308 



ZOOLOGY. 



comparison of that figure with fig. 154 will show the nature of the 

 flexion and extension of the abdomen. 



10. Endophragmal System. A further development of the plan of 



Fig. 153. MEDIAN SECTION OF CRAYFISH, WITH ABDOMEN EXPENDED 

 To show flexor and extensor muscles. 



exoskeletal ingrowths for muscular attachment is seen in the thorax, 

 where a complicated series of hard arches is formed in this manner, 

 constituting what is known as the endophragmal system (fig. 154). 

 This is arranged as follows. 



Fig. 154. MEDIAN SECTION OF CRAYFISH, WITH ABDOMEN FLEXED. 

 To show endophragmal system. 



From the side-wall of the thorax a paired metameric series of 

 ingrowths (endopleurites) passes inwards towards the middle line, 

 forking in a Y-shaped manner, the ends of each Y being in contact 

 with corresponding ends of the next Y in front and behind. At 

 the same point of contact they are joined by the top of one of 



