CONDITIONS OF LIFE 239 



to ascend the vertical walls of the large pot-holes in the bed of 

 the stream (Fig. 930). In other hill-stream fishes the paired 

 fins also take part in the formation of an adhesive disc, generally 

 being placed more or less horizontally at the sides of the body, 

 as in the Bornean Sucker {Gastromyzon) , in which there are as 

 many as twenty-six to twenty-eight rays in each pectoral fin, 

 and twenty or twenty-one in each pelvic (Fig. 35c). 



Other modifications of the mouth and associated structures 

 are connected with the food and methods of feeding in these 

 waters, and may be directly traced to the habit of stripping 

 vegetable slime or weeds from the surfaces of rocks and stones. 

 The snout is nearly always broad and flat, and the mouth, 

 generally crescentic in outline, lies on the under side of the 

 head. In rapidly flowing water barbels would be a hindrance, 

 and these are reduced to minute proportions or absent alto- 

 gether. In the toothless Cyprinids the jaws may be sharp and 

 cutting at their edges, and in some species they are covered 

 with a strong horny sheath. In the Cat-fishes of the family 

 Sisoridae the jaws are often armed with broad bands or patches 

 of minute teeth of various shapes, which act after the manner 

 of miniature rasps. A remarkable Cyprinid {Gyrinocheilus), 

 found only in the mountain streams of the Malay Peninsula 

 and Archipelago, has taken to feeding solely on mud, and the 

 pharyngeal teeth, so characteristic of other members of this 

 tribe, have entirely disappeared, the pharyngeal bones them- 

 selves being vestigial, and the horny pad at the base of the skull 

 absent. The lips surround the mouth and form a funnel-like 

 sucker, serving not only to scoop up the mud, but also to enable 

 the fish to cling to stones and other objects. The diet has also 

 led to changes in the internal organs, and, since the proportion 

 of nutritive matter in mud is very small, large quantities must 

 be swallowed at a time, and so the intestine has become very 

 much elongated, being about fourteen times the length of the 

 fish itself. 



When attached by the mouth to stones in the stream bed, or 

 engaged in feeding, it is obvious that these fishes are often 

 unable to take in water through the mouth for breathing 

 purposes in the usual manner. In the Malay Cyprinid just 

 mentioned, this difficulty is overcome by having the external 

 gill-opening on each side divided in such a way that the water 

 flows in through the upper part and out through the lower, 

 and a similar arrangement is adopted by some of the Cat-fishes. 

 In other hill-stream forms, however, the method of breathing 



