EXCRE TOR Y S YSTEM. 509 



lungs arise from an outgrowth of the gut, as does also 

 the swim-bladder of many Fishes, though it usually lies 

 on the dorsal surface, has rarely more than a hydrostatic 

 function, and usually has a blood supply different from 

 that of the lungs. In Dipnoi and some "Ganoids" it is 

 supplied by a pulmonary artery arising from the sixth aortic 

 arch. There is probably a homology between lung and 

 swim-bladder. 



Excretory system. The development of this is always compli- 

 cated. In the embryos of Vertebrates at an early stage there are always 

 traces of a. pronephros, or so-called head-kidney. This is perhaps seen 

 in its most primitive condition in Amphioxtts, where, as already de- 

 scribed, there is a series of tubules, segmentally arranged, opening on 

 the one side into the body cavity by several flame-cells, and on the 

 other into the atrial chamber, i.e. the exterior. On the surface of 

 each tubule a vessel connecting the sub-intestinal vein with the dorsal 

 aorta forms a vascular plexus the so-called glomus. Such a con- 

 dition of parts is never in its entirety found in the Craniata. There 

 the tubules open not directly to the exterior, but into a longitudinal 

 pronephric or segmental duct, and they are usually few in number ; 

 but in their segmental arrangement, as shown by the blood supply, 

 and in the presence of glomera, they agree entirely with those of Am- 

 phioxus. In connection with the glomera, it may be noted that while 

 the blood supply usually comes directly from the dorsal aorta, it has been 

 shown by Paul Mayer and Riickert that in the embryos of Selachians 

 connecting vessels occur between the dorsal aorta and the sub-intestinal 

 vein, which form rudimentary networks on the tubules of the pronephros. 

 This shows a very striking correspondence with the conditions seen in 

 Amphioxus. 



The pronephros develops from the parietal mesoblast at the junction 

 of the muscle segments and the unsegmented body cavity (see Fig. 270) 

 in the anterior region, and varies greatly in its degree of development. 

 In Myxine and Bdellostoma it persists rq, adult life, though apparently, 

 at least in part, in a degenerate condition, and is said to be the functional 

 excretory organ of the little (degenerate ?) fish Fierasfer and some other 

 Bony Fishes. In most Bony Fishes, and in Amphibia, it is merely a 

 larval organ, but is then large and important. In Elasmobranchs and 

 Amniota, except Crocodiles and Turtles, it is from the first rudimentary 

 and functionless. 



The origin of the segmental or pronephric duct is still undetermined. 

 It usually arises from the mesoblast, in some cases growing backwards 

 directly from the rudiment of the pronephros, while in others the sur- 

 rounding mesoblast takes an important part in its formation ; in Elasmo- 

 branchs, in Mammals, and in the chick, a connection with the epiblast 

 has been described by various observers. Rtickert is of opinion that it 

 originally arose by the fusion of the outer ends of the pronephric 

 tubules, and that the occasional connection with the ectoderm indicates 

 the position of former excretory pores (cf. Amphioxus}. 



