The First FoundcUio7i of the Lung of Ceratodus. 319 



XXIII. The First Foundation of the Lung of Ceratodus. 

 (Prelimiuary Notice.) By Gregg Wilson, Ph.D., D.Sc. 



(Read 21st March 1900.) 



Ceratodus, the lung-fish of Queensland, is interesting as 

 being one of the few modern fishes in which lung-breathing 

 supplements the ordinary respiration by gills. It has one 

 lung, in the position of the swim-bladder of ordinary fishes ; 

 while the closely-related Frotopterus of Africa, and Lepido- 

 siren of South America, have each got two lungs. These 

 three forms (the Dipnoi) seem to constitute a connecting 

 link between Pisces and Amphibia, yet the relationship is 

 not clear, and some zoologists prefer to connect the amphi- 

 bians with such primitive Ganoid forms as Folypterus. The 

 real affinities will only be fully apparent after a study of the 

 embryology of the different groups has been made, and as 

 yet little has been done in that direction, either with 

 Dipnoans or with the primitive Ganoids. Semon's splendid 

 Ceratodus material has yielded results as to early stages and 

 the development of fins, teeth, etc., but there is still much 

 to be done in the study of the Dipnoi. 



During a recent visit to Australia I was fortunate enough 

 to get a series of eggs of Ceratodus, and developmental 

 stages up to three months after hatching ; and I am now 

 working up this material, with a view to helping in the 

 determination of the affinities of the Dipnoi. 



One of the first organs to attract my attention was the 

 lung of Ceratodus. Most zoologists believe that the lungs of 

 amphibians and higher vertebrates are homologous with the 

 swim-bladder of fishes, yet no one has explained why the 

 former arise as a ventral outgrowth from the gut, while the 

 latter is dorsal in its origin ; no one has proved a change of 

 position in the foundation of either structure. The only 

 thing to reconcile us to the proposed homology is the fact 

 that in certain fishes the swim-bladder is not connected in 

 the adult with the dorsal side of the gut, but with the lateral 

 or ventral wall ; in Erythrinus it shows a lateral connection, 

 while in Polypterus and Calamoichthys the opening is 

 ventral. 



