THE MACHINERY OF ALIMENTATION, 51 
tained within it, and which is represented by tho various 
divisions of the alimentary canal, with its appendages ; 
by the apparatus for the distribution of nutriment; and 
by two apparatuses for getting rid of those products 
which are the ultimate result of the working of the whole 
organism. 
And here we must trench somewhat upon the province 
of Morphology, as some of these pieces of apparatus are 
complicated ; and their action cannot be comprehended 
without a certain knowledge of their anatomy. 
The mouth of the crayfish is a longitudinally elongated, 
parallel-sided opening, in the integument of the ventral 
or sternal aspect of the head. Just outside its lateral 
boundaries, the strong mandibles project, one on each 
side (fig 3, B; 4); their broad crushing surfaces, which 
are turned towards one another, are therefore completely 
external to the oral cavity. In front, the mouth is over- 
lapped by a wide shield-shaped plate termed the upper 
lip, or labrum (figs. 3 and 6, 1b); while, immediately be- 
hind the mandibles, there is, on each side, an elongated 
fleshy lobe, joined with its fellow by the posterior 
boundary of the mouth. These together constitute the 
metastoma (fig. 3, B; mt), which is sometimes called 
the lower lip. A short wide gullet, termed the ceso- 
phagus (fig. 6, oe), leads directly upwards into a spacious 
bag, the stomach, which occupies almost the whole cavity 
of the head. It is divided by a constriction into a large 
anterior chamber (cs), into the under face of which the 
R2 
