vii DIGESTION 373 



the gizzard (Fig. 93) : when they comract 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 produced 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 straining 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 

 absorption of the digested products, are performed by 

 a pair of large glands (Fig. 93, Ir), lying one on either 

 side of the gizzard and anterior end of the intestine. 

 They are formed of finger-like sacs or cceca, 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 consisting 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 



