68 THE CRAYFISH [CH. II 



eventually give rise to smooth, flat, and slightly concave 

 stones, known as gastroliths 1 , or popularly as "crabs' eyes" 

 in crab and lobster. Before the moult actually occurs the 

 stones are cast off into the cavity of the stomach, and are 

 either ground down in the gastric mill or removed with 

 the lining of the stomach when the moult takes place. 

 These stones are chiefly composed of calcium carbonate, 

 and it seems probable that their substance is derived from 

 the old shell which is about to be cast off, and indeed that 

 the lines along which the shell splits open are thus pre- 

 pared and rendered weaker and less resistant by the 

 removal of their mineral component which is then 

 excreted by the wall of the stomach. When the time for 

 the moult arrives the shell splits across the back at the 

 junction of the thorax with the abdomen; other splits 

 of varying magnitude and extent occur along the limbs. 

 The crayfish, lying on its side, withdraws the cephalo- 

 thorax and its limbs first, and then the abdomen, and 

 remains in hiding and helpless for a few days until the 

 new shell has hardened. It is interesting to note that 

 stores of glycogen (a food substance allied to starch and 

 sugar) are laid up in the connective tissues prior to the 

 moult. Provision is thus made for the period of helpless- 

 ness when the animal is unable to procure fresh food 

 of any kind. 



If frightened or injured a crayfish will often throw 

 off one or more of its limbs, especially if the alarm is 

 caused by one of these being seized : the separation takes 



1 Vide Irvine and Woodhead, Proc.R. Soc. Edinb. xvi. p. 330; Herrich, 

 Bull. U. S. Fish. Commis. p. 93, 1895. 



