202 Proceedings of the Royal Physical Society. 



that this groove appears only late in development is against 

 its morphological significance. 



The problem is more difficult as regards the swim-bladder 

 of Actinopterygians. This is throughout dorsal in relation 

 to gut : its blood supply is derived from the dorsal aorta. It 

 is, however, important to notice that in a considerable number 

 of Teleosts — e.g., Erythrinus — and other Characinids, the open- 

 ing of the swim-bladder is lateral, and that in many Teleosts 

 the first rudiment is not exactly median. These are indica- 

 tions of transitional conditions, and suggest that a wandering 

 round the gut has taken place. Important evidence bearing 

 on this question is afforded by the manner in which the 

 organ is supplied with blood. In the typical lung, and also 

 in the "swim-bladder" in Dipnoans and Crossopterygians, 

 the blood is supplied by means of a pulmonary artery on 

 each side, arising from the sixth aortic arch. In the case of 

 the typical Actinopterygian, on the other hand, the blood 

 supply of the swim-bladder is derived from branches of the 

 coeliac artery and branches of the dorsal aorta situated 

 posterior to this, an apparently fundamentally different 

 arrangement. Now, as we know that the swim-bladder 

 arises as a small localised rudiment from the pharyngeal 

 wall, and grows back secondarily, any blood supply derived 

 from farther back branches of the dorsal aorta, and 

 particularly any blood supply whose vessels arise over 

 a considerable antero-posterior extent of the dorsal aorta, 

 must be looked on as secondary. That this is so, and that 

 the blood supply of the swim-bladder was primitively the 

 same as that of the typical lung, is indicated further by the 

 fact that in at least one Actinopterygian, Amia, such a blood 

 supply is still retained. 



Upon the whole, the view that seems most tenable regarding 

 the phylogeny of lung and swim-bladder is this : — 



1. lb arose as a median ventral diverticulum. 



We must conclude this from its always arising in this posi- 

 tion in ontogeny, except in the specialised Actinopterygians. 



2. Its ventral position renders it probable that the 

 structure functioned at first as a lung rather than as a 

 hydrostatic organ, and this is supported by the fact that in 



