E}7ibri/ology of certain of fJbe Loiuer Fishes. 197 



arch in its early stages is, in fact, the vessel of the external 

 gill. This relation of aortic arch to external gill constitutes, 

 to my mind, a weighty piece of morphological evidence of 

 the archaic nature of these organs. 



Phylogenetic Development. 



Palaeontology so far has afforded next to no evidence 

 regarding external gills in the more ancient vertebrates. 

 This is not surprising when we bear in mind (1) that until 

 their morphological importance is appreciated, palaeonto- 

 logists are not likely to direct any special attention to the 

 search for traces of them, and (2) that external gills, com- 

 posed almost or quite entirely of soft tissue, would not be 

 likely to leave any conspicuous traces behind them. 



In this connection it is of interest to note, however, that 

 there is already some evidence to indicate that external 

 gills have in at least certain cases possessed a supporting 

 skeleton, e.g., in Dolichosoma, Fritschi found remains of an 

 extensive skeleton projecting from the branchial region, and 

 which can only be explained on the supposition that it 

 served as a support for large external gills. Along with 

 this evidence of palaeontology, we have the fact discovered 

 by Budgett,^ that in the young larval Polypterus there 

 is a segmented rod of cartilage projecting from the hyoid 

 arch, and serving to support the base of the external gill. 



Evolution of Organs of Eespiration. 

 It is one of the fundamental characters of living 

 substance, that it needs to carry on respiratory exchanges 

 with the surrounding medium. The process of life is to a 

 certain extent a process of slow combustion, and, like any 

 other combustion, it needs a supply of oxygen to enable it to 

 go on. As in other forms of combustion, carbon dioxide is 

 produced, a substance unfavourable to the combustion 

 process, and which actually stops that process if it is 

 allowed to accumulate. In any piece of living substance 

 then, these respiratory exchanges take place between it and 

 the fluid medium in contact with it — for living protoplasm 



1 Fauna Gaskohl., Bd. II. s. 114. 



2 Trans, Zool. Soc. Loncl., voL xvi. p. 32 . 



