EXCRETORY ORGANS. 87 



cavity. These portions of the body cavity are called nephro- 

 tomes, and accurately correspond at first with the segments of 

 the embryonic muscular system. The pronephros is the anterior 

 end of this excretory organ, which is developed before the rest 

 to meet larval needs. The serial homology between the prone- 

 phros and the hinder part of the excretory system was for many 

 years denied, partly because of a certain difference in structure 

 and partly because there is usually a gap between it and the 

 front end of the rest of the organ. But the differences in struc- 

 ture are very small, in some cases indeed (e.g. Lepidosteus) do 

 not exist ; and vestigial nephridia have been found in the gap 

 between the two organs (cf. especially Price's researches on the 

 development of the excretory organs of Bdellostoma). Finally, 

 Brauer's recent researches on the development of the excretory 

 organs of Gymnophiona remove all doubt on the point. 



The hinder part of the excretory system differs from the 

 pronephros mainly in the fact that the glomerulus — the vas- 

 cular tuft which secretes the fluid part of the urinary excre- 

 tion — is segmented into portions, one for each nephridium (or 

 kidney tubule), instead of forming a continuous structure as in 

 the pronephros ; and the portion of the body cavity containing 

 each of these is partly shut off from the rest to form the 

 malpighian body of the kidney tubule. This malpighian capsule, 

 however, frequently, though not always, retains its connection 

 with the rest of the body cavity by the so-called nephrostome. 

 The internal opening is retained in most Elasmobranchs, but 

 is lost in Teleostei, Dipnoi, Ganoidei and Marsipobranchii. 

 This hinder part of the excretory system becomes in 

 Elasmobranchii much reduced in front and largely developed 

 behind. In consequence of this it is described as consisting 

 of the mesonephros in front and the metanephros behind ; but 

 this differentiation is not found in other fishes. 



As in all Vertebrates, the longitudinal duct (archinephric duct) 

 is the first part of the excretory organs to appear. The prone- 

 phros is developed in connection with the front end of this duct, 

 so that the duct is at first the pronephric duct. The Miillerian 

 duct is found in all Pisces with the probable exception of the 

 Marsipobranchii and the Teleostei, and in all cases it becomes 

 the oviduct in the female, but is reduced in the adult male. In 

 Ganoids the longitudinal duct joins the oviduct (Miillerian) 



