CONDITIONS OF LIFE 237 



but others have been brought about by one or more of the other 

 factors just mentioned. 



All hill-stream fishes are necessarily bottom-living forms, 

 and their bodies are generally much flattened from above 

 downwards, sometimes being almost leaf-like (Fig. 35c). The 

 Loaches {Cobitidae) form an exception, but their small size and 

 narrow cylindrical form are admirably adapted for creeping 

 into holes and crevices beneath stones. The lower surface of the 

 body in other forms is almost invariably flat, and in order to 

 allow of the adhesion of this surface to that of rocks or stones, 

 the scaly covering is much reduced below, especially in the 

 region of the chest and belly. At the same time, the absence 

 of larger predaceous fishes has obviated the necessity for armour 

 above, and thick scales, scutes, and spines are normally absent. 

 In the Loricariids, for example, the bony scutes with which the 

 whole head and body are covered in the numerous lowland 

 species are entirely wanting in those of the streams of the 

 Andes. Where the current is rapid, a smooth flat lower surface 

 may not be sufficient to obtain a hold during periods of heavy 

 rains, and many fishes have developed a more elaborate 

 adhesive apparatus. In some Asiatic Cat-fishes {Glyptosternum, 

 Pseudecheneis) the skin of the lower surface is puckered up into 

 grooves and ridges, these being generally most prominent on 

 the thorax or on the under side of the outer ray or rays of the 

 pectoral or pelvic fins. These ridges seem to act as a mechanical 

 friction device, designed simply to prevent the fish from slipping, 

 but where the skin is produced into loose folds a more or less 

 effective vacuum may be created by raising or depressing the 

 folds, the apparatus functioning in much the same way as the 

 sucker of a Remora {cf. p. 70). In some Cyprinids the skin 

 covering the lower surface of a few of the outer rays of the 

 paired fins is greatly thickened, in places forming cushion-like 

 pads which enable the fish to chng to the surface of a rock. 

 In others, some sort of adhesive disc working on the vacuum 

 principle is developed, generally taking the form of a rounded 

 or oval structure composed of a pad-like central portion sur- 

 rounded by a membranous flap. As a rule this lies close behind 

 the mouth, the surrounding membrane being formed by the 

 modification of the lips. In the Loaches {Cobitidae) and Suckers 

 {Gastromyzonidae) it is the mouth itself that forms the disc, the 

 lips being greatly swollen and divided in the middle, so that 

 when pulled outwards away from the mouth they pro\ide a 

 ring-like sucker. Some of the Asiatic Cat-fishes {Glyptostemum, 



