348 



HISTOLOGY 



middle tubule; on the other hand, it merges slowly into the wall of 

 the wide, large tubule, so that no sharply defined boundary is formed. 

 *, Its cytoplasm is dif- 



ferentiated into an in- 

 ner and an outer layer. 

 The inner layer is ho- 

 mogeneous except for 

 excretion particles 

 which it may contain. 

 The outer layer of 

 cytoplasm is distinctly 

 striated. The striae 

 extend radially from 

 the periphery to the 

 inner layer of finely 

 granular cytoplasm. 



The general striated 



** Q*' appearance is distorted 



FIG. 310. Sections of earthworm's tubule below "ampulla." f n 1 Pr pc K v PY rrptmn 

 ex.gr., excretion particle ; ex. vac., excretion vacuole. X 870. ai P iace ^ D 7 excr( 



bodies and vacuoles 



(Fig. 309, ex.}. The nuclei of this region are large and oval. They 

 have the same general structure as those seen in Figures 308 and 310. 

 This region of the tubule has apparently the power to elaborate excretion 

 products. The syncytium of the wide, large tubule lacks the inner layer 

 of "cell substance" or cytoplasm; otherwise it appears to be a structure 

 similar to the "ampulla." Figure 310 was taken from a section near the 

 ''ampulla." Here excretion vacuoles and an excretion body are shown, 

 ex vac. and ex. gr. These become less and less frequent as the tubule 

 leads from the "ampulla." The striae also become gradually less distinct 

 farther from the ampulla. The muscular duct is well developed. Its 

 diameter is about the same as that of the wide, large tubule. It is yet 

 uncertain as to whether its lumen is intra- or inter-cellular. The cyto- 

 plasm of its wall is difficult to demonstrate. Certain sections may show 

 it bearing as many as three nuclei in a single section; others show no 

 nuclei and little cytoplasm. The muscles are distributed throughout 

 the wall of the muscular duct in various directions. "The muscular 

 duct penetrates the body wall, the muscles appearing to be continuous 

 with those of the cuticular layer; a slight invagination of the epidermis 

 meets the nephridial lumen, and puts the latter into communication with 

 the exterior." 



The closed nephridium of the Crustacea. The nephridial tubule 

 is free of solenocytes, but in the higher forms it becomes com- 

 plex. 



