vii DIGESTION 373 



are inserted on the gizzard (Fig. 93) : when they contract 

 they move the mill in such a way that the three teeth meet 

 in the middle line and complete the comminution of the food 

 begun by the jaws. The separation of the teeth is effected 

 partly by the elasticity of the mill, partly by delicate muscles 

 in the walls of the gizzard. The posterior division of the 

 gizzard forms a strainer : its walls are thickened and pro- 

 duced into numerous setae, which extend quite across the 

 narrow lumen and prevent the passage of any but finely 

 divided particles into the intestine. Thus the gizzard has 

 no digestive function, but is merely a masticating and strain- 

 ing apparatus it is therefore not a stomach either in the 

 embryological or in the physiological sense. On either side 

 of its anterior division is found, at certain seasons of the 

 year, a plano-convex mass of calcareous matter, the gastrolith 

 or " crab's-eye," which possibly merely serves to store up 

 reserve calcareous material for use after the next ecdysis. 



The digestion of the food, and to some extent the absorp- 

 tion of the digested products, are performed by a pair of 

 large glands (Fig- 93, lr\ lying one on either side of the 

 gizzard and anterior end of the intestine. They are formed 

 of finger-like sacs or c&ca, which discharge into wide ducts 

 opening into the mid-gut, and are lined with glandular 

 epithelium derived from the endoderm of the embryo. 

 The glands are often spoken of as the liver, but as they 

 have a complex function, and the fluid they secrete is able 

 to digest proteids, it is better to call them simply digestive 

 glands. The crayfish is largely carnivorous, its food con- 

 sisting of decaying animal as well as vegetable matter. 



The digestive organs and other viscera are surrounded by 

 an irregular cavity, which is in free communication with the 

 blood-vessels and itself contains blood. This cavity is not 

 lined by epithelium, and is to be looked upon as a large 



