AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



The extremely short mid-gut (m.g) is not lined by cuticle. Its 

 dorsal wall is produced upwards and forwards into a short blind 

 tube, the ccecum (cce). A large brownish three-lobed digestive 

 gland (h.p) (liver, hepato-pancreas) lies on each side within the 

 cephalo-thorax, and opens by a short, wide duct (bile duct) (b.d) 

 into the mid-gut, of which it was originally an outgrowth. 



Fig. 28. ALIMENTARY CANAL AND EXCRETORY ORGANS or CRAYFISH. 

 A, Alimentary canal ; m, mouth ; Ib, labrum ; mt, metastoma ; ce, gullet : 

 ca and py, cardiac and pyloric ends of stomach; c.o, cardiac sclerite; 

 the horizontal shading just below reference-line shows the commence- 

 ment of the ptero-cardiac sclerite, which tapers to a point below; 

 py.o, pyloric sclerite, running into the zy go-cardiac sclerite, which 

 joins the ptero-cardiac ; a.g.m and p.g.m, anterior and posterior 

 gastric muscles; m.g, mid-gut; cce, caecum; h.p, digestive gland; b.d, 

 duct of the same ; t, intestine ; a, anus. B, Left renal organ ; gl, 

 glandular part ; bl, bladder ; st, style passed into renal opening ; 

 an and an', antennule and antenna ; op, eye ; r, rostrum. 



The hind-gut or intestine (i). which succeeds the mid-gut, is a 

 narrow thin-walled tube running back in the middle line, between 

 the abdominal flexor and extensor muscles, to open by the anus (a). 

 It is lined by cuticle, and six slightly-twisted longitudinal ridges, 

 covered by minute elevations (papillae) project into its cavity. 



The alimentary canal is lined throughout by epithelium one 

 layer of cells thick, which in the fore and hind guts secretes a 



