i g22.] S. L. Hora : Fish of Mountain Torrents- 43 



surface of the head and body are firmly and closely applied to 

 the rock to which it may be clinging at the time, and this also 

 will make respiration difficult. In all probability the following 

 factors help hill-stream fishes in respiration : — 



(i) The water in the hill-stream is better oxygenated and is 

 purer than that of a sluggish stream in a flat country. 



(ii) By reducing the gill-openings, the fishes are enabled 

 to retain water in their gill-chambers for a comparatively longer 

 time. 



(iii) The inner rays of the pectoral fins in fishes of rapid 

 streams are held in constant motion when the fish rests against 

 a piece of rock. The movements of these rays may help respira- 

 tion in two ways : — 



(a) The blood may be oxygenated in the rays themselves, 



or (b) they may force water in and out of the gill-opening. 



The following quotation from Mr. Chapin's notes given by 

 Nichols and Griscom ' on the mechanism of respiration in Enchi- 

 lichthys dybowskii (Vaillant) when clinging to rocks is very interest- 

 ing : — " Two examples were brought alive in a basin where they 

 stuck fast to the smooth enamel surface. When thus attached, 

 the water for respiration enters by the back of the mouth, and 

 the movement of the gills often makes the whole fish quiver or 

 move slightly back and forth. Natives say they cling to rocks and 

 eat algae. They can swim rapidly. The mouth is here drawn 

 as though slightly extended ; while sucking, it of course contracts." 

 The above observations were made on fishes in a state of captivity 

 and require confirmation. The sucker by means of which the fish 

 adheres appears from the figure to surround the mouth completely, 

 and it is probable that the fish uses both lips for adhesion as in the 

 Indian hill-stream forms. The posterior jaw is in almost all cases 

 more highly specialized for rasping the algal slime from the rocks 

 than the anterior jaw and under the circumstances detailed above, 

 it seems highly improbable that water can enter the gill cavities 

 from the back of the mouth when the fish is either feeding on 

 algal slime or clinging to a rock. 



With the reduction of the gill-openings and the backward 

 shifting of the mouth on the under surface considerably behind 

 the tip of the snout, the branchiostegal rays and membranes are 

 greatly reduced. Usually these structures on the two sides of a 

 fish such as Labeo rohita meet and overlap on the under surface, 

 but in hill-stream fishes, with the exception of those belonging to 

 the genera Glyptothorax and Laguvia, they form an obtuse angle 

 on the under surface, if they meet at all. 



8. The air-bladder. — The bladder in the hill-stream forms 

 shows considerable degeneration and in 1893 Bridge and Haddon* 



1 Nichols and Griscom, Bull. Amer. Mus. Nat. Hist. XXXVII, p. 720, pi, 

 Ixxvi, fig. 3 (1917). 



2 Bridge and Haddon, Trans. Phil. Soc. London, vol. 184, part I (B), p. 

 305 ft8Q3). 



