44 Records of the Indian Museum. [Voi,. XXIV, 



attributed the reduction to the following causes, which bear repeti- 

 tion even to-day. They say : — "The causes that have led to the 

 degeneracy of the air-bladder in so many forms are in many in- 

 stances not difficult to trace, and, as in so many Physoclist 

 Teleostei, the assumption of a purely ground habit of life is prob- 

 ably the most important one. Not a few of the genera of 

 Siluridae abnormales inhabit the comparatively shallow waters of 

 rapidly flowing mountain streams and torrents often living at a 

 considerable altitude, and in general habit are not unlike our 

 common English Loaches. Many are provided with an adhesive 

 apparatus on the ventral surface of the body between the pectoral- 

 fins for attachment to stones, so that they may be enabled to 

 withstand the force of mountain torrents. Such fishes when not 

 in motion by the exercise of their fins probably rest upon, or 

 attach themselves to, the river bottom, and the utter uselessness 

 and probable harmfulness of an air-bladder as a hydrostatic organ 

 under such conditions is no doubt the cause of its degenerate and 

 rudimentary conditions in such Siluroids as Sisor, Pseudecheneis, 

 Glypto sternum, Euclyptostemum, Exosloma, .1 mblyceps , etc." I 

 have dealt with this interesting organ at some length in my pre- 

 vious paper and have shown in the case of the genus Garra that 

 the reduction in the organ is directly proportional to the strength 

 of the current of the streams in which the fish live. 



In almost all the highly specialized hill-stream forms such as 

 certain species of Loaches, Homalopterid fishes and the forms 



included under the Silurid 

 genera, Glyptothorax , 

 Glyptosternum and Pseu- 

 decheneis the bladder is, 

 divided into two lateral 

 chambers (fig. 6) which 

 are more or less connect- 

 ed with each other by a 

 short, narrow transverse 

 tube. Moreover the blad- 



Text-fig. 6. — Air-bladder of Glyptothorax der is wholly or partially 



madraspatanus. encapsuled by a bony 



case in almost all cases. 

 Q. Special modifications of the skin. — Under this heading I 

 include the diverse forms of modifications exhibited by the skin in 

 the formation of adhesive organs. The simplest form of special- 

 ization occurs in Cyprinid fishes, where the skin covering the under 

 surface of the few outer rays of the paired fins is greatly thickened 

 and becomes cushion-like in places. By these cushion-like pads 

 the fishes are enabled to cling to rocks and hold their own against a 

 rapid flow of water. In the Silurid genera the skin instead of being 

 plain is thrown into grooves and ridges. Such striated portions of 

 skin may occur anywhere on the under surface of the fish but are 

 generally found in the anterior third of the body. I have found 

 such striated surfaces on the barbels, on the sides of the mouth, 



