28o AN INTRODUCTION TO THE STUDY OF FOSSILS 



these is united to its fellow by a hinge-joint, hence movement 

 can occur in but one plane, as at the knee and elbow in man; 

 but since the axis of the hinge is different at successive joints, 

 each entire leg can move in all directions. Movement is effected 

 by an extensor and a flexor muscle fastened to the sides of each 

 tube-like section and inserted in the sides of the proximal end 

 of the next outer section; the movable arm of the pincer is 

 similarly worked. 



Digestive system. — The food of the crayfish is varied ; it may 

 be animal or plant, living or dead ; it is largely decaying animal 

 matter. On account of their need of lime they are fond of stone- 

 wort (Chara). The smaller particles of food caught by the 

 toothed bases (gnathobases) of the three anterior pairs of tho- 

 racic feet are carried forward by them to the mouth, as are the 

 larger pieces caught by the huge claws. There the food is 

 torn into bits by the toothed edges of the strongly calcified 

 mandibles situated at its sides. Anteriorly the mouth is 

 bounded by a single plate, — the upper lip (labrum or hypo- 

 stome) ; posteriorly by a pair of delicate lobes, — the lower lip. 

 The food passes from the mouth through the oesophagus to the 

 gastric mill, a division of the stomach ; this is lined with chitin 

 and has projecting into it from its walls three calcified teeth 

 (see Fig. 125). The mill is so moved by two pairs of muscles 

 extending from it to the thoracic skeleton that the three teeth 

 meet in the middle and thus complete the comminution of food 

 begun by the mandibles. The food then passes to the smaller 

 or pyloric division of the stomach ; the numerous hairs which 

 extend across this division act as a sieve to prevent all but the 

 very fine food from passing on into the intestine. The stomach 

 is thus merely a masticating and straining apparatus ; digestion 

 takes place in the small intestine. Into this is poured from the 

 single pair of digestive glands a yellow fluid which digests both 

 the proteids and fats, combining thus the function of both 

 pancreas and liver of the vertebrates. The undigested remnant 

 passes out through the large intestine and finally through the 



