THE ALIMENTARY CANAL AND ITS APPENDAGES. 53 



Section IV. The Alimentary Canal and its Appendages. 

 PLJLTE IV. Figs 18. 



The o?>ophagus commences at the posterior extremity of the 

 pharynx, and curving sharply backwards, pierces the cephalic 

 ganglion and passes through the cephalo-thoracic opening, above 

 the great nerve trunk, and beneath the dorsal vessel. As soon 

 as it enters the thorax it divides into two branches of equal 

 diameter. One passes upwards and immediately opens into the 

 proventriculus ; the other runs the whole length of the thorax, 

 beneath the chyle stomach, and passes into the abdomen under 

 the intestine, above the great nerve, and between the salivary 

 glands, to all of which it is closely bound by a plexus of 

 tracheiil vessels, in the meshes of which the nerve cells of the 

 great sympathetic ganglion lie. As soon as it enters the abdo- 

 men it opens into the sucking stomach or crop. 



The oesophagus may be said to have four coats ; the most external of 

 circular and the second of longitudinal muscular fibres ; the third con- 

 sists of membrane insoluble in liquor potassse; and the fourth of 

 pavement epithelium. The membranous coat exists throughout the 

 whole alimentary canal, and bears the epithelial lining ; like the 

 protoderm it is continuous, and insoluble in hot solutions of caustic 

 potash. 1 have no doubt that a new layer is formed in the pupa on the 

 exterior of the old, which is shed like the integument of the larva, but 

 this has not been demonstrated; it differs from the protoderm in develop- 

 ing epithelial cells upon its free surface, but I believe it is continuous 

 with it. 



The sucking stomach consists of a pair of almost hemis- 

 pherical sacs, united anteriorly in the mesial line ; it serves as a 

 reservoir for the food when first taken. There is no organ 

 homologous to this stomach in the larva, but it is developed 

 from a bud which appears in the pupa on the under side of the 

 oesophagus. The organ in the maggot which performs the same 

 function is a large flask-shaped sac, which opens directly into 

 the pharynx, and the oesophagus commences from the under 

 side of its neck. The neck of this sac is not surrounded by the 

 great ganglia, but is anterior to them ; and the sac is dorsal to 

 all the other organs in the larva. When the maggot is feeding 



