240 



COMPARATIVE ANATOMY 



CHAP. 



medicinalis), wliich opens externally tlirougli a pore. That part of the nephridial 

 canal which comes after the funnel is distinguished by the fact that fine, branched, 

 and often anastomosing canals enter its intracellular principal lumen. The cells of 

 this portion are in fact perforated by branching canals. In Sirudo the funnel is 

 closed towards the blood sinus in which it lies. 



The nephridia in the genera Pontobdella, Branehellicm, and Pisdcola difier 

 much from the above, as they form in each segment a, complicated network of 

 canals which are always intracellular, this network opening outwardly by 2 apertures 

 and entering the blood sinuses of the body through 2 funnels. 



The nephridia of other Oligochmta show (Fig. 160) gi-eat correspondence with 



those of the Eirudinea. The funnel of a ne- 

 phridium projects from the anterior wall of each 

 dissepiment into the cavity of the segment lying 

 anterior to that in which the rephridium lies. 

 Starting from the funnel, the nephridial canal, 

 which is everywhere intracellular, first passes 

 through the dissepiment, forms more or less 

 complicated loops in that segment of the body 

 cavity which lies posteriorly (in these coils we 

 can generally distinguish several different por- 

 tions), and finally emerges through a terminal 

 portion into a vesicle which opens outwardly. 

 This vesicle is often provided with muscular 

 walls. The funnel and external aperture of a 

 nephridium thus always lie in two diS'erent 

 segments ; the two external apertures of a pair 

 of nephridia lie in the same segment as the 

 inner funnels of the pan* of nephridia which 

 -Etu come next in order posteriorly. This position 

 of the inner and outer apertures of the nephridia 

 in 2 consecutive segments is maintained even in 

 those cases where, as in the middle body seg- 

 ments of Phreatothrix, the nephridial canal 

 passes through several dissepiments, running 

 back from its ciliated funnels through several 

 segments ; it then forms a loop and bends for- 

 wards again. In the nephridia of the Chceto- 

 gastridce the ciliated funnels are wanting. In 

 them, as in the Hirudinea, numerous branched 



Fig. 160. — Nepliridium of an Oil 

 gochKte, diagrammatic, tr, Funnel ; t^ty, 



dissepiment ; 



glandular ; ngi, non- 



glandular portion of the nephridial duct ; 

 d), terminal vesicle ; Iw, hody wall (partly and anastomosing intracellular canals enter the 

 after Vejdovsky). central canal. In a species of Acanthodrilus 



[Lumbricidce) there are typically 4 pairs of nephridia in each segment (even in 

 the genital segments). There is said to be a similar arrangement of nephridia in 

 the anterior segments of PericTueta mirdbilis. 



The permanent nephridia of the Polychceta are tubes with cellular walls ; their 

 often ciliated central canal is thus as a rule, in opposition to that of the 

 Oligochmta and Hirudinea, intercellular. (Intracellular nephridial canals, however, 

 also occur. ) The nephridial tube is almost always so bent that we can distinguish 

 in it two limbs, one centripetal, at whose inner end lies the funnel, which is mostly 

 wide open and provided with cilia, and another centrifugal, which opens outwardly 

 by breaking through the body wall. The nephridia lie in the nephridial or renal 

 chambers of the body cavity which have already been described (p. 213), and they may 



