220 MANUAL OF ZOOLOGY SECT, x 



If, as seems probable, the eye-stalks and antennules are to 

 be looked upon as belonging to a preoral region corre- 

 sponding to the prostomium, of Nereis, then it will be seen 

 that the body of the crayfish consists of a prostomium, 

 eighteen metameres, and a telson. The prostomium bears 

 eye-stalks and antennules ; the first four metameres are fused 

 with the prostomium to form the head, and bear the an- 

 tennae, mandibles, first maxillae, and second maxillae; the 

 next eight metameres (5th-i2th), constitute the thorax, 

 and bear the three pairs of maxillipeds and the five pairs of 

 legs; the remaining six metameres (i3th-i8th), together 

 with the telson, constitute the abdomen, and bear five pairs 

 of pleopods and one of uropods. 



The digestive organs (Fig. 125) are somewhat complicated. 

 The mouth lies in the middle ventral line of the head, and is 

 bounded in front by the labrum, at the sides by the mandi- 

 bles, and behind by a pair of delicate lobes, the paragnatha. 

 It leads by a short wide gullet () into a capacious stomach, 

 which occupies a great part of the interior of the head, and 

 is divided into a large anterior or cardiac division (c. s) , and 

 a small posterior or pyloric division (ps) ; the latter passes 

 into a narrow and very short small intestine (md), from 

 which a somewhat wider large intestine (M) extends to the 

 anus (an), situated on the ventral surface of the telson. 



In the cardiac division of the stomach the chitinous lining 

 is thickened and calcified in certain parts, so as to form a 

 complex articulated framework, the gastric mill, on which 

 are borne a median and two lateral teeth, strongly calcified 

 and projecting into the cavity of the stomach. Two pairs 

 of strong muscles arise from the carapace, and are inserted 

 into the stomach ; when they contract they move the mill in 

 such a way that the three teeth meet in the middle and com- 

 plete the comminution of the food begun by the jaws. The 



