242 COMPARATIVE ANATOMY CHAP. 



hind-gut, and on the other are in open communication with the body 

 cavity by means of numerous ciliated funnel apertures, one of which 

 always lies terminally. An excretory function is ascribed to them. 

 Whether they represent a pair of modified nephridia cannot at 

 present be decided. The fact that they are supplied with numerous 

 funnel apertures ought not to stand in the way of such a view, as 

 typical permanent nephridia in the Poli/chceta (Capitdla) and Oligochceta 

 (Anacha'ta) may be provided with accessory funnels. 



Organs which may with certainty be pointed to as nephridia have 

 until now not been observed in the Myzostomidae. 



The problem of the morphological relations between the permanent nephridia, 

 the provisional trunk nephridia, and the embryonic head nephridia, is still unsolved. 

 It is closely connected with the questions as to the significance of segmentation in 

 the Annulate body and the morphological significance of the body cavity and the 

 mesoderm. It is very probable that the permanent segmentally-arranged nephridia in 

 all the Annulata are homologous. The histological difference between the nephridia 

 of the Hirudinea and Oligochceta on the one hand and those of the Polychccta on 

 the other, which consists in the fact that in the former the nephridia are perforated 

 rows of cells and in the latter as a rule tubes with epithelial walls, would in that 

 case be unessential, as also would be the absence or presence of branchings. It is 

 further probable that the provisional trunk nephridia are morphologically equivalent 

 to the permanent nephridia. They are distinguished from the latter only in that 

 they appear earlier and disappear in proportion as the permanent nephridia appear 

 and assume their functions. The homology between provisional trunk nephridia 

 and permanent nephridia is supported by the circumstance that no case has as yet 

 been known in which permanent nephridia have attained development in a segment 

 where provisional nephridia have previously appeared and then disappeared. C'upi- 

 /'//<', in which in the tenth and eleventh segments both provisional and permanent 

 nephridia develop, is an exception, but only an apparent exception, for we have seen 

 that in this animal several pairs of permanent nephridia occur in one segment. 

 It is probable that the embryonic head nephridia are homologous with the 

 trunk nephridia (provisional and permanent). The whole nephridial apparatus 

 would then have to be judged of in the following way. Originally a pair of nephridia 

 occurs in each segment of the segmented Annulate body, even in the head segment. 

 All the pairs of nephridia are segmentally homologous with each other. The larva 

 or embryo of the now living Annulata consists of the embryonic head segment and 

 the unsegmented rudiment of the trunk. The differentiation of the trunk occurs 

 from before backward ; the first and oldest segment to be developed is the first trunk 

 segment ; then follows the second trunk segment, and so on. In correspond- 

 ence with this, the pair of nephridia of the head segment first appears (em- 

 bryonic head kidneys), then the pairs of nephridia of the anterior trunk segments, 

 several of which (provisional trunk nephridia) may degenerate gradually as new 

 nephridia, the permanent trunk nephridia, begin to form behind them. The cause 

 of the disappearance of the head and the provisional trunk nephridia in the 

 course of development is perhaps to be found in the fact that the foremost body 

 segments in the adult animal are crowded with organs (pharynx, brain, etc.) Besides 

 this, in the whole animal kingdom oi'gans which attain development very early and 

 function during larval or embryonic life show a tendency to degenerate egirly, as 

 if they were soon worn out. Further in the OliyoclieKta (excepting the Lumbrici<?<f) 

 the nephridia in the genital segments degenerate when the various sexual organs 

 attain development. 



