EXCRETORY ORGANS. 567 



bunch of fine tubes. The whole organ resembles in many respects 

 the excretory organ of the Platyelminthes. 



In the larva of Bonellia Spengel has described a pair of provisional 

 excretory tubes, opening near the anterior end of the body, which are 

 probably homologous with the provisional excretory organs of Echiurus 

 [vide Vol. L, fig. 162 C, se). 



Discophora. As in many of the types already spoken of, per- 

 manent and provisional excretory organs may be present in the 

 Discophora. The former are usually segmentally arranged, and 

 resemble in many respects the excretory tubes of the Chsetopoda. 

 They may either be provided with a peritoneal funnel (Nephelis, 

 Clepsine) or have no internal opening (Hirudo). 



Bourne' has shewn that the cells surrounding the main duct in 

 the medicinal Leech are perforated by a very remarkable network of 

 ductules, and the structure of these organs in the Leech is so peculiar 

 that it is permissible to state with due reserve their homology with 

 the excretory organs of the Chsetopoda. 



The excretory tubes of Clepsine are held by Whitman to be 

 developed in the mesoblast. 



There are found in the embryos of Nephelis and Hirudo certain 

 remarkable provisional excretory organs the origin and history of 

 which are not yet fully made out. In Nephelis they appear as one 

 (according to Robin), or (according to BUtschli) as two successive 

 pairs of convoluted tubes on the dorsal side of the embryo, which 

 are stated by the latter author to develop from the scattered meso- 

 blast cells underneath the skin. At their fullest development they 

 extend, according to Robin, from close to the head to near the ventral 

 pucker. Each of them is U-shaped, with the open end of the U 

 forwards, each limb of the U being formed by two tubes united in 

 front. No external opening has been clearly made out. Fiirbringer 

 is inclined from his own researches to believe that they open laterally. 

 They contain a clear tluid. 



In Hirudo, Leuckart has described three similar pairs of organs, 

 the structure of which he has fully elucidated. They are situated 

 in the posterior part of the body, and each of them commences 

 with an enlargement, from which a convoluted tul)e is continued 

 for some distance backwards ; the tube then turns forwards again, 

 and after bending again upon itself opens to the exterior. The an- 

 terior part is broken up into a kind of labyrinth ic network. 



The provisional excretory organs of the Leeches cannot be identi- 

 fied with the anterior provisional organs of Polygordius and Echiurus. 



Arthropoda. Amongst the Arthropoda Peripatus is the only 

 form with excretory organs of the type of the segmental excretory 

 organs of the Chsetopoda^. 



1 " On the structure of the Nephridia of the Medicinal Leech." Quart. J. of 

 Micr. Science, Vol. xx. 1880. 



2 Vide F. M. Balfour, " On some points in the Anatomy of Peripatus Capensis." 

 Quart. J. of Micr. Science, Vol. xix. 1879. 



