14 ARTHUR E. SHIPLEY. 



behind the internal, and opens also into the bladder. The 

 opening to the exterior is surrounded by a thickened ring of 

 connective tissue with muscle-fibres intermingling, the latter 

 forming a sphincter. The walls of the passage are folded and 

 lined with cubical epithelial cells. The communication between 

 the internal opening and the bladder is eflfected by means of a 

 short passage, the epithelium of which is ciliated. The walls 

 of the bladder itself are formed of a single layer of cubical 

 cells, a middle coat of irregularly arranged muscle-fibres, and 

 an external investment of peritoneum. The relations of the 

 bladder and its openings will be evident from the diagram, fig. 

 18. The walls of the bladder are very elastic, they contain 

 many muscular fibres, and are lined with cubical epithelial 

 cells. 



The tubular portion of the kidney is a backward prolonga- 

 tion of the bladder, and is attached from the anterior half of 

 its course to the body wall by a mesentery, its posterior half 

 being free. The tube possesses anteriorly a simple lumen, 

 which is broken up posteriorly by a number of septa, producing 

 an appearance which reminds one of that presented by the 

 interior of a frog's lung, the transition between the two regions 

 is very gradual. 



The epithelium lining the tubular portion of the kidney 

 is generally one cell thick ; it is produced internally into a 

 series of long papillse, which are separated from one another 

 by a series of depressions (see figs. 19 and 20). 



The cells forming the papilke are extremely long, and are 

 loaded with fine, yellowish granules. In specimens killed 

 during the functional activity of the organ these papilla-cells 

 are furnished at their inner extremities with a series of large 

 thin-walled vesicles, which appear to be thrown off from time 

 to time into the lumen of the kidney (fig. 20). 



The granules, with which the kidney-cells are loaded, appear 

 to decrease in number as the vesicles are approached ; and it 

 seems possible that the excretory products of the nephridial 

 cells are stored up in the vesicles before being thrown, together 

 with the vesicles themselves, into the nephridial tube. The 



