40 



with the dorsal border of the sixth endosternite. Between 

 this arthrophragm and the seventh endopleurite there is 

 a large foramen. 



; 



Fro. 6 Diagrammatic plan 

 of the endophragmal 

 system to show the 

 muscle chambers. (The 

 corresponding parts 

 are shaded similarly 

 throughout). 



1 7 = thoracic arthro- 

 phragms 1 to 7. 



I. P. VI I. P. = pleural 

 muscle chambers of 

 thoracic somites 1 7. 



1.8. VII. 8. = sternal 

 muscle chambers of 

 thoracic somites 1 7. 



VIII.P.S. = pleuro-sternal 

 muscle chambers of last 

 thoracic somite. 



Ep. = epimeron. 



E.s. = endosternite. 



PI. endopleurite. 



St. = sternum. 



ra. = median plate. 



(2) ANTERIOR POST-ORAL ENDOPHRAGMAL SYSTEM 

 (Text fig. 7). 



This consists of the last three thoracic and the two 

 post-oral cephalic arthrophragms. 



The nature of the epimera and sterna in this region 

 naturally decides the form and extent of the corresponding 

 arthrophragms . 



The apparently great differences between the endo- 

 phragmal system of this region and that of the posterior 

 thoracic somites is due to three main causes. 



