252 BONY FISHES ix. 3- 



will take either form of food. The parrot-fishes (Scaridae) have a beak 

 and a grinding mill of flattened plates in the pharynx. With this they 

 break up the corals, rejecting the inorganic part from the anus as a 

 calcareous cloud. The Cyprinidae, including many of our commonest 

 freshwater fishes (goldfish, carp, perch, and minnow), have no teeth 

 on the edge of the jaw, hence the name 'leather-mouths'. There are, 

 however, teeth on the pharyngeal floor, biting against a horny pad 

 on the floor of the skull. These fishes are mainly vegetarians, but many 

 take mouthfuls of mud and extract nourishment from the plants and 

 invertebrates it contains. 



4. Protective mechanisms of bony fishes 



In general teleosts depend for protection against their enemies on 

 swift swimming, powerful jaws, good receptors, and brain. The 

 majority of them have thus been able to abandon the heavy armour 

 of their Palaeozoic ancestors. In many cases, however, subsidiary 

 protective mechanisms have been developed, and are especially pro- 

 minent in fishes that have given up the fast-swimming habit and taken 

 either to moving slowly among weeds or to life on the bottom. These 

 developments are a striking example of the way in which, following 

 adoption of a particular mode of life, appropriate subsidiary modifica- 

 tions take place, presumably by selection of those varieties of struc- 

 ture that are suited to the actions of the animal. 



These protective devices may be classified as follows : 



1. Protective armour of the surface of the body. 



2. Sharp spines and poison glands. 



3. Electric organs. 



4. Luminous organs. 



5. Coloration. 



5. Scales and other surface armour 



The typical cycloid teleostean scales have already been described. 

 They form a covering of thin overlapping bony plates, providing some 

 measure of protection, but not interfering with movement. The 

 hinder edges of the scales are sometimes provided with rows of spines, 

 and are then said to be ctenoid. In many fishes the scales bear 

 upstanding spines and possess a pulp cavity, which recalls that of 

 denticles. In the tropical globe-fishes (or puffers) and porcupine- 

 fishes (Diodon) these spines are very long and sharp and the puffers 

 are able to inflate themselves and cause the spines to project outwards, 

 a very effective protective device. In a few fishes the scales have 



