TIE 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 maxille (4, 6) 
the mandibles (4), the antenne (3), the antennules (4), 
the eyestalks (1), and the six somites to which they are 
attached (I—V1I), lie in front of the boundary and com- 
pose the head. 
Another important point to be noticed is that, in frout 
of the mouth, the sternum of the antennary somite (fig. 
43, ITT) 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 (LJ) is at right angles to the latter ; and 
that of the eyes (J) 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 whatis 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, 
M2 
