372 



THE CRAYFISH 



CHAP. 



etft 



the enteric canal developed 

 from the enteron of the em- 

 bryo : the gullet and gizzard 

 (fore-gut) arise from the 

 stomodaeum, the hind-gut 

 from the proctodaeum (p. 

 204). Thus only a very 

 small portion of the enteric 

 epithelium is endodermal. 



In the anterior division 

 of the gizzard the chitinous 

 lining is thickened and cal- 

 cined in certain parts, so as 

 to form a complex articu- 

 lated framework, usually 

 known as the " gastric mill," 

 on which are borne one 

 median and two lateral 

 teeth, strongly calcined 

 and projecting into the 

 cavity of the gizzard. Two 

 pairs of strong muscles 

 arise from the cara- 

 pace, and are inserted 



on 



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. (Nat. size.) 

 aa. antennary artery ; ab. abdomen ; 

 an. anus ; 6. d. aperture of right diges- 

 tive duct exposed by removal of gland ; 

 bf. 4, cheliped ; bm. ventral nerve- 

 cord ; cs. anterior division of gizzard ; 

 cth. cephalothorax ; em. dorsal inns 

 cles ; fm. ventral muscles; g. br.nn , 

 h. heart ; ltd. hind-gut ; Ir. left di.^cs- 

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

 lateral ostium of heart ; on. ophthalmic 



artery ; oaa. dorsal abdominal artery ; oe. gullet ; pi. 1-5, pleopods ; pi. 6, uropod ; 



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



t. (below an) telson ; uaa. ventral abdominal artery ; v. d. spermiduct ; vdo. male 



ycnital aperture. (From Lang, after Huxley.) 



