LEYDIG ON DAPHNIID2B. 29 



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 Arti- 

 culata 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 natu- 

 ralists 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 Miiller's Arch., 1860, pt. i.*~), 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, Hackel, 



* M. Baur, however, refers to MM. Kolliker and Hackel 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 over- 

 looked Mr. Huxley's article (in Todd's " Cyclopaedia") on Tegumentary Organs, in which 

 this theory is propounded. 



