440 ZOOLOGY SECT. 



the cells lining the tube are active in separating out such waste 

 matters, and are loaded with granules and concretions. The com- 

 munication which they establish with the exterior also enables 

 the nephridia to take on the additional function of reproductive 

 ducts, as will be more particularly noticed in discussing the repro- 

 ductive system. The outer part of the tube is usually more or 

 less dilated : the inner narrower part in some cases consists of a 

 row of perforated cells, the lumen being intra-cellular. The ex- 

 ternal apertures are nearly always ventral in position ; in Polynoe 

 and its allies they are situated on short tiibercles or papilla'. 

 Sometimes the nephridia are confined to a certain region of the 

 body. In some families of Polychseta there is a single pair of large 

 nephridia in the anterior region of the body, with or without 

 smaller pairs in the posterior segments, the former alone appear- 

 ing to have an excretory function. In Sternaspis only a single 

 pair of nephridia is present. In the great majority of the Poly- 

 chseta the nephridia have the function of reproductive ducts as 

 well as of excretory organs. In the Oligochseta the ordinary 

 nephridia do not possess this function : usually they are simple 

 elongated and coiled tubes, a pair or sometimes more than one in 

 each segment ; but in some these are replaced or supplemented 

 in certain of the segments, or in all, by a branching system of 

 tubes with scattered ciliated funnels. In one genus, though paired 

 nephridia are present, they do not open directly on the exterior, 

 but communicate with a branching system of tubes consisting of 

 four principal longitudinal vessels and segmentally repeated trans- 

 verse branches. Sometimes the ordinary nephridia are not de- 

 veloped in the segments lodging the reproductive organs, their place 

 being there taken by three pairs of tubes (mesonephridia) which 

 become modified to give rise to the reproductive ducts ; but ordinary 

 nephridia may be present in these segments as well. In some 

 Ohgochaeta the nephridia of the most anterior segments open 

 into the mouth or pharynx, and have apparently taken on the 

 function of digestive glands (peptonephridia). 



The mode of development of these tubes varies in different 

 instances. In some cases they are formed, like the excretory tubes 

 of Platyhelminthes and Trochelminthes, from cells of ectodermal 

 origin (ecto-nephridia). In others their source is mesodermal, each 

 tube being formed as a funnel-like outgrowth of the peritoneum 

 (peritoneal funnel) which subsequently comes to open on the 

 exterior (mesonephridia). In other cases, again, both ectoderm 

 and mesoderm take part in their formation, the peritoneal funnel 

 becoming connected with an ingrowth of the ectoderm (diplo- 

 n< t >liridia). The ordinary nephridia of the Oligochseta are ecto- 

 uephridia; those of the Polychseta for the most part diplonephridia. 



The permanent nephridia of the adult Chsetopod are preceded 

 in the larva by a series of provisional or < ////// -i/onic nephridia of a 



