358 



THE CRAYFISH 



the enteron of the embryo : the 

 gullet and gizzard (fore-gut) 

 arise from the stomodsum, 

 the hind-gut from the proc- 

 todeum (p. 204). Thus only 

 a very small portion of the 

 enteric epithelium is endo" 

 dermal. 



In the anterior division of 

 the gizzard the chitinous 

 lining is thickened and cal- 

 cified 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 gizzard. 

 Two pairs of strong muscles 

 arise from the carapace, and 

 are inserted into the gizzard 

 (Fig. 86) : when they contract 



Fig. 86. — Dissection of Fresh-water Cray- 

 fish made by removing the exo- 

 skeleton with the appendages, and the 

 muscles, digestive gland and e.vcretory 

 organ, of the right side. 

 aa. antennary artery ; ah. abdomen ; a7i. 

 anus ; b.d. aperture of right digesti\'e 

 duct e.xposed by removal of gland ; bj. 

 4, cheliped ; bn. ventral nerve cord . 

 cs. anterior division of gizzard ; ctli. 

 cephalothorax ; cm. dorsal muscles ; 

 fnt. ventral muscles; g. brain; h. 

 heart ; hd. hind-gut ; Ir. left digestive 

 gland ; md. mid-gut ; o. right lateral 

 ostium of heart ; oa. ophthalmic artery 



oaa. dorsal abdominal artery ; a\ gullet ; //. i — 5, pleopods ; pL 6, uropod ; i>s. 



posterior division of gizzard ; .y. a. sternal artery ; t. (near heart), spermary ; t. 



(below anus), telson ; uaa. ventral abdominal artery ; v. d. spermiduct ; vdo. 



male genital aperture. (From Lang, after Huxley.) 



