VI DIGESTION 359 



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 sets, 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 straining apparatus — in fact it is not a stomach at 

 all in either the embryological or in the physiological sense. 

 On each side of the anterior division is found, at certain 

 seasons of the year, a plano-convex mass of calcareous 

 matter, the gastrolith or " crab's-eye," which apparently 

 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. 86, Ir), lying one on each side of the 

 gizzard and anterior end of the intestine. They are formed 

 of finger-like sacs or caca, 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 the 

 yellow fluid they secrete digests proteids as well as fat, the 

 name hepato-pancreas is often applied to them, or they may 

 be called simply digestive glands. The crayfish is car- 

 nivorous, its food consisting largely of decaying animal 

 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 



