330 STRUCTURE OF THE LARVAL POLYPTERUS. 



IV. — Ueinogenital Oegaxs. 



The excretory system of this larva is of especial interest, for here we find that 

 a regular metamorphosis is taking place. Tlic larval pronephros, though still 

 functional, shows signs of degenerating, while the mesoncphros is still fairly simple 

 in its arrangement, and in some respects is still in a larval condition. 



The pronephros consists of a rather small glomus almost filling the pronephric 

 chamber which lies close by the aorta (PI. XXXIV. fig. 9, Gl.) ; from this a narrow 

 tube embedded in lymphoid tissue, the pronephric duct, passes outwards dorsal to the 

 posterior cardinal vein and towards the body-wall (PL XXXIV. fig. 9, pn.d.) ; on 

 passing between the dorsal and ventral lateral muscles it suddenly dilates, turns 

 headwards, loops round the brachial nerve, and, in a mass of lymphoid tissue lying in 

 the angle between the anterior ends of the dorso-lateral and ventro-lateral muscles and 

 the skin, becomes greatly convoluted. In the successive convolutions the duct 

 becomes narrower and narrower, occasionally becoming dilated and then suddenly 

 narrower again ; finally there leaves the convoluted mass a very fine duct 

 (PI. XXXIV. fig. 9, s.d.), and this, running along the outer side of the posterior 

 cardinal vein, passes backwards and inwards until it comes to lie just over the 

 peritoneum. Two segments behind the glomus of the head-kidney it gives off the 

 first kidney-tubule, and thence passes straight to the hind end of the body-cavity 

 as the segmental duct, ever increasing in size until it meets its fellow to form the 

 urinary sinus (text-fig. 4, u.s.). 



The smallness of the pronephric chamber and the dilatation of the convoluted 

 portion of the pronephric duct must be regarded as signs of degeneration. In fact the 

 connection between the pronephros and the mesonephros could not be traced on 

 the left side, while on the right side it was done with the utmost difficulty. The 

 glomus itself on the left side was very small, and the anterior end of the coiled 

 pronepric duct could not be traced all the way to it. 



On the right side there appeared to be traces of a peritoneal funnel leading to 

 the pronephric chamber (PI. XXXIV. fig. 9, P.f.); but with the thick sections into 

 which I had cut this region (10 ^u,) I was unable to satisfy myself that it actually 

 opened into this chamber. 



In specimens 9 cm. in length all trace of the pronephros had disappeared, there 

 remaining in its place a small mass of lymphoid tissue. 



When we compare the pronephros of Polypterus with that of the Ganoids and 

 Teleosteans which have been described, there can be no hesitation in saying that 

 it resembles most closely the pronephros of Amia described by Jungersen i, where 

 there is one pronephric chamber, one peritoneal funnel, and one opening of the 

 pronephric duct into the pronephric chamber. To draw a close comparison between 



' Zool. Aiiz. 1894, p. 246. 



