26 THE DIODON. [chap.M 



of manganese and iron. In the Orinoco it occurs on tfl 

 rocks periodically v/ashed by the floods, and in those pai^ 

 alone where the stream is rapid; or, as the Indians sdfl 

 "the rocks are black where the waters are white." HeH 

 the coating is of a rich brown instead of a black coloi^B 

 and seems to be composed of ferruginous matter alonfl 

 Hand specimens fail to give a just idea of these brov^B 

 burnished stones which glitter in the sun's rays. Th^H 

 occur only within the limits of the tidal waves ; and ^| 

 the rivulet slowly trickles down, the surf must supply t^M 

 polishing power of the cataracts in the great rivers. 19 

 like manner, the rise and the fall of the tide probably answer 

 to the periodical inudations ; and thus the same effects are 

 produced under apparently different, but really similar, cir- 

 cumstances. The origin, however, of these coatings of 

 metallic oxides, which seem as if cemented to the rocks, 

 is not understood ; and no reason, I believe, can be assigned 

 for their thickness remaining the same. 



One day I was amused by watching the habits of the 

 Diodon antennaius^ which was caught swimming near the 

 shore. This fish, with its flabby skin, is well known to 

 possess the singular power of distending itself into a nearly 

 spherical form. After having been taken out of water for 

 a short time, and then again immersed in it, a considerable 

 quantity both of water and air is absorbed by the mouth, 

 and perhaps likewise by the branchial orifices. This process 

 is effected by two methods : the air is swallowed, and is then 

 forced into the cavity of the body, its return being prevented 

 by a muscular contraction which is externally visible ; but 

 the water enters in a gentle stream through the mouth, 

 which is kept wide open and motionless ; this latter action 

 must, therefore, depend on suction. The skin about the 

 abdomen is much looser than that on the back ; hence, 

 during the inflation, the lower surface becomes far more 

 distended than the upper ; and the fish, in consequence, 

 floats with its back downwards. Cuvier doubts whether 

 the Diodon in this position is able to swim ; but not only 

 can it thus move forward in a straight line, but it can 

 turn round to either side. This latter movement is effected 

 solely by the aid of the pectoral fins ; the tail being collapsed, 

 and not used. From the body being buoyed up with so 

 much air, the branchial openings are cut of water, but a 

 stream drawn in by the mouth constantly flows through 

 them. 



