MUSEUM OF COMPARATIVE ZOOLOGY. 317 



been con firmed by a large number of observers, 1 and it has been further 

 shown that individual cells, having become charged with excreted con- 

 crements, loosen from the layer to which they belong, and float freely in 

 the ccelom, whence they are discharged through the nephridia. The 

 chloragogen layer covering the blood-vessels appears moreover from its 

 anatomical relations to be a portion of the visceral mesoderm, and it 

 has been shown to arise ontogenetically from that layer (Houle, 'SO, 

 pp. 201, 252, 200). The chloragogen cells are frequently distributed 

 upon special vascular processes, thus forming distinct glandular organs. 

 Similar in function is probably the glandular envelope of the ventral 

 vessel in Polyophthalamus (E. Meyer, '82, p. 81 G), and cases may be 

 found among Polyclnetes in which definite peritoneal glands are present 

 (Grobben, '88, pp. 255 et seq., Eisig, '87, pp. 227, 245, 681). I should not 

 wish to assert a strict homology between the glomus and the masses of 

 chloragogen cells ; yet it seems to me likely that the latter represent an 

 early differentiation of the splanchnic mesoderm of which we have more 

 specialized developments both in Annelids and in Vertebrates. 2 (3.) The 

 efferent conduits take their origin from the ccelom by means ot a series 

 of ciliated funnel-shaped openings, the nephrostomes. (4-.) The nephro- 

 stomes lead into transverse convoluted canals, along the course of which a 

 large part of the excretion takes place. (5.) The nephridial tubes arise 

 from the parietal peritoneum. (6.) They are typically strictly meta- 

 meric, one pair of tubules being developed in each metamere. The 

 deviation from this typical metamerism to which I have already referred 

 in the case of Vertebrates is paralleled by similar conditions in Capitella 

 (Eisig, '87, p. 50-4). (7.) The development of the Chtetopod nephridia 

 resembles in general that shown by those of Vertebrates. Both the 

 pronephric and mesonephric tubules arise as a series of metameric 

 outgrowths from the somatic mesoderm. In Polychretes this is evidently 

 the mode of origin of the nephridia. In Oligochastes the development is 

 more doubtful, but the method of origin described by Bergh ('00) is in 

 essence the same as that known in Polychsetes, and the mode of devel- 

 opment maintained by Wilson ('87, pp. 185, 186. and '80, pp. 410 et seq.) 

 may be interpreted so as not to be in fundamental opposition with such 

 a method. 



There is one feature in regard to which the nephridia of most Anne- 



i E. g. Timm ('83, pp. 122, 120): Kiikenthal ('85, p. 330); Meyer ('87, p. 648). 



2 A further analogy is possibly to be found in the fact that in Amphibia certain 

 cells early loosen from the wall of glomus and fall into the ccelom, leaving inter- 

 vals between the remaining cells of the epithelium. 



