VII 



THE MASTERY OF THE WATER. BONY FISHES 



1 . Introduction : the success of the bony fishes 



The acanthodians and some other of the late Silurian and Devonian 

 gnathostome fishes possessed bony skeletons; from these, or some 

 placoderm animals like them, may have been derived not only the 

 elasmobranchs but also the bony fishes and the lung-fishes, which 

 gave rise to the land animals. These presumed descendants of the 

 placoderms can be divided into three groups : first the elasmobranchs, 

 secondly, the crossopterygians, the lobed-fin or lung-fishes, including 

 the Devonian forms that led to the amphibia, and thirdly, the actino- 

 pterygian or rayed-fin fishes, culminating in the modern bony fishes. 

 In Devonian times the Crossopterygii and Actinopterygii were very 

 alike and both, like the placoderms, contained bone. The term bony 

 fishes or Osteichthyes is often applied to these two groups together, 

 since they have some features in common and distinct from the 

 elasmobranchs. 



The great group of Actinopterygii, which, for all the importance of 

 the elasmobranchs, must be reckoned as the dominant fish type at the 

 present time, includes most of our familiar fishes, perch, pike, trout, 

 herring, and many other types of 'modern' fish. In addition there are 

 placed here some surviving relics of the stages that have been passed 

 before reaching this condition, such as the bichir, sturgeons, bow-fin, 

 as well as related fossil forms. 



Many groups of animals have been successful in the water; Crus- 

 tacea, for instance, are very numerous and so are cephalopod molluscs 

 and echinoderms, but the success of the bony fishes surpasses that of 

 all others. From a roach or perch in a stream, to a huge tunny or a vast 

 shoal of herrings in the sea, they all have the marks of mastery of the 

 water. They can stay almost still, as if suspended, dart suddenly at 

 their prey or away from danger. They can avoid their enemies by 

 quick and subtle changes of colour. Elaborate eyes, ears, and chemical 

 receptors give news of the surrounding world and complex be- 

 haviour has been evolved to meet many emergencies. Reproductive 

 mechanisms may be very complex, involving elaborate nest-building 

 and care of the young; social behaviour is shown in swarming move- 

 ments, which may be accompanied by interchange of sounds (p. 217). 



