606 MONTGOMERY— MORPHOLOGY OF THE [April 24, 



nephridia. Indeed, the segmental duct of Vertebrates appears to 

 have originated in this class. 



11. An ectoblastic tube (though this portion may be very small) 

 joining with retroperitoneal entomesoblast, and the latter con- 

 necting with a coelomostome ; these are generally either urogenital 

 or homodynamous with genital ducts, and the cavity is usually 

 intercellular. The inner end is widely open at least in the embryo. 

 These correspond to type 10, with the addition of a coelomostome. 

 In this type fall the nephridia of the Mollusca, and those of most 

 Polychasta. As mentioned above, the segmental organs of the 

 Sipunculida, Ectoprocta, Brachiopoda, Echiurida and Myzostomida 

 probably belong either here or with type 8. Essentially homologous 

 are the mesonephroi, therefore probably also the metanephroi, of 

 the Vertebrates, which consist to great extent of peritoneal funnels. 

 And Boveri has argued that the gonads of the Leptocardii may be 

 homologous with these mesonephroi. 



12. Non-tubular peritoneal differentiations of excretory nature. 

 Here are the so-called ciliated funnels of the Holothurians, that 

 are not funnels (coelomostomes) at all, and the widely represented 

 peritoneal glands (phagocytic organs, chloragogue in parte). 



13. Non-tubular retroperitoneal mesentoblastic cell masses. 

 With these belong a variety of structures the development of most 

 of which has been little examined, such as the excretophores of the 

 Hirudinea and the fat-body of Insects (the latter perhaps repre- 

 senting, as Wheeler has suggested, the remains of nephridia). 



(d) Entoblastic Excretory Organs. 



14. These are relatively few in number and seldom have an 

 exclusively excretory function. In the first place there are tubular 

 evaginations of the mid-gut, as the Malpighian vessels of Arachnida, 

 then the mid-gut coeca of the Polycladidea and Amphipoda and 

 probably of the Arachnida ; these are all essentially homologous. 

 The whole mid-gut has been shown to be excretory in the Collem- 

 bola, Dinopliiliis and the Ectoprocta; it seems to be specially so 

 only when other excretory organs are wanting, and in that case 

 there is either periodical moulting of the lining of the mid-gut 



