64 ADAM SEDGWICK. 



Here, however, an apparent difficulty presents itself. In the 

 Vertebrata the excretory organs (which probably were primi- 

 tively segmental i) open not to the exterior direct, but into a 

 longitudinal canal which opens behind into the alimentary 

 canal ; while in the Invertebrata each of them opens direct 

 to the exterior. 



As an explanation of this difficulty I suggest that in the 

 Vertebrate ancestors the primitive alimentary cavity acquired 

 a well arranged system of ducts, by which the peripheral 

 excretory matters were carried to the part of the alimentary 

 canal near the hind end of the primitive mouth (future anus), 

 that in consequence the excretory pores were not wanted, and 

 were either never developed or if developed lost. As confir- 

 matory evidence I may refer (1) to the circular canal of the 

 Medusae, which might easily be conceived transformed into the 

 Vertebrate segmental duct, the excretory organs themselves 

 being developed from the outer part of the radial canals; (2) to 

 the method of development of the anterior and least modified 

 part of the Vertebrate excretory organ. In the osseous fishes 

 and Amphibia the segmental or pronephric duct arises as a 

 groove of the body cavity, and is therefore a direct product of 

 the archenteric endoderm. In most Vertebrates the develop- 

 ment of the segmental duct is much modified ; but I pointed 

 out some years ago that we can only get an intelligible expla- 

 nation of the connection between the excretory tubules and 

 the duct of the kidney by supposing that they originally 

 developed in continuity, both as specialised parts of the body 

 cavity, and that this method of development is repeated in the 

 case of the anterior part of the kidney of Ichthyopsida, and in 

 a more modified manner in the Amniota. 



Turning to the Invertebrata, we find that the development 

 is not direct from the coelom, but from solid masses of cells ^ 



' Elasmobranchs. For discussion of this question, vide Sedgwick, "Early 

 Development of Wolffian Duct," * Quart. Journ. of Mic. Sci.,' vol. xxi, 1881. 



* Very various accounts are given of the origin of the Invertebrate ex- 

 cretory organs. I reserve a critical examination of these facts until I have 

 worked out the development of the nephridia of Peripatus. 



