igoS] EXCRETORY ORGANS OF METAZOA. 605 



two main kinds of these: (i) The ectoblastic portion very small, 

 and the end sac representing only a portion of the ccelom of a seg- 

 ment, as in the case of the salivary glands, nephridia, and genital 

 ducts of the Protracheata. And (2) the ectoblastic portion rela- 

 tively larger, the end sac being a whole coelomic sac, as in the case 

 of the coxal glands of Arachnids, Xiphosura, Crustacea, the salivary 

 glands of Diplopods, and the antennal, maxillary and maxillipedal 

 glands of Crustacea. 



10. An ectoblastic tube joining with retroperitoneal mesento- 

 blast, the latter neither joined with a coelomostome nor serving as 

 a genital duct ; the inner end is either quite closed or else has a small 

 opening (nephridiostome) into the ccelom; the cavity is usually 

 intracellular. Here belong the larval nephridia of the Hirudinea, 

 and the definitive nephridia of the Hirudinea, Oligochaeta and some 

 Polychjeta ( Phyllodocidae, Glyceridge, Nephthyidae, Capitellidas, 

 and perhaps the Nereidse). Probably the anal kidneys of Echiurids 

 belong here, and perhaps also the nephridia of the Nematoda. In 

 essential agreement with this type is the pronephros of the Verte- 

 brata, which also consists of a retroperitoneal mesentoblastic tube 

 whose inner end opens secondarily into the coelom (not by a peri- 

 toneal funnel) and whose outer end joins with the segmental duct 

 that is of either mesentoblastic or ectoblastic origin. Possibly the 

 nephridia of the Leptocardii are also homologous, as Boveri has 

 suggested, but nothing is know-n of their development; it will be 

 recalled that Boveri homologized the atrial chamber of the Lepto- 

 cardii with the segmental duct of the Vertebrata.^^ There is no 

 homology between the segmental duct of Vertebrates and the longi- 

 tudinal canals of the Polychsetes Lanice and Ploiiiiia, for the latter 

 seem to be formed by a late fusion of the secretory portions of the 



"As to the ph3'logeny of this segmental dnct, Balfour considered it to 

 be the foremost modified pronephric tubule, and Field has accepted this 

 view. Haddon (1886) and Beard (1887) suggested that the pronephroi first 

 opened separately into an open ectoblastic groove, that later closed to become 

 the segmental duct. Riickert (1888) also concluded that originally the pro- 

 nephric tubules opened independently to the exterior, and that they ex- 

 tended through the whole trunk ; he maintained that the segmental duct 

 arose by the meeting and fusion of their lateral ends, that is, by a back- 

 ward growth of collective tubules. 



