GENERAL CONSIDERATION*. 



391 



Arthropoda (Protostraca) and, through the latter, to the Annelida 

 (cf. Vol. ii., p. 315, and Vol. iii., p. 427). 



The Insecta represent the highest grade of development of this 

 phyletic series. That they are more highly specialised than the 

 Myriopoda can be seen in the sharper demarcation of the different 

 regions of the body, the fixation of the number of body-segments, 

 and the development of a new locomotory system, the wings. 



The marking-off of the three regions which can be distinguished 

 in the body of the Insect (head, thorax, and abdomen) seems to 

 be foreshadowed in the Myriopoda. Here also we find an anterior 

 region, the head, sharply distinguished from 

 the rest of the body. Further, of the 

 trunk-segments that follow this region, the 

 anterior (thoracic) segments may be dis- 

 tinctly differentiated from those which 

 follow (the abdominal region) ; thus, by 

 way of example, we may recall the fact 

 that in the Diplopoda the thoracic segments 

 do not unite to form double segments, as 

 is the case with the other trunk-segments. 

 We have, however, already pointed out 

 (p. 236) that the region here distinguished 

 as the thorax cannot be entirely identified 

 with the thorax of the Insecta, since, in 

 the Diplopoda, a limbless segment is inter- 

 calated between the three limb -bearing 

 segments of the thorax (Fig. 121 B, p. 235, 

 and Fig. 122, p. 237), a modification not 

 found in the Insecta. 



Although the division of the body into 

 regions can also be recognised as indicated 



in the Myriopoda, it is much more distinctly marked in the Insecta. 

 The boundary between the thorax and the abdomen especially is 

 much more distinct. This is connected with the division of labour 

 between the two regions. In the Insecta, the most important loco- 

 motory organs are concentrated in the thoracic region. This has led 

 to the greater rigidity of the thorax and the development of large 

 masses of muscle, while the softer, more extensible abdominal region 

 is the receptacle for almost all the vegetative organs. Into this region 

 have shifted the most important parts of the intestinal canal and of 

 the respiratory and circulatory systems, as well as the genital organs. 



Fig.192.— Scolopendrella im- 

 maculata (after Latzel, from 

 Lang's Text-book). 



