142 THE MORPHOLOGY OF THE COMMON CRAYFISH. 



scaphognathite lies, clearly indicates the natural boundary 

 between the head and the thorax. It has further been 

 observed that there are, in all, twenty pairs of ap- 

 pendages, the six hindermost of which are attached to 

 the abdomen. If the other fourteen pairs are carefully 

 removed, it will be found that the six anterior belong to 

 the head, and the eight posterior to the thorax. 



The abdominal region may now be studied in further 

 detail. Each of its seven movable segments, except the 

 telson, represents a sort of morphological unit, the repe- 

 tition of which makes up the whole fabric of the body. 



If the abdomen is divided transversely between the 



Fig. Se.—Astacus ftuviatilis.— A transverse section through the nine- 

 teenth (fifth abdominal) somite ( x 2). e.m., extensor muscles ; /.«.., 

 flexor muscles ; gn. 12, the fifth abdominal ganglion ; h.g., hind-gut ; 

 i.a.a., inferior abdominal artery ; s.a.a, superior abdominal artery ; 

 pi. XIX, pleura of the somite ; st. XIX, its sternum ; t. XIX, its 

 tergum ; e.p. XIX, its epimera ; 19, its appendages. 



