266 Annals Entomological Society of America [Vol. XV, 



distinct layers; upon the inside is the lining or the intima, then the 

 large celled wall and the covering which is muscular, Plate XVIII, 

 Figs. 7 and 8. The dorso-lateral and lateral tubules reach the surface 

 posterior to the ventral pair. 



Intestine: The transition to intestine in H. analis is accomplished 

 with a mere constriction and change in cellular structure, but in L. 

 indivisus there is a valve formed as the cesophageal valve is formed, but 

 reversed in its components, the upper part of the fold is slightly columnar 

 in its cellular structure but covered with intima in a thin coat, the intima 

 becomes thicker and the cells cubical as the folds turn toward the 

 intestinal wall, Plate XV, Fig. 10. At the posterior edge of segment 

 seven the cubical wall changes and becomes large celled with large 

 nuclei, Plate XVI, Fig. 2. In H. analis this continues to the anterior 

 margin of the ninth segment, Plate XVI, Fig. A at 2. At the beginning 

 of the ninth segment the epithelial wall changes to a cubical condition 

 which changes only as the wall becomes differentiated into the blood 

 gills and returns to the cubical condition when this function ceases. 

 The whole length of the intestine is thrown into six longitudinal folds 

 which begin in the sixth segment, Plate XVI, Fig. 6, and is traceable 

 through all the sections to the rectum where the semi-valve occurs 

 and the four blood gills arise. The folds do not continue into the rectum. 

 In L. indivisus transition in the eighth segment from the cubical celled 

 condition to the large celled, which marks the end of the intestine in 

 this form, is a distinct rectal valve not different in character or forma- 

 tion from the one into the intestine from the mesenteron; the cells of 

 the wall of the rectum are large with large nuclei and this condition 

 continues to within .5 mm. of the anus. It is possible to consider the 

 rectum of H. analis as beginning at this level and the semi-valve as only 

 the transition into the blood gills, but the gross anatomical structure 

 seems to bear the former interpretation. 



Musculature: The oesophagus is surrounded with bundles of longi- 

 tudinal muscles, outside of which is a layer of banded circular muscles 

 and outside of these six pairs of longitudinal muscles so noticeable in 

 gross structure. At the anterior end of the proventriculus the inside 

 layer of longitudinal muscles cease and the proventriculus is imme- 

 diately surrounded by circular muscles, (see Noyes 1915), which con- 

 tinue as a sheath of varying thickness for the full length of the alimen- 

 tary canal as far as the rectum. 



Mesenteron: The six pairs of longitudinal muscles break up into a 

 layer of longitudinal muscles, this layer is composed of large bundles 

 upon the outside and small ones upon the inside next to the thin cir- 

 cular muscles. See Plate XVI, Fig. 7. 



Intestine: At the anterior end of this division where the cells of 

 the epithelium are columnar but covered with intima, a heavy circular 

 muscle arises pushing some of the longitudinal muscles outside and 

 enclosing the smaller ones, Plate XVI, Fig. 6. In a very few sections 

 of .008 mm. in thickness this muscular wall is complete, Plate XVI, 

 Fig. 5. The inner layer of circular muscles becomes thicker as we 

 advance toward the posterior end of the canal and the inner longitudinal 



