36 NATURAL ARRANGEMENT OF INSECTS. 



The most perfect vertebrate animals are, of course, 

 the quadrupeds ; and these, like the most perfect apte- 

 rous insects,, are destitute of wings. The possession, 

 however, of those members, together with a complete 

 metamorphosis, are the grand characteristics of the An- 

 nulosa ; and more especially of the Ptilota, or winged 

 insects. The analogy between the Annelides and the 

 Radiata is remarkably strong ; for the greater part of 

 the former have their limbs or members radiating as 

 from a common centre ; they have all the outward ap- 

 pearance, in fact, of the Radiata, although their internal 

 structure is widely different. The most simply con- 

 structed animals in creation are the Acrita, and the 

 most simple of all Annulosa are the Fermes. So closely, 

 indeed, do these two groups represent each other, that, 

 in the present ignorance which pervades their history, 

 we hardly know to which of these groups certain genera 

 belong. Last of all we have the Cirrhipedes, with their 

 shelly covering, representing the Mollusca, or shellfish; 

 the grand characteristic of that class being the testa- 

 ceous cohering with which the whole body of the animal 

 is protected. We allude, of course, to those groups of 

 the Mollusca which stand at the head of the class, as 

 the Gasteropoda and the Dithyra, and which take prer- 

 cedence above all the diversified animals which form the 

 aberrant groups, scarcely any of which are testaceous. 

 We have already shown how erroneous are those con- 

 clusions which some writers have come to, respecting 

 the little dependence that can be placed on characters 

 drawn from empty shells; and we shall now venture to 

 make the assertion, that, if these coverings did not exist in 

 the typical groups of the Mollusca, in other words, if 

 no molluscous animals had shells, we should at once 

 pronounce the group to be artificial, because every analogy 

 in nature shows that one of the aberrant types lives under 

 the covering of some substance, generally harder than 

 itself. 



(32.) The metamorphosis of the Cirrhipedes, as we 

 have before intimated, was first made known by Dr. 



