256 MYRIOPODA. 



at a stage with few segments and with only three well developed 

 pairs of legs. The Chilopoda when hatched always have a large 

 number of segments and pairs of legs, some even possessing the 

 full adult number. We should feel inclined to consider this as 

 the more primitive condition, especially as Peripatus also possesses 

 the full number of segments when hatched, did not the whole 

 organisation make it appear doubtful which of the two divisions, 

 the Chilopoda or the Diplopoda, is the more primitive. 



The invagination of the embryo, or ventral flexure of the germ- 

 band of the Chilopoda, as well as their further development, seems 

 to take place in a primitive way, as it appears to be merely a 

 consequence of increase in length, while the early bending of the 

 germ-band of the Diplopoda does not admit of such a natural 

 explanation, but must rather be regarded as a derived condition. 

 On the other hand, the cylindrical form of the Diplopodan body 

 seems to represent a more primitive condition, since the Chilopodan 

 embryo also is cylindrical and becomes flattened dorso-ventrally only 

 after hatching. 



Whereas, in the Chilopoda, each body-segment carries a pair of 

 limbs, in the Diplopoda we see every two segments fusing together to 

 form one, which is then provided with two pairs of limbs. Ontogeny 

 has shown that for every segment of the Diplopoda, two primitive 

 segments and two ganglia appear as rudiments ; the double nature 

 of these segments can thus no longer be questioned. In this we 

 certainly have a secondary character in the Diplopoda. The mouth- 

 parts of this division, nevertheless, are far simpler than those of the 

 Chilopoda, in that the former probably possess only one pair of 

 maxillae, while in the Chilopoda two more pairs of extremities are 

 drawn in to assist this pair in the work of mastication. The tracheal 

 system is simpler in the Diplopoda and more complicated in the 

 Chilopoda, but, on the other hand, a more primitive condition of 

 the genital organs is found in the latter, the genital glands first 

 appearing dorsally to the intestine (as in Peripatus) and retaining 

 this position, while in the Diplopoda they are found ventrally to 

 the intestine. In the former, the genital aperture belongs to the 

 penultimate body -segment, whereas, in the latter, it lies near the 

 anterior end of the body, between the second and third trunk- 

 segments. It can hardly be doubted that the position of the genital 

 aperture at the posterior end of the body represents the primitive 

 condition, and that in other cases that condition has been modified. 



When it is further added that the Diplopoda appear palaeonto- 



