Emhviioloijij of certain of the Loiocr Fuhes. 201 



If the former, we should probably conclude that their function 

 was from the beginning a simple respiratory one ; they would 

 be looked on simply as local enlargements and outgrowths 

 of the primitively respiratory gut surface, which finally 

 developed an opening to the exterior, connected with the 

 causing a more perfect flow of water over the respiratory 

 surface. If, on the other hand, they first acquired their 

 directly respiratory surface as clefts, then we must assume 

 that in their first beginnings they had some other function. 

 Now, I think it is not impossible that such a significance 

 may be found in connection with the external gills. For the 

 functioning of the external gills, it is essential that the water 

 over their surface should undergo frequent renewal. It seems 

 perfectly possible that the clefts should have developed in 

 relation to this need ; supplies of fresh water being pumped 

 through them over the external gills. Were this view shown 

 to be correct, we should regard the direct respiratory activity 

 of the walls of the clefts as secondarily acquired— most prob- 

 ably by the spreading inwards of respiratory ectodermal 

 epithelium along the walls of the clefts. What looks very 

 like such a spreading inwards of ectoderm to form the 

 respiratory lining of the gill-cleft, may be seen in the 

 ontogeny of various water-breathing vertebrates. 



Then finally, in connection with the pharynx, we have 

 the swim-bladder and lungs. In development, each of 

 these arises as an outgrowth of the pharyngeal wall : in the 

 case of the lungs, a medioventral outgrowth ; in the case of 

 the swim-bladder, of a Teleost, mediodorsal. The question of 

 the homology of the swim-bladder and lungs has been much 

 discussed. The knowledge which we now possess appears 

 to render quite untenable the position of those who dispute 

 the homology of the two organs in at least certain groups. 

 In the case of Crossopterygians and Dipnoans, the swim- 

 bladder arises exactly as do typical lungs, and in the adult 

 condition the blood supply is identical. In this case it 

 would seem impossible to doubt the homology. Wieders- 

 heim recently found in Lepidosiren a mediodorsal groove 

 on the pharyngeal wall, which he interpreted as a last 

 vestige of a true dorsal swim-bladder : the fact, however, 



