X] LUMBRICUS 147 



directions. Thus some of it descends to the sub-neural vessel by 

 a vessel situated in each septum. Some of it streams out to the 

 gut and enters the plexus of fine vessels surrounding it in which it 

 absorbs the products of digestion and passes by other vessels into 

 the dorsal vessel; and some of it streams out to the nephridia, is 

 there relieved of its nitrogenous excreta, after which it passes on to 

 the parietal vessel. Blood from neural vessels and nephridia passes 

 upwards in the parietal vessel to pour into the dorsal vessel. 



In the earthworm the excretion of the waste nitrogenous 

 material is carried out by the nephridia. They are distributed 

 throughout the body, one pair being situated in each segment, 

 except the last segment and the first three, which have no nephridia 

 (7, Fig. 60, and 2, Fig. 61). The nephridia differ from those 

 of Platyhelminthes, Nemertinea and Rotifera, (1) in being un- 

 branched, since each consists of a single coiled tube, and (2) in 

 that each opens into the coelom at its inner end by a funnel termed 

 the nephrostome. This nephrostome has cilia on its funnel-shaped 

 rim, and these flicker with an untiring movement. The nephro- 

 stome does not lie in the same segment as the rest of the tube but 

 pierces the anterior septum, and projects into the cavity of the 

 segment in front, somewhere near the sub-intestinal vessel. Thus 

 each segment contains a funnel-shaped opening and a tube which 

 opens externally, but they do not belong to the same nephridium. 

 The tube is not straight but is coiled and lies as a white glistening 

 tangle close to the septum in front. Close inspection shows it to 

 be completely enveloped in a loose fold of the peritoneum forming 

 the hinder wall of the septum. 



When we examine a nephridium through a microscope we see 

 that the walls of the tube are very richly supplied with minute 

 blood-vessels. The tube is really a cord of glandular cells placed 

 end to end and traversed by a minute cavity. It is these cells 

 which take up the waste nitrogenous matter from the blood and 

 convey it out of the body. The part of the nephridium nearest the 

 external opening is swollen so as to form a bladder. The cavity 

 is here intercellular instead of piercing the cells themselves, and 

 surrounding it is a muscular wall by the contraction of which the 

 contents are from time to time expelled. 



The blood thus takes digested food to the living cells all over 

 the body and brings from them certain nitrogenous excreta to the 

 nephridia, which cast them out of the body. But the nephridia 

 also exert some action on the other great fluid of the body the 

 coelomic fluid which bathes all the organs of the body. It has 



102 



