EXCRETORY SYSTEM. 50 
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 Amphioxus, 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, ze. the exterior. On the surface of 
each tubule # 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 4m- 
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 AGyxine and Bédellostoma it persists in 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 /zerasfer 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. Riickert 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). 
