XI. 



PHYLUM ARTHROPODA 



535 



The digestive organs are con- 

 structed on the same general plan as 

 those of Apus, but present many strik- 

 ing differences (Fig. 443). The mouth 

 lies in the middle ventral line of the 

 head, and is bounded in front by the 

 labrum, 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 

 gizzard (sometimes termed stomach), 

 which occupies a great part of the in- 

 terior of the head, and is divided into 

 a large anterior division (cs), and a 

 small posterior division (ps) : the latter 

 passes into a narrow and very short 

 portion of the intestine, the mid-gut 

 (md), from which the rest of the in- 

 testine (hind-gut, hd) extends to the 

 anus (an), situated on the ventral sur- 

 face of the telson. 



The outer layer of the enteric canal 

 consists of connective-tissue containing 

 striped muscular fibres : within this is 

 a single layer of columnar epithelial 

 cells. In the gullet and gizzard, and in 

 the hind-gut, the epithelium secretes a 

 layer of chitin, which thus constitutes 

 the innermost lining of those cavities. 

 It is proved by development that the 

 mid-gut, which has no chitinous lining, 

 is the only part of the enteric canal 

 developed from the mesenteron : the 

 gullet and gizzard arise from the stomo- 

 dseum, the hind-gut from the procto- 

 daeum. Thus a very small portion of 

 the enteric epithelium is endodermal. 



In the anterior division of the gizzard 

 the chitinous lining is thickened and 

 calcified in certain parts, so as to form 

 a complex articulated framework, the 



gastric 



on which are borne a 



median and two lateral teeth, strongly 

 calcified and projecting into the cavity 

 of the gizzard. Two pairs of strong 

 muscles arise from the carapace, and 

 are inserted into the gizzard : when 



I 10. 443. Astacus fluviatilis, 

 dissection from the right side. 

 aa. antennary artery ; ab. abdo- 

 men ; an. anus ; b. d, aperture of 

 Uuct of right digestive gland ; 

 If. 4, cheliped ; bm, ventral nerve- 

 cord ; cs. anterior division of 

 gizzard ; cth. cephalothorax ; 

 em, dorsal muscles ; fm, ventral 

 muscles ; ff. brain ; h. heart ; 

 fid, posterior part of intestine ; 

 Ir, left digestive gland ; md, 

 mid-gut ; o. ostium of heart ; oa. 

 right lateral (sternal) artery ; oaa, 

 dorsal abdominal artery ; oe, 

 gullet ; pi. 1 5, pleopods ; pi. 6, 

 uropod ; ps. posterior division of 

 gizzard ; sa. sternal artery ; t. 

 testis and telson ; uaa, ventral ab- 

 dominal artery ; vd. vas deferens ; 

 vdo, male genital aperture. (From 

 Lang, after Huxley.) 



