40 



with the dorsal border of the sixth endosternite. Between 

 this arthrophragm and the seventh endoplenrite there is 

 a large foramen. 



Pig. 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. 



LP. - VII. P. = pleural 

 muscle chambers of 

 thoracic somites 1 — 7. 



I.S. — VI 1. 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. — endoplenrite. 



St. = sternum. 



m. = 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. 



