THE THEORY OF THE SKELETON. 163 



limbs (11 — 14), and the eight somites of which they are 

 the appendages (VII — XIV), lie behind this boundary 

 and belong to the thorax. The two pairs of maxilla? (5, 6) 

 the mandibles (£), the antennae (3), the antennules (2), 

 the eyestalks (1), and the six somites to which they are 

 attached (I— VI), lie in front of the boundary and com- 

 pose the head. 



Another important point to be noticed is that, in front 

 of the mouth, the sternum of the antennary somite (fig. 

 43, III) is inclined at an angle of 60° or 70° to the direc- 

 tion of the sterna behind the mouth. The sternum of the 

 antennulary somite (II) is at right angles to the latter ; and 

 that of the eyes (I) looks upwards as well as forwards. 

 Hence, the front of the head beneath the rostrum, though 

 it looks forwards, or even upwards, is homologous with the 

 sternal aspect of the other somites. It is for this reason 

 that the feelers and the eyestalks take a direction so dif- 

 ferent from that of the other appendages. The change 

 of aspect of the sternal surface in front of the mouth, 

 thus effected, is what is termed the cephalic flexure. 



Since the skeleton which invests the trunk of the cray- 

 fish is made up of a twenty-fold repetition of somites, 

 homologous with those of the abdomen, we may expect 

 to find that the appendages of the thorax and of the head, 

 however unlike they may seem to be to those of the ab- 

 domen, are nevertheless reducible to the same funda- 

 mental plan. 



