458 STRUCTURE OF VERTEBRATA. 



Respiratory System. 



In BalanoglossuS) Tunicates, and Amphioxus, the walls of 

 the pharynx bear slits, between 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, and such also is 

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

 on the dorsal surface, has rarely more than a hydrostatic 

 function, and has a blood supply different from that of the 

 lungs. That lung and swim-bladder are homologous is by 

 no means certain, but the comparison is plausible. 



Excretory System. 



The development of this is always complicated. In the 

 embryos of Vertebrates at an early stage there are always 

 traces of a pronephros or so-called head kidney. In its 

 most developed condition this consists of 3-7 pairs of 

 segmentally arranged tubules, which communicate on the 

 one hand with the body cavity, and on the other by a 

 segmental duct with the exterior. The tubules are com- 

 parable to, and perhaps homologous with, the nephridia of 

 Annelids. The pronephros persists, although apparently in 

 a somewhat degenerate condition, in Myxine and Bdello- 

 stoma ; in Bony Fishes and Amphibia it forms a large and 

 functional organ in early life; in Elasmobranchs and onwards 

 it is from the first rudimentary and functionless. 



