NEPHRIDIAL TISSUES 



349 



In the lobster, Homarus vulgaris, the nephridium has an end-sac and 

 a middle convoluted tubule both of which eliminate waste products. 

 The third and terminal region has been expanded to form a retaining 

 vessel for the fluids excreted. The vessel delivers its contents through 

 a pore to the exterior. The entire tubule is covered by a tunica 

 propria of connective tissue, and is lined with a non-ciliated colum- 

 nar epithelium. The cells of the end-sac are irregular and have the least 

 dense reticular cytoplasm. Distally they bear large vacuoles in which 

 excretion products are found. The nuclei are in the basal third of the 

 cell (Fig. 311). The epithelium of the convoluted middle part of the 



FIG. 311. Section through parts of two regions of lobster's nephridium. ex.p., excretory 

 products; ex.v., excretory vacuoles. X 425. 



tubule is composed of stout cells with very distinctive features. The 

 nuclei are spherical and have a conspicuous reticulum. There may be 

 two nuclei in a single cell. The ends of the cells bear a well-defined 

 cuticula. Beneath this there is a zone of reticular cytoplasm which 

 tends to be striated toward the middle of the cell. The basal zone of 

 cytoplasm is highly striated. These striae lie at right angles to the base 

 of the cell. Vacuoles containing excretion products are found within the 

 cytoplasm (Fig. 311, at ex. p.}. When these cells become highly active 

 the cytoplasm at their free ends becomes highly vacuolated, and the 

 cuticula becomes greatly broken in its contour. The cells lining the 

 storage region or "bladder" of the tubule lack a cuticula. The vacu- 

 olization here is at the basal end of the cells instead of at the free 

 ends. The marginal zone of cytoplasm is dense. Toward the base a 



