372 



THE CRAYFISH 



ctft 



part of the enteric canal deve- 

 loped from the enteron of the 

 embryo : the gullet and gizzard 

 (fore-gut) arise from the stomo- 

 daeum, the hind-gut from the 

 proctodaeum (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, usually known as 

 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 



FIG. 93. Dissection of Fresh-water Cray- 

 fish, made by removing the exo- 

 skeleton with the appendages, and the 

 muscles, digestive gland and excretory 

 organ, of the right side. 

 . antennary artery ; ab. abdomen ; an. 

 anus ; b. d. aperture of right digestive 

 duct exposed by removal of gland ; bf. 

 4, cheliped ; bm. ventral nerve cord ; 

 cs. anterior division of gizzard ; cth. 

 cephalothorax ; ejn. dorsal muscles ; 

 fm. ventral muscles; g. brain; //. 

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

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

 ostium of heart ; oa. ophthalmic artery. 



oaa. dorsal abdominal artery ; oe. gullet ; pi. 15, pleopods ; pi. 6, uropod ; ps. 



posterior division of gi/^ard ; s. a. sternal artery ; t. (near heart), spermary ; t. 

 "(below an) telscn ; uaa. ventral abdominal artery; v. d. spermiduct ; vdo. 



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



