601 



most important of which are specimens of hake (Urophycis sp.), and 

 scup (Stenotomus chrysops). 



The material for this study was obtained chiefly by the use of the 

 lobster - rearing apparatus with modifications for the rearing of fishes 

 as devised by Mead ('08), at the Wickford Experiment Station of the 

 Rhode Island Commission of Inland Fisheries. With this apparatus 

 several of the species named above have been successfully reared, and 

 thus a consecutive series of the larval and adolescent stages has been 

 secured. Most of the technical work of this investigation was done 

 in the laboratories of Brown University under the direction of Dr. 

 A. D. Mead. 



I. Comparative Anatomy. 



The form and structure of the swim-bladder varies extremely in 

 the different groups of Teleosts. But it appears to be possible to 

 reduce these very different forms of the organ to a few important 

 morphological types. These types, arranged in series in the order of 

 the specialization of the organ, may be diagrammatically represented 

 as in Figures 1 to 6. The following descriptions of the different types 

 are generalized and refer only to the more essential anatomical and 

 histological characteristics. No account is taken of the many remar- 

 kable special relations of the swim- bladder in particular species, since, 

 in most cases, these conditions appear to be of no general morpho- 

 logical significance. The descriptions of certain of these types are not 

 primarily intended to refer to the swim - bladder of any particular 

 species, but are rather to be taken as essentially true for whole groups 

 of fishes. The limits of most of these groups are probably not de- 

 finitely known at the present time. 



The most primitive type of swim-bladder is represented by that 

 of the salmon (Fig. 1). It is a simple sack-like bladder with an open 

 pneumatic duct leading from the anterior of the organ into the oeso- 

 phagus (Corning, '88). Another primitive type is found in the Cypri- 

 niidae (carp, Fig. 2). The swim-bladder in these fishes is hourglass- 

 shaped, with an anterior and a posterior chamber, of which the latter 

 is connected with the oesophagus by the long, narrow pneumatic duct. 

 For reasons to be stated later, the Clupeoid swim -bladder is to be 

 considered a modification of this latter type. The lining of the swim- 

 bladders of all these primitive forms consists essentially of a simple, 

 undifferentiated epithelium. 



Closely related to these primitive types is the swim -bladder of 

 the pickerel (Esox, Fig. 3). This consists of a much elongated 



