152 Mr. W. S. MacLeay on the Anatomy of the 



of Philosophical Comparative Anatomy is fomided on the maxim of the 

 various organs undergoing different degrees of developement in different 

 animals. 



It is well known that certain Ametabola, instead of gettmg 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 Squ'dla among Crustacea, and 

 are most confluent in the neighbouring class of Arachnida. Not only 

 the several segments which compose the head in Squilla, become conflu- 

 ent in Arachnida, but sometimes the whole head with the body.f But 



kingdom, must be carefully distinguished from the mere comparison of organs. 

 The latter is the Principe des Connexions of GeofFroy-St.-Hilaire, which many 

 centuries ago Aristotle explained and described under the name of the Arrange- 

 ment of Organs Kar'avaXoyiav. 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, 

 which is the province of the naturalist. The naturalist, on tli^ 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 

 Connexions by inspiration, as he tells us, (Phil. Anat. p. 30.), we are certainly 

 justified in believing that Aristotle miist also have been inspired before him. 



* This number of segments I have stated in the " Hotcb Entomologies," 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 typicallj' four, are 

 therefore of course, only to be considered as secondary. 



t 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 Linnaeus, pretty much where the learned Swede left that most hete- 

 rogeneous group. Like Linnfeus, 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 Linnaeus, who pointed out this distinction, although perhaps 

 in an 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 Linnaeus so much 



