1910] Alimentary Canal of Psychoda Alternata 291 



Posteriorly the constricted ileum projects into the expanded 

 colon so as to form a slight valve (Figs. 2 and 33, v.) Just 

 in front of this valve is a strong sphincter composed of five or 

 six strong strands of circular muscles. The colon is slightly 

 shorter than the ileum but its diameter is twice as great. Its 

 epithelium is very thick, consisting of large cubical cells, of uni- 

 form finely granular nature, with distinctly striate inner curv- 

 ing margins which project into the lumen (Figs. 35 and 36, ep.) 

 The cell walls are seldom distinct to the base. The nuclei are 

 very large, and oval, with large nucleoli and fine chromatin 

 granules. The basement membrane is very delicate and diffi- 

 cult of detection in most preparations. The circular muscles 

 are strongly developed and are so fitted into pockets in the epi- 

 thelium that their outer surface is just even with that of the 

 epithelium (Fig. 36, c. m.) Longitudinal muscles have not 

 been observed either in longitudinal or cross sections of the 

 colon as well as the rectum. Sections of close-fitting nerve 

 fibers were at first mistaken for longitudinal muscles. The 

 chitinous intima is but slightly developed and closely adheres 

 to the epithelium. Along the surface of the intima and espec- 

 ually down in the grooves between the cells is a layer of a finely 

 granular deposit which stains blue with haematoxylin. This 

 deposit is present in all of my preparations, which together with 

 the enormous development of the epithelium, would suggest 

 some association with absorption and I am of the opinion that 

 in this larva much of the absorption of digested food takes 

 place here. 



The circular muscles of the anterior end of the rectum are 

 much more strongly developed than are those of the colon 

 (Fig. 37, c. m.) They lie on the surface of the epithelium and 

 form a continuous layer. Farther caudad the individual bands 

 decrease in strength rapidly. These circular muscles form a 

 sort of sphincter about the rectum. The nuclei of the circular 

 muscles are very large and are surrounded by undifferentiated 

 protoplasm. The basement membrane is more strongly devel- 

 oped than in the colon. The epithelium is greatly reduced 

 especially in the caudal portion where the cells are long and thin 

 except opposite the nuclei, where they are thickened so as to 

 protrude slightly into the narrow lumen. In the anterior region 

 of the rectum the epithelium is thrown into six longitudinal 

 folds (Fig. 37, ep.) The epithelium of the rectum is continuous 



