152 Mr. W.S. MacLeay on the Anatomy of the 
of Philosophical Comparative Anatomy is founded on the maxim of the 
various organs undergoing different degrees of developement in different 
animals. 
It is well known that certain Ametabola, instead of getting wings, 
acquire an additional number of segments to their body, but it is not so 
well understood, that the contiguous class of Crustacea have generally a 
tendency to adopt a typical number* of segments. These typical seg- 
ments, for example, are all distinct in Squilla among Crustacea, and 
are most confluent in the neighbouring class of Arachnida, Not only 
the several segments which compose the head in Squzlla, become conflu- 
ent in Arachnida, but sometimes the whole head with the body.t But 
kingdom, must be carefully distinguished from the mere comparison of organs. 
The latter is the Principe des Connexions of Geotfroy-St.-Hilaire, which many 
centuries ago Aristotle explained and described under the name of the Arrange- 
ment of Organs kar’avadoyrav. The comparison of animals is one thing, and 
the comparison of their organs is another. The last is the province of the 
comparative anatomist, who is not always, as we know, versed in the first, 
whith is the province of the naturalist. The naturalist, on tlre other hand, 
cannot compare animals together without some degree of comparison of their 
organs. If Geoffroy, therefore, arrived at the first idea of his Principe des 
Connewions by inspiration, ashe tells us, (Phil. Anat. p. 30.), we are certainly 
justified in believing that Aristotle must also have been inspired before him. 
* This number of segments I have stated in the “‘ Hore Entomologice,” to be 
fifteen, allowing three for the head, and twelve, as usual, for the body; but 
there is good reason to believe, as I shall hereafter shew, that even Crustacea 
may be reduced to the ordinary number of primary segments, which is thirteen, 
The segments of the head, which are sometimes three, but typically four, are 
therefore of course, only to be considered as secondary. 
+ Mr. Kirby, from having through life devoted his attention to winged in- 
sects, has, in his “Introduction to Entomology,” remained, with respect to the 
Aptera of Linnzus, pretty much where the learned Swede left that most hete- 
rogeneous group. Like Linneus, he divides them according to their number 
of feet, and in one respect, as to the distinction of the head from the thorax, 
he is even behind Linnzus, who pointed out this distinction, although perhaps 
inan improper way. It is to be hoped that our indefatigable countryman will 
take up the study of this important branch of Entomology with his usual 
energy, and not leave the pages which relate to the Aptera of Linneus so much 
