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 
saa, 
Fic. 36.—Astacus fluviatilis.—A transverse section through the nine- 
teenth (fifth abdominal) somite (x 2). ¢.m., extensor muscles ; f.m., 
flexor muscles ; gn. 12, the fifth abdominal ganglion ; h.g., hind-gut ; 
i.a.a., inferior abdominal artery ; s.a.a, superior abdominal artery ; 
pl. XIX, pleura of the somite ; st. XIX, its sternum ; ¢. XTX, its 
tergum ; ep. XZX, its epimera ; 19, its appendages. 
