x PHYLUM ARTHROPODA 219 



sometimes counted as appendages serially homologous with 

 the antennae, legs, etc. 



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

 be looked upon as belonging to a pre-oral region corres- 

 ponding 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 antennae, 

 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 re- 

 maining 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. 1 16) are somewhat complicated. 

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

 bounded in front by a shield-shaped process the labium 

 or upper lip, at the sides by the mandibles, and behind by 

 a pair of delicate lobes, the paragnatha. It leads by a short 

 wide gullet (oe.} 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 (cs.\ and a small posterior 

 or pyloric division (ps.) : the latter passes into a narrow and 

 very short small intestine (ind.\ from which a somewhat 

 wider large intestine (hd.) 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 



