with Observations on the General Arrangement of the Ailiculata. 269 



in some of the Hymenoptera Mr. Westwood* and myself f have observed 

 fifteen. During the changes of the insect this number is gradually reduced 

 by the aggregation and anchylosis of some of the segments to form particular 

 divisions or regions of the body, in the construction of which some of the 

 segments become enlarged, and others are atrophied or almost obliterated. 

 In the Myriapoda, on the contrary, the young animal invariably comes from 

 the ovum with its smallest number of segments, and in most of the genera 

 this seldom exceeds nine ; although before the Myriapod has arrived at its full 

 growth, it acquires, in some species, nearly eight times the original number ; 

 a definite number of new segments being constantly in the course of formation 

 between the antepenultimate and penultimate segments of the body. This is 

 the great characteristic of the class, which distinguishes the Myriapoda from 

 Insecta, Arachnida and Crustacea, and approximates them to the Annelida, in 

 which a similar addition of parts takes place. The Myriapoda are also distin- 

 guished from Insects by a permanent anatomical character, the number of 

 segments and legs in the adult animal. There are never fewer than twelve 

 segments and eleven pairs of legs in any genus of Myriapoda. In some 

 genera the latter even amount to one hundred and sixty ; while no insect, 

 even in the larva state, has more than eight pairs, five of which are rudimen- 

 tary, and disappear as soon as the four anterior segments have acquired their 

 full growth, and the insect undergoes its metamorphosis, when its legs are 

 reduced to three pairs, and the insect passes into a higher state of deve- 

 lopment. These are the considerations which have led me, with Leach, 

 Latreille, and others, in opposition to the high authority of M. Brandt, to 

 separate the Myriapoda from the true Insects, and to place them as a class 

 immediately before the Annelida. 



It has been customary with most zoologists to make the organs of nutrition 

 the basis of classification in the Invertebrata, because these are more distinctly 

 marked in the lowest organized species. This is the view adopted by Cuvier, 

 Owen, and some other anatomists and zoologists. But although I hesitate 

 to differ from these high authorities, I cannot help regarding the organs of 



* Transactions of the Entomological Society of London, vol. ii. p. 124. 



t Article Insecta in Cyclopaedia of Practical and Comparative Anatomy, part 17. vol. ii. October 

 1838. 



2 n2 



