we are now speaking of the mouth 
alone is the instrument of capture. 
(310.) We are thus led to the feet of this type, the 
absence or slight developement of which, in natatorial 
forms, is almost universal. As Fishes constitute the 
pre-eminent natatorial type of vertebrated animals, so we 
find that those groups which represent them in other of 
the vertebrated circles have the feet transformed, as it 
were, into fins. How beautifully is this exemplified in 
the whales and porpoises (forming the natatorial order 
of quadrupeds), the swimming order of birds, and 
the Elaniosauri, or fin-footed reptiles! Analogies, if 
they are true, are universal ; and thus we find the same 
general structure, under different modifications, both in 
the Mollusca and in the Annulosa. The Pteropoda, 
the Cephalopoda, and the Crustacea, or crabs, employ 
the same organs for swimming: these animals, above all 
others in their respective circles, are the most expert 
swimmers, and the most adapted for inhabiting a watery 
element. Even when we descend to more minute 
groups, the same general structure, under new and sur~ 
prising modifications, can be distinctly traced. The 
apodal larve, for instance, of all the Ptilota or winged 
insects, are natatorial types ; while the order Neuroptera, 
in the circle of Ptilota, is precisely of the same de- 
scription, being analogous to the Crustacea, or crabs. 
(311.) Il. As to the economy of the aquatic types, 
we have already premised that they are almost entirely 
carnivorous, — a habit which is naturally to be expected 
