Ill 



EXTERNAL GILLS 



157 



the mandibular arch which has lost its respiratory and taken on a 

 supporting function. 



While external gills occur within three main subdivisions of the 

 Vertebrates, namely Teleostomatous fishes (Crossopterygians — the 

 most archaic of existing Teleostomes), Lung-fishes, and Amphibians, 

 there are two main groups — Elasmobranchs and Amniotes — in which 

 they are conspicuous by their absence. Having regard to the 

 tendency of the organs in question to disappear (as in the cases 

 already alluded to amongst the Amphibia) their absence in a 

 special group would not 

 in any case constitute 

 strong evidence that they 

 were never present in 

 the ancestors of that 

 group. As it happens 

 however there is in the 

 two groups mentioned 

 a definite cause which 

 seems quite competent 

 to account for the dis- 

 appearance of external 

 gills, namely the de- 

 velopment of a new 

 organ — the yolk - sac 

 with its highly developed 

 vitelline network of 

 blood-vessels — which in 

 addition to its primitive 

 function must neces- 

 sarily also function as a 

 very efficient organ of 

 respiratory exchange and 

 so render any pre-exist- 

 ing respiratory organ no 

 longer necessary. 



Taking into consideration the presence of external gills in three 

 archaic groups of Vertebrates it seems to the present writer to be 

 clearly indicated that these organs are a very ancient characteristic of 

 the Vertebrate phylum. The only alternative indeed is to regard 

 them as having become evolved independently in the three groups 

 in which they occur. It is difficult to accept this as in any way 

 probable having regard to the similar morphological relations of the 

 organs in question. 



It might be suggested that somewhere on the course of a large 

 blood-vessel, such as an aortic arch, would be a most natural place 

 for the development of a new respiratory organ. Such a suggestion 

 however is entirely fallacious for simple physical reasons : for new 

 breathing organs will tend to become evolved not on the course of a 



Fig. 88. — Three stages in larval development of a newt 

 (Triton taeniatus) as seen from above. (After Bgert, 

 1913.) 



6, balancer ; e.g, external gill of first branchial arch. In Fig. A 

 wlfat loolcs like a posterior external gill is the pectoral limb. 

 In Figs. B and C the external gills have been cut away leaving 

 only their basal stumps. 



