xxv NEPHRIDIA 285 



nephridium (Fig. 69) is an extremely delicate tube consisting 

 of two divisions bent at right angles. The outer division is 

 placed vertically, lies in the thickness of the body-wall, and 

 opens externally by a minute aperture, the nephridiopore 

 (Figs. 68 and 69, Nph. p). The inner division is horizontal 

 and lies in the ccelomic epithelium ; passing forward it pierces 

 the septum which bounds the segment in front (Fig. 68, 

 A and B), and then dilates into a funnel shaped extremity or 

 nephrostome (Nph. st\ which places its cavity in free com- 

 munication with the ccelome. The whole interior of the 

 tube as well as the inner face of the nephrostome is lined 

 with cilia which work outwards. 



FIG. 69. A nephridium of Polygordius, showing the cilia lining the 

 tube, the ciliated funnel or nephrostome (Nph. st), and the external 

 aperture or nephridiopore (Nph. p}. (After Fraipont.) 



A nephridium may therefore be defined as a ciliated tube, 

 lying in the thickness of the body-wall and opening at one 

 end into the ccelome and at the other on the exterior of 

 the body. 



In the higher worms, such as the earthworm, the nephridia 

 are lined in part by gland-cells, and are abundantly supplied 

 with blood-vessels. Water and nitrogenous waste from all 

 parts of the body pass by diffusion into the blood and are 

 conveyed to the nephridia, the gland -cells of which withdraw 

 the waste products and pass them into the cavities of the 

 tubes, whence they are finally discharged into the surround- 

 ing medium. In all probability some such process as this 

 takes place in Polygordius. 



