-506 



ZOOLOGY 



SECT. 



tUt 



e.0. 



t- 



401. Astacus fluyiatilis, 



dissection from the right side. 

 mi. antennary artery; al>. abdo- 

 men ; an. anus ; b. (/. bile duct ; 

 If. -4, cheliped ; lnn, ventral nerve 

 cord ; cs. cardiac division of 

 stomach ; cth. cephalo-thorax ; 

 em, dorsal muscles; fin, ventral 

 muscles ; g, brain ; It. heart ; lul, 

 large intestine; Ir. liver; md, 

 small intestine ; o. ostium ; ore. 

 ophthalmic artery ; oaa, superior 

 abdominal artery ; <c, gullet; 

 />/. l~i, pleopods ; pi. 6, m-opod ; 

 jix, pyloric division of stomach 

 sa. sternal artery; t. testis and 

 tulsi.n ; I'IKI, inferior abdominal 

 artery; vd. vas deferens ; vdo, 

 male genital aperture. (From 

 Lung, after Huxley.) 



proximal edge of the segment to be 

 moved, the smaller on the extensor 

 (ext.), the larger on the flexor (/.) 

 side, in each case half-way between 

 the two hinges, so that a line join- 

 ing the two muscular insertions is 

 at right angles to the axis of articu- 

 lation. 



The digestive organs are con- 

 structed on the same general plan 

 as those of Apus, but present many 

 striking differences (Fig. 401). 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 stomach, which occupies 

 a great part of the interior of the 

 head, and is divided into a large an- 

 terior or cardiac division (c. s), and 

 a small posterior or pyloric division 

 ( i is) : the latter passes into a narrow 

 and very short small intestine (md), 

 from which a somewhat wider large 

 intestine (hd) extends to the anus (an.), 

 situated on the ventral surface of the 

 telson. 



The outer layer of the enteric canal 

 consists of connective tissue contain- 

 ing striped muscular fibres : within 

 this is a single layer of columnar 

 epithelial cells. In the gullet and 

 stomach, and in the large intestine, 

 the epithelium secretes a layer of 

 chitin, which thus constitutes the 

 innermost lining of those cavities. It 

 is proved by development that the 

 small intestine, which has no chitinous 

 lining, is the only part of the enteric 

 canal developed from the mesenteron : 

 the gullet and stomach arise from 

 the stomodseum, the large intestine 

 from the proctodseum. Thus a very 

 small portion of the enteric epithelium 

 is endodermal. 



