i6 A HISTORY OF FISHES 



(Fig. 40D), provide excellent examples. The compressed body, 

 on the other hand, although unknown in Selachians, is much 

 more common in Bony Fishes. Often it is shortened as well, 

 and, flexibility of the body being no longer an absolute neces- 

 sity, many of these forms are able to afford heavy protective 

 armour of some kind. The brilliant little Butterfly-fishes 

 {Chaetodontidae) of tropical coral reefs are excessively quick in 

 their movements, in spite of their short, deep, flattened bodies, 

 and ihey rely largely on their agility to escape being eaten, 

 coupled with the fact that their deep bodies and strong, spiny 

 fins make them awkward mouthfuls to swallow (Fig. 83G, d). 

 The beautiful Angel-fish (Pterophyllum) of the rivers of South 

 America, a familiar object in aquaria, has a very much com- 

 pressed and almost circular body, and the large fins have 

 some of the rays drawn out into lengthy filaments (Fig. 8c). 

 It is a very slow swimmer, spending most of its time suspended 

 almost motionless in mid-water, and relies on its remarkable 

 resemblance to the water plants among which it lives to escape 

 detection. The Flat-fishes [Heterosomata) , a group which 

 includes such well-known edible forms as the Halibut, Turbot, 

 Plaice, and Sole, all have very much flattened bodies, and, 

 like the Rays, spend much of their time on the sea floor, where 

 their mottled coloration harmonises with the ground on which 

 they lie and renders them inconspicuous (Figs. 8b; 40A-G). 

 In the minds of many people the Plaice and the Skate are 

 lumped together as "Flat-fishes," but it is obvious that the 

 resemblance between the two fishes is a purely superficial one. 

 Both have taken to a life on the bottom, where a flattened 

 body is a decided advantage, but the Skate has become flattened 

 from above downwards, whereas the Plaice is compressed from 

 side to side. In other words, the colourless lower surface of the 

 former which rests on the bottom is the true lower or ventral 

 side, whereas in the Flat-fish this surface represents the right 

 or left side. 



The Globe-fishes or Puffers (Tetrodontidae) and their relatives 

 the Porcupine-fishes {Diodontidae) provide examples of fishes 

 with shortened, rounded bodies, in which the consequent loss 

 of swimming power is compensated for by the development of 

 some sort of bodily armour in the form of spines or small 

 prickles [cf. p. 98). In addition to their spiny covering, these 

 fishes possess the power of swallowing water or air and thereby 

 inflating the body like a balloon. When thus inflated they are 

 fond of floating passively with the currents, more often than 



