24 CHARLES DARWIN 



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

 ing the inflation, the lower surface becomes far more dis- 

 tended than the upper; and the fish, in consequence, floats 

 with its back downwards. Cuvier doubts whether the Dio- 

 don 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 out of water, but a stream drawn in 

 by the mouth constantly flows through them. 



The fish, having remained in this distended state for a 

 short time, generally expelled the air and water with con- 

 siderable force from the branchial apertures and mouth. It 

 could emit, at will, a certain portion of the water; and it 

 appears, therefore, probable that this fluid is taken in partly 

 for the sake of regulating its specific gravity. This Diodon 

 possessed several means of defence. It could give a severe 

 bite, and could eject water from its mouth to some distance, 

 at the same time making a curious noise by the movement 

 of its jaws. By the inflation of its body, the papillae, with 

 which the skin is covered, become erect and pointed. But 

 the most curious circumstance is, that it secretes from the 

 skin of its belly, when handled, a most beautiful carmine- 

 red fibrous matter, which stains ivory and paper in so per- 

 manent a manner that the tint is retained with all its bright- 

 ness to the present day: I am quite ignorant of the nature 

 and use of this secretion. I have heard from Dr. Allan of 

 Forres, that he has frequently found a Diodon, floating alive 

 and distended, in the stomach of the shark; and that on 

 several occasions he has known it eat its way, not only 

 through the coats of the stomach, but through the sides of 

 the monster, which has thus been killed. Who would ever 

 have imagined that a little soft fish could have destroyed 

 the great and savage shark? 



March i8th. We sailed from Bahia. A few days after- 

 wards, when not far distant from the Abrolhos Islets, my 

 attention was called to a reddish-brown appearance in the 

 sea. The whole surface of the water, as it appeared under a 



