343 THE ANATOMY OF INVERTEBRATED ANIMALS. 



embryo, and not from the subsequently-added segments. 

 New segments, each giving rise to two pairs of limbs, are 

 developed by sixes in the germinal region between the penul- 

 timate segment and the hindermost of the newly-formed seg- 

 ments, until the full number of the adult is complete. 



In all other Chilognatha of which the development has 

 been traced, the young, at first, possess only three pairs of 

 functional legs ; and one of the four segments which follow 

 the head is apodal. According to Fabre, the apodal seg- 

 ment in JPolydesmus complanatus is the second, and not, as 

 in lulus, the third. 



In the Chilopoda the young leave the egg with seven 

 (Lithobius, Scutigera) or a greater number of pairs of ambu- 

 latory limbs. Scolopendra is said to be viviparous. The 

 early stages of development of Geophilus have been de- 

 scribed by Metschnikoff. 1 Complete yelk-division takes 

 place, and when the young leaves the egg it has a cylindrical 

 body, like that of one of the Chilognatha, and possesses many 

 pairs of limbs. Newport 2 has pointed out that, in Geophi- 

 lus longieornis, the basilar segment is formed by the conflu- 

 ence of four somites, of the appendages of which only two 

 are ultimately developed. Thus the basilar segment of the 

 head of the Chilopoda appears to correspond very closely 

 with the four somites which follow the head in the Chilogna- 

 tha. Under these circumstances, the difference in the posi- 

 tion of the reproductive apertures in the two groups is ex- 

 ceedingly remarkable. 



Fossil Myriapoda occur both in the tertiary and secondary 

 formations, and there seems no reason to doubt that the 

 Xylobius sigillarice discovered in the coal of Nova Scotia by 

 Lyell and Dawson is to be referred to this group. 



The Ixsecta. — Notwithstanding the vast number and the 

 singular diversity of form of Insects, the fundamental unity 

 of their structure is remarkable, and, in this respect, the group 

 exhibits a striking contrast to the Crustacea. 



The division of the body into three regions — head, thorax, 

 and abdomen — is usually well marked, not only by the peculiar 

 modifications which the cephalic and thoracic somites under- 

 go, but by the attachment of the three pairs of ambulatory 



1 Zeitschrlft fur wiss. Zoologie, 1875. 



2 Monograph of the class Myriapoda, order Chilopoda. (" Transactions of 

 the Linnsean Society," xix.) 



