EXCRE TOR Y S YSTEM. 489 



which the blood is exposed in superficial blood vessels to 

 the purifying and oxygenating influence of the water. 



In Cyclostomata, Fishes, all young and some adult Am- 

 phibians, there are not only clefts on the walls of the pharynx, 

 but gills associated with these. On the large surface of the 

 feathery or plaited gills, the blood is exposed and purified. 



In Reptiles, Birds, and Mammals, traces of gill-clefts 

 occur in the embryos, but without lamellae or respiratory 

 function. In the embryo the blood is purified, as will be 

 explained afterwards, by aid of the foetal sac known as the 

 allantois ; and after birth the animals breathe by lungs. 

 All adult Amphibians also have lungs, to which the lung 

 or swim-bladder of Dipnoi is physiologically equivalent. 



The gill-clefts arise as outgrowths of the endodermic gut 

 which meet the ectoderm and open. The ventral paired 

 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 has a blood supply different from that of the 

 lungs. There is no demonstrable 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 v. 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 funnels, 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 condition 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, in their internal openings, 

 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. 235) 

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



