THE ALIMENTARY CANAL. 



b. The Alimentary Canal of the Blow-fly Larva. 



55 



General Structure. — The wall of the whole alimentary tube 

 exhibits a peritoneal, a muscular, and an epithelial coat. 



The Peritoneal Coat (PL I., Figs. 2„pt, and 6,/)) is the most 

 external. It consists of stellate mesoblastic cells, endothelial 

 plates, and tracheal vessels ; it forms part of the wall of a 

 great blood sinus, which surrounds the whole alimentary 

 tract. 



The Muscular Coat consists of two layers, an external 

 longitudinal and an internal circular set of striated fibres. 

 Those of the pharynx and rectum resemble the somatic 

 muscles ; but the remainder of the muscular coat consists of 

 flat branching bands of less distinctly striated fibres, without 

 any myolemma, which do not exceed 8'^ to lo" in their broadest 

 transverse diameter. 



The Epithelial Coat consists of flat polygonal cells in the 

 stomodcGum and proctodaeum, and of cubical or columnar cells 

 in the rest of the alimentary tube. In the stomodaeum and 

 proctodaeum there is also a thick cuticular intima ; in the 

 mesenteron this intima is either absent or much thinner, 

 but the cells secrete a mucoid layer on their inner surface. 

 In the larvae of some Nematocera, however, a distinct cuticular 

 lining can be seen in the living animal, separating the coarser 

 particles of food from a clear fluid, which intervenes between 

 the cuticle and the epithelium. Hammond [24] describes the 

 same thing in the chyle stomach of the Crane-fly larva. 



It appears to me that the difference between the mesenteron 

 and the stomod^um and proctodaeum does not consist in the 

 absence of a cuticular layer in the former, but in its non- 

 adherence to the cells. In the anterior and posterior sections 

 of the gut the cuticular intima is only shed at an ecdysis, 

 whilst that of the mesenteron is shed during digestion, and 

 probably only forms a net-like mucoid envelope around the 

 coarser particles of food material. 



Basement Membrane. — There is a very thin cuticular mem- 

 brane between the epithelial cells and the muscular coat ; and 



