100 
SYSTEM OF NATURE. 
The remaining articulates are not thus to be circum¬ 
scribed : the respiratory and circulating systems are so 
different in the various component groups, that they cannot 
be regarded as affording any satisfactory character. They 
agree with the winged insects in the possession of jointed 
legs, but differ in possessing also the vegetable power neces¬ 
sary to their reproduction when lost. They have no trace 
of wings, even in a rudimentary or undeveloped state. 
They are entirely without adult metamorphosis, for it will 
be recollected that the metamorphosis— so well known in 
the young of Myriapoda, and discovered by Mr. Thomp¬ 
son in the young of Crustacea — is what might almost be 
termed a foetal metamorphosis, and is rather analogous to 
that of the young kangaroo than to that of the full grown 
caterpillar of a moth or butterfly. In strict compliance 
with scientific truth, metamorphosis is but a mode of that 
change common to all organized beings, but the mode 
becomes a character, and constitutes a difference more 
striking than the introduction of a new series of organs; 
so that, in this fact alone, there is a barrier erected to the 
integrity of the articulates as a province. 
I have already said that I regard it a test of the equality 
and natural character Of groups, that they shall submit 
to corresponding subdivision. Now it scarcely admits 
of a doubt that, if we separate the Pterota, the remain¬ 
ing articulates are divisible into four groups, three of 
them being identical with Latreille’s classes, — Crustacea, 
Arachnides and Myriapoda,—and the fourth corresponding 
with the first division of his fourth class, and comprising 
his Thysanoura and Parasita, being equivalent to the In¬ 
sect a ametabola of Leach. It is somewhat remarkable 
that each of these groups is almost invariably divided into 
