CH. II] THE CRAYFISH 53 



than let go of its prey when once seized by the chelae. 

 This tenacity is of great value to it in wearing down the 

 strength of a struggling opponent : on the other hand, a 

 few spasmodic jerks of its abdomen suffice, as a rule, to 

 take the animal to a place of safety. 



Food and Digestion. The nature of the food has 

 already been mentioned. On reaching the ' stomach ' it 

 undergoes preparation for the action of the digestive fluids 

 to which it is subsequently exposed. The ' stomach ' con- 

 tains a number of ossicles, three of which carry teeth 

 projecting into the cavity. These teeth are caused to 

 clash together and thus triturate the food by the action of 

 muscles, some of which are extrinsic and attached to the 

 outer covering of the body, while others lie in the wall of 

 the ' stomach ' itself. At the hinder end of the organ is 

 a sieve of crossed bristles which allows only finely divided 

 matter to pass through. The action of this ' gastric mill ' 

 may be most easily understood by constructing a card- 

 board model as follows : 



Cut out a piece of card shaped as in Fig. A. Along 

 ab, cd, ef, hi, and mn cut just the surface of the card with 

 a penknife ; do the same, but on the opposite face of the 

 card, along gk and lo. Then bend slightly downwards the 

 triangular pieces 2, 2 ; turn 9, 9 under the piece 6, 5, 6 

 until the lower surfaces of 9, 9 are flat against that of 

 6, 5, 6 : stitch the shaded part of 9, 9 firmly by thread or 

 fine wire to 6, 5, 6 ; then bend the unshaded part of 9, 9 

 till at right angles to the shaded part, using lo as hinge- 

 line. These projecting pieces of 9, 9 then represent the 

 lateral teeth. 



