EXCRETION 



147 



remainder of the tube lies, into the preceding compartment. 

 Thus the nephrostome is in one segment and the nephridiopore 

 in the following. Since the body cavities are filled with a 

 colorless fluid or lymph containing corpuscles, excretory products 

 dissolved in this fluid or in the form of solid particles can be 

 drawn in by ciliary action through the nephrostome and eventually 

 expelled to the outside through the nephridiopore. However, 



since the tubules as a whole are 

 richly supplied with blood ves- 

 sels, excretory material is prob- 

 ably also absorbed directly from 

 the blood through the walls of 

 the tubule. 



Kidney of the Frog.— The 

 kidneys of the frog lie against the 



Sffife* 



Fig. 93. Fig. 94. 



Fig. 93. — Nephridium of earthworm, schematic, neph, nephrostome; se, 

 portion of septum; 1, neck and long narrow coiled part of tubule; 2, 3, 4, 5, 

 includes two wide portions connected by narrow tube; 6, ejaculatory duct; 7, 

 nephridiopore. (From Hesse and Doflein after Maziarski.) 



Fig. 94. — Section of nephrostome of Rana pipiens. o, opening into nephro- 

 stome from ventral surface of kidney; v, vein, with contained blood corpuscles, c. 



dorsal wall of the posterior half of the body cavity, one on either 

 side of the mid-line (Fig. 17). The ventral surface of the kidneys 

 and their edges only are covered by the peritoneum, the dorsal 

 surface being in direct contact with the body wall. The kidneys 

 are therefore retroperitoneal. Each is flattened and oblong in 

 shape and reddish in color. The yellow longitudinal band on the 

 ventral face of the kidney is an adrenal body, which has nothing 

 to do with the excretory function of the kidney. From the outer 

 edge of each kidney a ureter passes back to open into the cloaca 

 on its dorsal side near the mid-line. The functional unit of the 

 kidney is the uriniferous or renal tubule, each of which bears a 



