Emhnjology of certain of the Loioer Fishes. 199 



portions of it taken into the gut. From these two primitive 

 breathing organs are developed the specialised breathing 

 organs which are so characteristic of the subgroups of the 

 Vertebrata. 



The general surface of the skin is the great original organ 

 of respiration. In groups where it still carries on this 

 function, we find very usually that the function becomes 

 accentuated locally. Special portions of the skin become 

 specially vascular, and then they tend to increase in area 

 so as to increase the respiratory surface. In this way we 

 find involutions of skin into interior of body, as, e.g., in 

 tracheae of insects, or we find projections developed from 

 the surface forming dermal gills, e.g., molluscs, chsetopods. 



In the lower vertebrate, probably in correlation with 

 increase in thickness, and more especially with the develop- 

 ment of a placoid dermal skeleton, the skin generally seems 

 to have lost its great respiratory significance, but there exist 

 in some of the most primitive vertebrates structures which 

 appear to represent ancient dermal gills. These structures 

 are the external gills mentioned already as occurring in the 

 young of Crossopterygians, Dipnoans, and Amphibians. 



The more usual view that these external gills are second- 

 arily developed structures of no phylogenetic importance 

 seems to be negatived — (1) by their very early appearance in 

 ontogeny; (2) by the importance of their blood supply — the 

 main aortic arch passing into them ; (3) and by the remark- 

 able constancy of their position in relation to the upper 

 end of the visceral arch. 



There is one obvious and strong argument against the 

 archaic nature of the external gills, and that is the fact 

 of their absence in certain of the admittedly primitive groups, 

 e.g., the Elasmobranchii. There are two considerations which 

 seem to me to diminish the weight of this objection. We 

 know that external gills are remarkably liable to injury 

 through disease or enemies. Apart from this being known 

 from observation, it is implied in the remarkable powers of 

 regeneration shown by these organs. They would therefore 

 tend to disappear in evolution if their function were taken 

 on by other organs. 



