OLIGOCHAETA. 



501 



times open into the coelom (Penchaeta, and in Odocliaetus in the 

 case of the nephridia which open into the rectum), and sometimes 

 they do not (Odo- 

 cliaetus, Megasco- 

 liilrx). The coelomic 

 openings when pre- 

 sent are numerous, 

 but whether they 

 have any numerical 

 relation to the ex- 

 ternal openings is 

 not clear from the 

 published accounts. 

 Nor is it clear 

 whether we are to 

 look upon the plec- 

 tonephric condition 



being 



due to 



as 



the presence of FIG. 411. Diagram showing the disposition of the nephridia of a 

 n 11 TTl eroil S nn 1 1 1 segment of Perichaeta aspergillum (from Perrier, after Beddard). 



p dorsal pore ; o external openings of nephridia : i intestine ; s 



nephridia in a Seg- setae ; w ventral blood vessel ; n nerve cord ; r nephridial net- 

 i -i work ; vd dorsal blood vessel. 



ment which have 



become connected together, or whether they are derived from the 

 modification of a single pair of nephridia. However this may be, it 

 appears certain that the plectonephric condition is preceded in the 

 embryo by a stage in which there is only one pair of ordinary 

 nephridia in each segment. It is supposed that the adult condition 

 in such forms has arisen by the multiplication of the original simple 

 tubes. The diffuse nephridia may be the only kind of nephridia 

 present; or they may coexist in the same segment with meganephridia 

 (Perichaeta armata, etc.) ; or meganephridia may be present in one 

 part of the body, and plectonephric nephridia in another part, the 

 two conditions gradually passing into one another ; e.g. in Megas- 

 colides australis there are plectonephridia without internal funnels 

 in the anterior segments, an intermediate condition in the middle 

 segments, while in the posterior segments the nephridia are mega- 

 nephric and possess coelomic funnels. It appears that in some cases 

 the nephridial network is continuous from segment to segment. 



In some Oligocliaeta the external opening of a certain number of 

 nephridia is into the alimentary canal. This is the case in the 

 Enchytra&idae, Odocliaetus multiporus, species of Rhinodrilus, Acantlio- 



