THE NATATORIAL FORM. 251 



to such animals as lived upon others, we accordingly 

 find that nearly all natatorial types are carnivorous : the 

 medusa), 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 Mottusca 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 larvae, 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.) II. 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 



