Although, therefore, the body of decapods and that of insects agree in consisting, theoretically of four, and practically of three, segments, there is this difference, that in the decapods the head and thorax have coalesced, while in insects it is the abdomen and post-abdomen which form a continuous series. It seems to me to be of great importance that we should use the words "thorax" and "abdomen" in one sense only; but when our nomenclature is thus corrected, it is a matter of little importance whether, with most naturalists, we divide the body of Articulata into three parts, with Leydig into four, with Zaddach into five, or whether, with Erichson, we consider the body of the insect as falling into three divisions, and that of the Crustacea, in which the abdomen is not continuous, into four.
Indeed, these divisions, though convenient, are still artificial, since the breaks which occur do not occupy the same place in all Articulata; and, even in the limits of one class, as for instance of the Crustacea or of the Insects, we find segments which in some families lose their usual attachments, and become more or less firmly united to one of the other two divisions.
In order, therefore, to make our nomenclature self-consistent, I should propose to confine the use of the word "thorax," in Crustacea, to the three segments which bear the maxillipeds (and which are homologous with the three thoracic segments of Insects); and to call the five leg-bearing segments of the higher Crustacea the "abdomen," since they correspond to the first five abdominal segments of Insects.
Prof. Leydig adopts the idea first suggested by Gruithuisen, and confirmed by Zaddach, that the valves of Daphnia are homologous with the anterior wings of insects; and as regards the mode of origin of the "cuticle," or outer chitinous membrane, he adheres to the view expressed by him in his article on Argulus (Zeitsch f. W. Zool, 1850, vol. ii., p. 325). According to this view, the outer chitinous investment is a secretion from the subjacent cellular (?) layer; and as most naturalists are agreed on this point, we may regard it as being pretty well established. M. Leydig, however, in 1855, expressed a decided opinion, that the chitinous skin of Articulata was to be considered as chitinised connective tissue. This is apparently a diametrically opposite statement; and we cannot wonder that most naturalists (see, for instance, M. Baur's interesting paper in Müller's Arch., 1860, pt. i.[1]), should have looked upon Leydig as having abandoned his previous idea. However this may be, it is satisfactory to find that the best authorities are now agreed in considering that the chitinous outer skin of Articulata is thrown off from the underlying cellular layer, although Leydig refers this layer to the class of tissues known as connective tissue, while Kolliker, Häckel,
- ↑ M. Baur, however, refers to MM. Kolliker and Häckel as being the first to regard chitine as an excretion from the subjacent cellular layer, rather than a modification of pre-existing tissue. M. Baur, like other continental naturalists, seems to have overlooked Mr. Huxley's article (in Todd's "Cyclopædia") on Tegumentary Organs, in which this theory is propounded.
