92 PROCEEDINGS OF THE ACADEMY OF [March, 



"Clock Face Notation") as the three folds of columnar epithelium 

 seen from sections 40 to 80 (Figs. 6 and 2). Histologically, too, 

 the epithelia of the folds 4, 8 and 12 of these two portions of the 

 gut appear similar. These folds have been described by Calvert in 

 the larva of this species (1911 b, p. 452, Plate XVII, Fig. 10). 



The rectum extends from about section 160 to section 380 (Fig. 6), 

 that is, through nearly all of segment VIII, all of segment IX, and 

 the anterior fourth of segment X. Figure 4 represents the appear- 

 ance of the rectum from section 200 to section 300. From within 

 outward can be seen: the chitinous intima, ?'; the folds of columnar 

 epithelium, e; the basement membrane, b; the non-continuous 

 circular muscular coat, cm; and the longitudinal strands of muscle, 

 Im. (These last are located at the positions 1, 3, 5, 7, 9, 11, of the 

 clock face notation, but do not stand out very clearly in the photo- 

 graph). Spaces between the basement membrane and the circular 

 muscle coat are filled with blood, and Malj^ighian tubules also 

 extend into them, e.g., in TS. 315, 319, 324, 327, 342, etc. In this 

 part of the rectum almost the entire lining of the gut consists of the 

 epithelium of the three longitudinal folds. The latter are, however, 

 separated by three small pigmented areas (Fig. 4). These are the 

 "non-cellular" bands described by Calvert in larva No. 1, (1911 b, 

 p. 452, Plate XVII, Fig. 10). I have examined the original slide 

 containing the rectum of larva No. 1 and found no trace of cells or 

 nuclei in these three areas. In Fig. 4, one of these pigmented bands 

 (mvpa) is seen to be midventral, the other two (rlpa and llpa) are 

 dorso-lateral in position. At section 320, where the longitudinal 

 folds 2, 6, 10 begin to appear in the pigmented bands, the cellular 

 nature of these areas becomes somewhat more apparent. (Compare 

 Cullen, 1918, fig. 1, and Jamieson, 1918, fig. 1.) 



Mecistogaster larva No. 1, described by Calvert, was older than 

 No. 3, the subject of this paper. Whether the age of the larva has 

 anything to do with the presence or absence of epithelial cells in the 

 pigmented rectal bands I am unable to say. It does not seem likely 

 that it is a matter of fixation as larvae No. 1, 2, and 3 were all fixed 

 in the same way (hot water) and at the same time. 



The three broad folds of the rectal epithelium and the three small 

 pigmented areas mentioned in the two preceding paragraphs are 

 homologous with similar folds and areas in the rectum of Thauma- 

 toneura (Calvert, 1915, p. 387) and Argia (Cullen, 1918, p. 77, Fig. 1). 

 The folds are, however, much shallower than in Thaumatoneura (Cal- 

 vert, 1915, Plate XVII, Fig. 1) and Argia talamanca (Jamieson, 1918, 



