582 THE ANIMAL KINGDOM. 



no excretory apparatus has been detected : (i) sacs, each with a single 

 funnel leading into the coelome, typically paired but restricted at the out- 

 side to four pairs in all, and opening externally on the ventral aspect ; (2) 

 sacs, with numerous funnels leading into the coelome, and opening exter- 

 nally with the rectum ; but there is reason to believe that this communica- 

 tion with the rectum is secondarily acquired \ The second kind of 

 nephridia (2 supra] is present only in Gephyrea Chaetifera, side by side 

 however with the first kind. The type of renal organ with ciliated funnels 

 is limited to groups in which a coelome is present, and is probably an 

 adaptation to it (Lang) ; and it is a noteworthy fact that the funnels so far 

 as is known are developed independently of their excretory tubes. The 

 Rotifera are said to be an exception to the statement, in so far that a 

 coelome is present, yet their nephridia belong to the first type. The same 

 is true of some Turbcllaria. Their coelome however is probably an 

 archicoele. Larval or provisional nephridia, commencing with flame-cells, 

 occur in many Polychaetan Trochospheres ; with the flame-cells aborted in 

 Polygordius among Arc hi- Annelida, and in Echiurus among Gephyrea, and 

 in a very rudimentary condition in Hiriidinea. A pair of these organs is 

 found in the head, and may be branched as in Polygordius and Echiurus. 

 A second pair of organs in Polygordius has a structure similar to the 

 first pair. A right and left longitudinal canal grows backwards from the 

 first pair or cephalic nephridia in Polygordius, and gives origin to the per- 

 manent nephridia, afterwards disappearing. Similar but persistent canals 

 connecting the permanent nephridia have been discovered in Polymnia 

 nebulosa ( = Terebella Meckelii). In the Polychaetan Trochospheres a pair of 

 these larval nephridia is found in the head, and one or more pairs in the 

 body. Provisional cephalic nephridia have also been observed in larval 

 Oligochaeta. It is possible that the excretory system of the Ttirbellaria, 

 especially the Triclada, with its more or less segmentally arranged apertures 

 as well as longitudinal canals, represents a primitive condition which 

 becomes modified in other cases (i) by the suppression of the longitudinal 

 canals, and (2) by the formation of intercellular funnels coupled with a 

 well-marked segmental arrangement of the parts persisting and of the 

 external apertures 2 . The relation which the nephridia acquire, either 

 temporarily or permanently, to the sexual organs is entirely secondary. 



1 The male Bonellia viridis has two simple nephridial tubes, each with a single internal funnel, 

 which open externally and independently, but near the posterior attachment of the intestine. The 

 corresponding organs in the larval Echiurus open similarly at first ; their subsequent connection to 

 the rectum is therefore secondary. It is doubtful how far the Gephyrean nephridia retain a renal 

 function. 



2 A very interesting discussion on these points will be found in Lang's ' Polycladen,' Fauna, &c. 

 des Golfes von Neapel, xi. 1884, pp. 674-79. The occasional presence of a longitudinal canal in 

 connection with segmental nephridia is especially noteworthy. The primitive segmental duct of 

 Vertebrata is perhaps to be derived from it. 





