646 



are not directly related to the morphologically primitive forms in which 

 the embryonic duct opens into the anterior end of the swim-bladder, 

 but are rather to be classed with the more highly specialized types 

 in which the pneumatic duct is posterior in the embryo. 



III. Morphology of the Swim-bladder. 



Of the results of this embryological investigation, the most im- 

 portant from the point of view of the general morphology of the organ 

 is, that the posterior chamber in the swim-bladder of Opsanus, Sipho- 

 stoma, etc., develops directly from the embryonic pneumatic duct. The 

 posterior chamber in these forms, therefore, represents the pneumatic 

 duct which has lost its connection with the oesophagus. 



This conclusion may be made the basis of a suggestive comparison 

 between the different types of the organ. In the eel, the pneumatic 

 duct has not lost connection with the oesophagus, while the histological 

 structure of its walls is like that of the posterior chamber in the Op- 

 sanus swim-bladder; on the other hand, the essential structure of the 

 oval could apparently be produced by the approximation of the trans- 

 verse partition in the Opsanus swim-bladder to its posterior wall with 

 the consequent obliteration of the cavity of the posterior chamber. 

 It is therefore natural to infer that in the phylogenetic development 

 of the swim-bladder, the posterior chamber is probably a transitional 

 stage between the pneumatic duct in the eel and the oval in the 

 highest type. The clue to the morphology of the swim -bladder in 

 Teleosts is thus to be found in the progressive reduction of the pneu- 

 matic duct, as a result of which there is produced, first, the pneumatic 

 duct in the eel with its rete mirabile, then the posterior chamber in 

 the Opsanus type, and finally, the oval in the most highly specialized 

 swim-bladder. 



The evolution of these structures is accompanied by a progressive 

 differentiation of the epithelial lining of the swim-bladder which results 

 in the development of the different types of the red gland. The epi- 

 thelium is simple and undifferentiated in the primitive swim-bladders. 

 In the eel, the cells of the epithelium have become columnar and are 

 thrown up into simple folds over the greater portion of the internal 

 surface. In the higher types, the differentiation and folding of the 

 epithelium takes place only in certain restricted areas, but the folds 

 have become correspondingly more complicated. The red gland which 

 is thus formed differs in complexity in different groups of fishes, but 

 in general it seems to be more highly developed in the type of swim- 

 bladder having the oval. 



