248 



BONY FISHES 



IX. i- 



jaws. On the other hand, deep-sea fishes that retain the swim-bladder 

 have a well-developed skeleton and powerful muscles (Marshall, 

 i960). 



2. Various body forms and swimming habits in teleosts 



Departures from the streamlined body form typical of pelagic fishes 

 have been very numerous; in nearly every case they are associated 

 with a reduction in the efficiency of swimming as such and the 

 development of some compensating protective mechanism (Fig. 149). 

 Lateral flattening, which is already a feature of all teleostean organiza- 

 tion, is carried to extremes in many types. Thus the angel-fish, 



P£5tl£!! 



Fig. 150. The angler-fish, Lophius. 



Pterophyllum, often seen in aquaria, is provided with long filaments 

 and a brilliant coloration, which, in its natural habitat (rivers of 

 South America), give it a protective resemblance to plants, among 

 which it slowly moves. The flat-fishes (plaice, sole, halibut, &c.) have 

 carried this flattening to extreme lengths. They feed on molluscs and 

 other invertebrates on the sea bottom and lie always on one side. 

 The upper side becomes darker and protectively coloured, the lower 

 side white. In order to have the use of both eyes the whole head is 

 twisted during the post-larval period. These forms are mostly poor 

 swimmers, but their coloration gives them a remarkable protective 

 resemblance to the background (p. 257). 



The John Dory (Zeus faber) has made a different use of lateral 

 flattening. The fish is so thin that its swimming is very slow, but 

 being inconspicuous when seen from in front it can approach close to 

 its prey, which it then catches by shooting out its jaws. 



Flattening in the dorso-ventral plane is less common among teleosts 

 than selachians. The flattened forms are mostly angler-fishes, of 



