with Observations on the General Arrangement of the Articulata. 281 



Invertebrata is extended along the ventral surface of the body, and it is that 

 portion in each segment which is first completed ; while in the Vertehrata it is 

 extended along the dorsal surface, which in like manner first acquires its de- 

 finite form. The dorsal surface of the Invertebrata is occupied by the vascular 

 system, and, like the abdominal surface, at which the nutrient vessels of the 

 embryo enter, in the Vertehrata, is the last portion of the external surface of 

 the body that is completed, as may be readily seen in the development of the 

 animal in the ovum. Consequently the dorsal portion of the tegument in the 

 Myriapoda, and other Articulata, is less early completed than the ventral, 

 although often developed to a much greater extent. 



Before proceeding further, I ought to state that the mode and order of 

 development of the dorsal and ventral surfaces, as now indicated, refer espe- 

 cially to what takes place in the original formation of the segments which 

 constitute the whole animal in the ovum. Before quitting the eg^ and the 

 condition of an embryo, the body, in most of the Myriapoda, is composed 

 of only nine segments ; although before the animal arrives at its adult state, 

 as first indicated by De Geer, it acquires a periodical addition of new seg- 

 ments. In this post-embryonic development each new segment makes its 

 appearance as a whole structure, immediately beneath the reduplicature of 

 the segment that precedes it ; and its composition of definite parts does not 

 become evident until it has nearly attained its full size as a finite part of the 

 body. 



The dorsal and ventral arches of each segment are united at their sides by 

 a portion of tegument, in which the appendages of the segments are always 

 developed. One portion of this tegument appears to be subsidiary to each 

 arc. The legs, and their basilar plates, are developed in that portion which 

 belongs to the ventral arch ; while the branchial tufts in the Annelida, as 

 already shown by my friend Professor Milne Edwards, the spiracles and 

 organs of respiration in the higher Myriapoda, and the spiracles and super- 

 numerary organs of locomotion, the wings, which are always connected with 

 the respiratory structures, in Insects, make their appearance in that portion 

 which belongs to the dorsal. 



Of these subsidiary structures, only those which belong to the ventral arch 

 are developed to any extent in the Myriapoda. Those which belong to the 



