44 HEAD KIDNEY OF TELEOSTEAN FISHES. 



When studying tlie extraordinary position of the air-bladder in 

 Dactylopterus volitans* I was struck by the very pronounced 

 head kidney, and by its peculiar position. In this fish the prone- 

 phros is entirely separated from the body kidney and is situated 

 anterior to the abdominal cavity in the same transverse plane as the 

 heart. In the paper referred to it is shown that the swimming 

 bladder of Dactylopterus is divided, on each side, into two main 

 portions, one large and muscular, the other thin-walled but sur- 

 rounded by bone and situated above and anterior to the muscular 

 portion. Below this secondary portion, and in front of the primaiy, 

 there is a cavity of inverted pyramidal shape, formed entirely of 

 bone except on the anterior aspect. 



The pronephros fills this cavity, its anterior surface coming in 

 contact with the extremely vascular membrane lining the posterior 

 portion of the branchial chamber. 



The body kidney is situated behind the large muscular portion 

 of the swimming bladder, and receives in a concavity the rounded 

 posterior end of the bladder. 



Communication between the two is, however, maintained by a 

 canal formed in the ventral surfaces of the anchylosed first four 

 vertebrae of the spinal column. This canal tunnels through what 

 would otherwise be the bases of the transverse processes, and so is 

 protected from any movements of the bladder which surrounds, and 

 lies largely above the spinal column in this region. 



The appearance of the pronephros when sectioned is represented 

 in fig. 1. It is apparently a functional kidney. Sections of the 

 body kidney give an exactly similar appearance, only in the majority 

 of sections a greater number of uriniferous tubules are present, 

 and no doubt this organ is capable of secreting more urine than the 

 other. In comparing the organs, I took at random, ten sections 

 from slides of head kidney sections, and ten from slides of body 

 kidney, and used a lens with a wide angle (Zeiss D). I counted 

 87 sectioned tubules for the head kidney, and 144 for the body 

 kidney ; i. e. a majority of 57 for the body kidney. 



This difference between the two organs may go to show that in 

 Dactylopterus the degeneration of the pronephros is only com- 

 mencing, but I think the conditions justify me in believing the organ 

 to have a renal function. 



In attempting to follow the course of degeneration I examined 

 Cyclopte7'us lumpus. Here the pronephros, although joined to the 

 mesonephros, is yet easily distinguishable from it. The single body 

 kidney somewhat resembles an elongated cone in form, the apex 



* On the Swimming Bladder and Flying Potvers of Dactylopterus volitans, Proc. Roy. 

 Soc. Edin., vol. xvii, 1890. 



