ANATOMY AND PHYSIOLOGY OF PATELLA VULGATA. 619 



epithelium in the fresh condition and in section, the connective tissue of the 

 folds is covered by epithelium Avhich is in several layers (PL CLI. fig. 35). 

 The lower cells are rounded or polygonal, and present a homogeneous protoplasm 

 crowded with granules of a light green or brownish tinge. A nucleus may 

 here and there be distinguished, but, as a rule, the density of the protoplasm, 

 and the manner in which it is filled with concretions, prevents it being possible 

 to do so. The upper cells of the epithelium are much larger (PL CLI. fig. 

 37), and present a large number of vacuoles. These cells are ciliated; audit 

 has been possible in some sections to make out distinctly the cilia, although 

 in by far the most cases they could not be made out. They are, however, 

 visible in the fresh condition by teasing. As stated by Von Jhering to occur 

 in Tethys, and as suggested by Cunningham in Patella, probably the process 

 of secretion is the absorption from the blood in the lacunar spaces in the walls 

 of the diverticula of the urinary matters, the presence of which in the cells 

 causes them to be vacuolated. Probably, as the lower cells become so filled, 

 they come to the surface, and burst into the lumen of the gland, where they 

 appear as granular debris, composed partly of the remains of the epithelial cells 

 themselves, partly of the concretions which they contained. The writer thinks 

 he has been able to make out the successive stages in this process in sections 

 which were found to show best mounted in balsam (PL CLI. fig. 37). (The 

 subject will be referred to in detail in its proper place in Part II.) The 

 epithelial layer varies in thickness at different points. On surface view the 

 polygonal outlines of the cells could be distinctly seen, and in sections mounted 

 unstained in balsam the separate cells were perfectly distinguishable. 



The right nephridium is of far larger size than the left nephridium. It 

 forms a large sac much darker in colour than the left kidney, and extending 

 round the viscera from almost the median line above quite to the median line 

 below, where it ends abruptly in a straight edge. It encircles the posterior 

 part of the genital gland, and rises over the coils of the alimentary canal behind. 

 It ends at the posterior part of the superficial coil of the intestine. In front it 

 is bounded by the anterior body wall, but passes in the form of a long tongue 

 behind the rectum, being bounded in that region behind by the anterior part of 

 the superficial intestinal coil. On dissection it is found to send a corresponding 

 tongue beneath the rectum (the " subanal tract " of Lankester and Bourne), 

 which is like the superficial tongue irregular in outline. Like the left kidney, 

 the right has two outlets — one to the exterior and one to the pericardium. 

 The canal opening to the exterior opens at the right renal papilla situated 

 to the right of the anal papilla. The opening into the pericardium can be 

 easily made out in a large specimen from the pericardial aspect. It lies 

 beneath and slightly behind the opening of the left kidney, and appears when 

 viewed from the interior of the pericardium as a longish pear-shaped slit lying 



