548 



ZOOLOGY 



SECT. 



FIG. 435. Astacus fluviatilis, 

 dissection from the right side. 

 aa. antennary artery ; ab. abdo- 

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

 duct of right digestive gland ; 

 If. It, cheliped ; Im, ventral nerve- 

 cord ; cs. anterior division of 

 gizzard ; cth. cephalothorax ; 

 CHI, dorsal muscles; fin, ventral 

 muscles ; g, brain ; h. heart ; 

 1x1, posterior part of intestine ; 

 Ir. left digestive gland ; md, 

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

 right lateral artery ; oaa, dorsal 

 abdominal artery ; ce, gullet ; 

 pi. 1 5, pleopods ; pi. C>, uropod ; 

 pit, posterior division of gizzard ; 

 sa. sternal artery ; t. testis and 

 telson ; uaa, ventral abdominal 

 art cry ; r<l. vas deferens ; r<>o, 

 male genital aperture. (From 

 Lang, after Huxley.) 



It will be seen that the body- 

 muscles of the Crayfish cannot be 

 said to form a layer of the body-wall, 

 as in Chaetopeds, the abdomen of Apus, 

 &c., but constitute an immense fleshy 

 mass, filling up the greater part of the 

 body-cavity, and leaving a very small 

 space around the enteric canal. 



In the limbs (Fig. 432) each podo- 

 mere is acted upon by two muscles 

 situated in the next proximal podo- 

 mere. These muscles are inserted, by 

 chitinous and often calcified tendons, 

 into the proximal edge of the segment 

 to be moved, the smaller on the ex- 

 tensor (ext.\ the larger on the flexor 

 (fl.) side, in each case half-way be- 

 tween the two hinges, so that a line 

 joining the two muscular insertions is 

 at right angles to the axis of arti- 

 culation. 



The digestive organs are con- 

 structed on the same general plan 

 as those of Apus, but present many 

 striking differences (Fig. 435). 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 (oc) into 

 a capacious gizzard (sometimes termed 

 stomach), which occupies a great part 

 of the interior of the head, and is 

 divided into a large anterior division 

 (c. s), 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 intestine (hind-gut, 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 epi- 

 thelial cells. In the gullet and gizzard, 



