ANNELIDA. 6 1 



organs). Each nephridium is a very long convoluted tube, of 

 varying calibre in its different regions, attached by a membrane 

 to the back of the septum bounding the front of the segment, 

 and placing the coelom in communication with the exterior. It 

 is thrown into three main folds running parallel to the septum in 

 a vertical direction, and forming an internal short loop, a middle 

 long loop, and a wide external end loop. The nephridium com- 

 mences with a ciliated funnel which lies in the preceding segment. 

 The margin of the funnel is formed by a row of elongated ciliated 

 cells, and its back by a very large crescentic cell. The slit-like 

 internal opening (nephrostome) of the nephridium is situated 

 within the funnel. The rest of the tube is divided into four 

 sections : (1) An extremely long and delicate narrow section which 

 runs back from the funnel through the septum, round the short 

 loop, up and down one side of the long loop, and back round the 

 short loop, after which it passes into (2) a much wider middle 

 section, contained entirely in the long loop, and followed by (3) a 

 wide section, which begins with a dilated part, runs along one 

 side of the long loop, round the short loop, and across to the end 

 loop, where it is succeeded by (4) a much larger muscular section 

 ("bladder") which opens to the exterior by a small ventral 

 aperture, the nephridiopore. The greater part of the nephridium 

 (i.e., from the funnel to (4) ) is made up of tubular " drain-pipe 

 cells," placed end to end, and the cavity of this part is therefore 

 intracellular i.e., within and not between cells. The tubular cells 

 differ in size and character in the different regions of the neph- 

 ridium. 



Cilia are found in parts of the narrow section and throughout 

 the middle section, while the cells of the middle and wide sections 

 are very large and full of excretory granules. The muscular 

 section is lined by cells (its cavity being therefore intercellular), 

 and its thick wall contains a network of muscle-fibres. 



The nephridium possesses a very rich network of capillary 

 blood-vessels, supplied from the ventral vessel and returning its 

 blood into one of the parietals. 



The nephridia are specially concerned with the excretion of 

 water and nitrogenous waste, the former entering them by the 

 ciliated funnels, and the latter being probably excreted by the 

 glandular drain-pipe cells of the middle and wide sections. It 

 is not unlikely that the chloragogenous cells have to do with 



