XIII 



EXCRETORY ORGANS 



357 



species the nephridia which open more dorsally are a little 

 different in structure from those which open more ventrally. 

 One set have a caecum, and the other have not. 



The nephridia of the terrestrial forms are enveloped by a richly 

 developed network of blood 

 capillaries, which is absent 

 in the smaller aquatic 

 genera. 



A very remarkable genus, 

 Brachydrilus, has lately been 

 described by Dr. Benham, 1 

 in which each segment has 

 two pairs of nephridia in- 

 stead of a single pair. More 

 recently, certain Australian 

 forms, which I propose to 

 unite on this account into 

 a genus Trinephrus, have 

 been discovered which have 

 no less than three distinct 

 and separate pairs in each 

 segment. 2 



In many Megascolicidae 

 there is a nephridial system 

 of a different character. In 

 Perichaeta when dissected 

 the nephridia appear, on 

 account of their minute size, 

 to be altogether absent. 



There is, however, in most Fig. 191. Section through body-wall of Megas- 



Periehaetidae, in many $~TTi 1^StSt^ 



Acailthodrilidae, and in 2, funnel ; 3, septum ; 7, external apertures. 



many Cryptodrilidae a mass 



of minute tubules which cover the inside of the body- wall, and 

 open on to the exterior by innumerable openings ; there may be 

 in a single segment one hundred or more of these external 

 orifices, which are scattered about irregularly. It is at present 

 uncertain whether these minute tubes are connected among 



1 Zool. Anz. xi. 1888, p. 72. 

 2 See Fletcher, P. Linn. Soc. X.S.JV. (2) iii. 1889, p. 1542. 



