THE BIOLOGY OF AN ANIMAL. 



wall (somatopleure). The inner wall, or splanchnic layer 

 (stpl.m), unites with the entoblast to constitute the wall of the 

 alimentary canal (splanchnopleure). An ingrowth of ectoblast 

 (stomodceum) takes place into the blastopore to form the pharynx, 

 and a similar ingrowth at the opposite extremity (proctodceum) 

 unites with the blind end of the arckenteron to form the anus 

 and terminal part of the intestine. 



As to its origin, therefore, the alimentary canal consists of 

 three portions, viz. : (1) the arckenteron, consisting of tke 



d.v j 

 ch s h V 



n 



n. 



s.i.v 



FIG. 38. Diagram of a cross-section of Lumhriciis, showing the relation of the 

 various organs, etc., to the germ-layers. Ectoblastic structures shaded with fine 

 parallel lines, entoblastic with coarser parallel lines, mesoblastic with cross-lines; 

 o/.c, alimentary canals; c/i, chloragogue layer; c<r, ccelom; c.m, circular muscles 

 of body-wall; c.ma, circular muscles-of alimentary wall; ep, lining epithelium of 

 alimentary canal; il.v, dorsal vessel; fij/i hypodermis or skin; l.m, longitudinal 

 muscles of body-wall ; l.m.a, longitudinal muscles of alimentary wall ; ?i, central 

 part of nerve-cord ; np, nephridium ; JM, sheath of nerve-cord ; p.c, peritoneal 

 epithelium ; r, reproductive organs ; g.i.v, sub-intestinal vessel. 



original entoblast; (2) the stomodaeum or pharyngeal region, 

 lined by ectoblast; and (3) the proctodaeum or hindmost part, 

 also lined by ectoblast. These three parts are called the fore- 

 gut (stomodseum), mid-gut or mensenteron (archenteron), and 

 hind-gut (proctodaeum), and it is a remarkable fact that these 

 same parts can be distinguished in all higher animals, not ex- 

 cepting man. 



The body now becomes jointed by the appearance of trans- 

 verse folds opposite the dissepiments, and the metamerism of the 

 body becomes evident on the exterior. The young worm has 

 thus reached a stage (E^ Fig. 36) where its resemblance to the 



