GENERAL CONSIDERATIONS. 255 



the condition found in Peripatus. The tracheae in the Myriopoda, 

 as in Peripatus, appear very late ; they are said to form only 

 during post-embryonic life, whereas in the Insecta they appear as 

 rudiments in an earlier stage of development. 



At first sight, the occurrence of larval forms provided with com- 

 paratively few segments and still fewer pairs of limbs might seem 

 to be a fact of great significance, all the more that these closely 

 resemble in appearance the young forms of the lowest Insects, viz., 

 the Thysanura. This brings us to the question whether the rich 

 segmentation of the Myriopodan body represents on the whole a 

 primitive condition, or whether it is a secondarily acquired char- 

 acter. We might answer that the racial form of the Myriopoda 

 was a homonomously segmented form, consisting like Peripatus of a 

 large number of segments, or we might, with Haask (No. 5) assume 

 that the large number of segments such as are now found in the 

 Myriopoda represents a later acquisition by these forms. The con- 

 tinuous lengthening of the body has been explained by the manner 

 of life of the Myriopoda, which is accompanied by such a develop- 

 ment of the body in the same way as in the Serpents among 

 Vertebrates. It is interesting to see how this lengthening of the 

 body leads to a modification of its morphological characters. In 

 those Chilopoda whose bodies consist of many segments, unpaired 

 •chitinous plates appear in the soft intersegmental ventral integument, 

 which in the shorter Chilopoda is only slightly developed, and as 

 the length and the number of the segments increase, these become 

 broad ventral plates, the unpaired scuta (Haase, No. -6). 



The common primitive form of the Insecta and the Myriopoda 

 has repeatedly been sought in some form approaching the Symphyla : 

 but Scolopendrella, to which, on account of its striking resemblance 

 to the Thysanura (Figs. 192 and 193), this great significance was 

 attached, shows as well as the latter certain peculiarities of organisa- 

 tion which prevent it as much as the Thysanura from being 

 regarded as a primitive form. We do not indeed doubt that the 

 iSymphyla as well as the Thysanura are very ancient forms, but 

 we would assume a racial form for the Myriopoda with still more 

 primitive organisation, the Symphyla being somewhat removed from 

 that form and the Thysanura still further. The differentiation of 

 a thorax, which is an important character of the latter, but which 

 is merely indicated in the Myriopoda, will be discussed in dealing 

 with the Insecta. 



The Diplopoda resemble most insect larvae in leaving the egg 



