THE MACHINERY OF ALIMENTATION. 51 



tained within it, and which is represented by the various 

 divisions of the alimentary canal, with its appendages ; 

 by the apparatus for the distribution of nutriment ; am] 

 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, lb) ; 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 oeso- 

 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 



