1910] Alimentary Canal of Psychoda AUernata 289 



regularly placed, with intervening spaces equal to the breadth 

 of the bands. These broad bands give way to narrower and 

 more widely separated ones, which continue back to the region 

 of tall slender epithelial cells. Sections of these strands show 

 their breadth to scarcely exceed the diameter of the nuclei of 

 the underlying epithelial cells. At the point where the large 

 epithelial cells give way to the tall slender ones, the strands 

 of circular muscles are greatly crowded together, having the 

 appearance of a slight sphincter (Fig. 22, c. m.) This crowding 

 of the circular muscles covers but a short area and posteriorly 

 gives way to a greater spacing of them, which extends back to 

 the base of the Malpighian vessels. 



The longitudinal muscles are not strongly developed in the 

 mid-intestine. They consist of from ten to twelve narrow, fiat 

 strands, which fit closely to the surface of the circular muscles 

 (Fig. 21 and 23, 1. m.) Opposite the point, where the circular 

 muscles are crowded together, these longitudinal strands are 

 greatly thickened, almost circular in cross section and stand out 

 from the circular bands more distinctly. 



Just at the union of the mid- and hind- intestine are the 

 five Malpighian vessels. In cellular structure they much more 

 closely resemble the epithelium of the mid-intestine than that 

 of the hind-intestine, but their glandular function will account 

 for this, however. From embryological studies it has been 

 shown that these vessels are outgrowths from the hind-intestine. 

 At the point of attachment of these vessels with the alimentary 

 canal they have a short, compact and slightly constricted 

 region (Fig. 26, m. t.) Here the cells are large and alternately 

 from opposite sides extend into the lumen of the vessels giving 

 it a zig-zag course. The nuclei of these cells are oval with small 

 nucleoli and fine chromatin granules. This constricted portion 

 gives way to the much expanded, thin- walled reservoir. The 

 epithelium of this region is very thin, with its inner surface del- 

 icately fibrillate and with valve-like projections here and there 

 extending into the lumen (Fig. 27, c.) Beyond the reservoir 

 comes the true glandular portion of the vessel which is much 

 constricted and, in life, is charged with a reddish brown pig- 

 ment (Fig. 27, a and b). Here the lumen is greatly reduced, 

 being almost closed by the finely granular and vacuolate cells. 

 These cells are very long, as can be seen from longitudinal sec- 

 tions, and as shown by some transverse sections, each cell 



