vm ARTHROPOD A 281 



MYRIAPODA 



Classification adopted 



[ Pauropoda 

 Progoneata . . -! Symphyla 



[Diplopoda 

 Opisthogoneat . . Chilopoda 



Before we consider what phylogenetic lessons are to be learnt 

 from the facts of Insect development which have been set forth in 

 the preceding pages, we may glance at what is known of the develop- 

 ment of those lower air-breathing Arthropoda known as Insecta 

 Myriapoda, in which every segment of the long body bears 

 appendages, and in which therefore regions corresponding to thorax 

 and abdomen are not differentiated from one another. 



This division constitutes, in fact, a lumber room for forms not at 

 all closely related to one another, and it is divisible into four 

 very distinct groups. Three of these the Symphyla, the Diplopoda, 

 and the Pauropoda differ from all Insecta, and from Peripatus also, 

 in having the genital openings situated in the anterior portion of 

 the body, not far from the head. They are termed on this account 

 Progoneata. The fourth group, the Chilopoda or Centipedes, agree 

 with both Insecta and Peripatus in having the genital opening 

 situated at the posterior end of the body just in front of the anus, 

 and are termed on this account Opisthogoneata. 



Of the development of Symphyla and Pauropoda nothing is 

 known. A certain amount of work has been done on the development 

 of Diplopoda or Millipedes, but the development of a Centipede, 

 Scolopendra, has been worked out in great detail by Heymons (1901) 

 and has yielded most interesting results. 



SCOLOPENDRA 



Scolopendra appears to be quite intermediate in i^s development 

 between Peripatus and Insecta, as was to be expected, and there can 

 be no doubt that in many of its features it represents a stage passed 

 through by Insecta in their ancestral history. Heymous' main results 

 concern (a) the segmentation of the egg and the formation of layers ; 

 (&) the segmentation of the body and the development of the 

 appendages ; (c) the development of the nervous system ; (cT) the 

 development of the coelom ; and (e) the invagination of the 

 embryonic rudiment into the yolk, and its subsequent evagination. 

 We shall say a few words on each of these points in order. 



(a) In the earliest stage examined the egg shows an incomplete 

 division of its substance into columnar peripheral segments which 

 abut on a central unsegmeuted portion. This reminds one of the 

 condition which Reichenbach found in the eggs of Astacus, which he 



