CH. Il] THE CRAYFISH 55 



downwards upon 3, using cd as hinge-line, and 4 must be 

 bent sharply on 5, using mn as hinge-line. Lastly, per- 

 forate the corner of 6, 6 and of 2, 2, and by a single wire 

 (to allow a certain amount of rotation) unite right hand 2 

 to right hand 6, and left hand 2 to left hand 6, in each 

 case 2 being outside 6. To do this 6, 5, 6 must be bent 

 like a bow, its right and left arms being thrust downwards 

 and inwards. The model will then be as in Fig. B. 



If now the pieces 8, 8 and 7, 7, which represent the 

 anterior and posterior gastric muscles, are pulled so as to 

 represent the effect of a muscular contraction the three 

 teeth come sharply together, but are separated again and 

 the whole model brought back to its original condition of 

 the elasticity of the cardboard. Of course in the actual 

 stomach of the crayfish the gaps between the ossicles 

 (vide Fig. C) are filled in with thin, flexible chitin. By 

 carefully adjusting the size and direction of the 3 teeth in 

 the model and further by hardening them with sealing-wax 

 or similar material, they may be made to grind bread, etc. 

 into small fragments. A sectional view is shown in Fig. D. 



The food in a state of fine division is exposed in the 

 intestine to the action of the digestive fluid secreted by 

 the so-called ' liver/ whose ducts open into the extremely 

 short mesenteron. The secretion has an acid reaction and, 

 in addition to ferments analogous in their action to those 

 produced by human salivary, gastric, and pancreatic glands 

 contains also a coloured substance called haematin, which 

 is an organic compound of iron. It appears that the 

 ' liver ' is not only a digestive gland but is also concerned 

 in the preparation and storage of pigment which is perhaps 



