1889.] 



Contributions to the Anatomy of Fishes, 



325 



In all cases the atrophied bladder is partially or completely enclosed 

 within osseous recesses. In one or two instances {e.g., Schilbichthi/s 

 and probably also Eutropiichthys) the air-bladder, although solid 

 mesially, nevertheless retains in each half traces of its original and 

 normal division into anterior and lateral compartments, but the ex- 

 treme thickness of its walls, and the small size of its internal cavities, 

 afford sufficient proof of its degenerate and functionless condition. 

 Solidification of the central portion of the bladder may reduce the cavity 

 of that organ to the condition of a circular canal of fairly uniform 

 calibre, surrounding a massive central pillar (Silondia). Two species 

 afford good examples of the extreme variability to which degenerate 

 structures are liable. In one {Ailia), the bladder assumes the 

 shape of a tubular horse- shoe, and is almost solid, except at its hollow 

 forwardly curved cornua ; in the other {Pimelodus scvpo) the organ is 

 solid mesially, and of its two pairs of forwardly curved lateral branches 

 one only is hollow and receives the insertion of the tripodes. In one 

 case {Gryptopterus micropus, and possibly also G. hexaptera) the bladder 

 consists of two partially separated lateral sacs, but its degenerate 

 character is betrayed by the partial obliteration of the cavity of the 

 posterior half by a network of fibrous bundles. 



The more frequent condition of the air-bladder in this group is in 

 the form of two simple, pyriform or globose, thin-walled, laterally 

 placed air-sacs, which are either quite distinct or connected by an 

 intermediate tubular portion {e.g., Glyptosternum, Getopis, Acrochordon- 

 ichthys, Bagarius, Akysis, Clarias, Saccobranchus, and Pimelodus 

 pulcher). 



The skeletal attachments of the air-bladder both to rigid portions 

 of the skeleton and to movable tripodes exhibit much the same 

 extent and kind of variation as we have already described in the case 

 of the anterior chamber of the more normal Siluroids. In such forms 

 as possess rudiments of lateral chambers and of transverse and longi- 

 tudinal septa the attachments of the air-bladder to the vertebral 

 column and its processes are quite normal, and even in many 

 members of this group where those structures are entirely wanting, 

 the skeletal attachments are in the main very similar to those of the 

 normal Siluroids. Thus, in those cases in which the air-bladder is 

 represented by two entirely distinct or mesially intercommunicating 

 sacs, the dorsal attachment of the primary transverse septum is repre- 

 sented either by the skeletal attachment of the dorsal edge of the 

 median portion of the posterior wall to the ventral surface and sides 

 of the complex centrum {Pimelodus puloher), or by a similar attach- 

 ment of the corresponding margin of the posterior wall of each lateral 

 air- sac to the transverse processes of the fourth or fifth vertebra 

 {Bagarius, Glyptosternum, Glarias). The median and tubular portion 

 of the air-bladder, when present, is always thin, and its firm attach- 



