SEA-FISHERIES LABORATORY. 825 



(1) The last five thoracic arthrophragms, beginning 

 in front and working backward (posterior thoracic). 



(2) The two post-oral cephalic arthrophragms and 

 the first three thoracic arthrophragms, beginning behind 

 and working forward (anterior post-oral). 



(1) Posterior Thoracic Endophragmal System 

 (PL III, fig. 18, and text, fig. 5). 



This consists of the arthrophragms of the last five 

 thoracic somites. 



The median plate commences at the posterior end of 

 the fourth thoracic sternum. At first it is extremely 

 shallow, but as it proceeds posteriorly it increases in 

 height. It is present in the last four thoracic somites. 



As in other parts of the endophragmal system, the 

 median plate is composed of two closely applied portions 

 of the exoskeleton. In the fifth thoracic somite these two 

 parts remain separate, and the cavity between them opens 

 to the exterior at the posterior end of the fourth thoracic 

 sternum. 



Each endosternite is at right angles to that part of 

 the sternum from which it arises, and similarly each 

 endopleurite arises at right angles to the epimeron. If 

 the sternum were horizontal throughout its entire length, 

 and also if the epimeral wall at each side were vertical, 

 the endophragmal system would be represented by a series 

 of vertical partitions arranged one behind the other. This 

 is the case in the Macrura. In the Brachyura, however, 

 neither the sterna nor epimera follow this arrangement. 

 The thoracic sternum is extremely convex antero- 

 posteriorly and has an extreme upward tilt at its posterior 

 end. The epimeral wall, instead of having a flat surface, 

 is extremely convex on its outer face. The shape of the 

 sternum and of the epimeral wall gives rise to much 



