312 MR. WESTWOOD ON THE SUPPOSED EXISTENCE 



to this gradual action, the greater portion of the f^ertehrata are subject to that par- 

 ticular species of ecdysis which Mr. MacLeay has termed incomplete, consisting 

 simply in the integuments, hairs, skin, feathers, &c., scaling off piece by piece, or 

 one by one, as distinguished from that complete change in the identity of the enve- 

 lope of other less perfect animals which prevails in various degrees amongst the 

 Annulosa and some few of the Fertehrata, and by means of which the entire envelope 

 of the animal is shed at once. And here we may be allowed to notice the rationale 

 of this complete shedding of the envelope which so peculiarly distinguishes the An- 

 nulosa. In them we find the internal vertebrae of the higher animals converted into 

 a hard horny or crustaceous external covering, to the inner surface of which the 

 muscles are attached. This covering would of course, from its very nature, offer an 

 insurmountable obstacle against the growth of the animal, were it persistent. It is 

 therefore necessary that, in order to ensure the due increase of size in the animal, 

 its old covering should be cast off and a new and enlarged one obtained. And this is 

 what takes place throughout the Annulosa, the shedding of the shell of the Lobster and 

 the moulting of the Caterpillar being but modified examples of the same principle. 



These modifications may be reduced to three principal heads, of which the Spider, 

 the Grasshopper, and the Butterfly may be cited as well-known examples. 



In the first of these, the animal is produced from the e^g in a form which it is 

 destined to retain throughout its existence, its only change consisting in a series of 

 moultings of the outer envelope, by which an increase of size, but not an addition of 

 new 'organs, is acquired. 



In the second, the animal at its exclusion exhibits the form which it retains through 

 life, but it is subject to a series of moultings, during several of the last of which cer- 

 tain new organs are gradually developed. 



In the third, the form of the animal at its exclusion from the egg is totally differ- 

 ent from that in which it appears in its imago state, this change of form taking 

 place during two or three of its final moultings, and consisting not only in the varia- 

 tion of the form of the body, but also in a complete change in the nutritive and di- 

 gestive systems, and in the acquisition of various new organs. This constitutes what 

 has generally been termed metamorphosis. 



Now, since the Ptilota of Aristotle are preeminently the types of the invertebrated 

 animals, and as such more distantly removed from the various groups of the Verte- 

 hrata than the remainder of the Invertehrata, (owing this preeminence not only to the 

 superiority of their instincts, but also to the development of organs of flight during 

 the latter moultings,) it will necessarily follow that those Annulosa which are less 

 typical, or, in other words, more nearly allied to the lowest of the higher groups of 

 animals, will not exhibit in so remarkable a degree those metamorphoses which, as 

 we have seen, the Ptilota so peculiarly undergo. 



Hence, since the organization of the Crustacea is more clearly analogous to that of 

 the Vertehrata than that of the Ptilota, we arrive at one of the chief grounds for the 



