vii. 1-2 THE TROUT 191 



Bony fishes abound not only in the sea but also in fresh water, 

 which has never been effectively colonized by cephalopods or elasmo- 

 branchs. They can exist under all sorts of unfavourable or foul- 

 water conditions and a considerable number of them breathe air and 

 live for a time on land. Perhaps the majority are carnivorous, but 

 others feed on every type of food, from plankton to seaweeds. 



To whatever feature of fish life we turn we find that the bony fishes 

 excel in it in several different ways in different species. It is small 

 wonder that with all these advantages they are excessively numerous. 

 There are some 3,000 species of living elasmobranchs, but more than 

 20,000 species of bony fish have been described. 



The number of individuals of some of the species must be really 

 astronomical. For instance, at least 3,000 million herrings are caught 

 in the Atlantic Ocean each year, so that the whole population there 

 can hardly be less than a million million. Again, it is estimated that a 

 thousand million blue-fish collect every summer off the Atlantic coast 

 of the United States and, being very voracious carnivores, they con- 

 sume at least a thousand million million of other fishes during the 

 season of four months. This gives some idea of the tremendous 

 productivity of the sea, and of the way the bony fishes have made 

 use of it. Needless to say, man has also made considerable use of the 

 bony fishes, which indeed provide, with the elasmobranchs, a not 

 inconsiderable portion of the total of human food. 



2. The trout 



Salmo trutta, the brown trout, may be taken as an example of a bony 

 fish; we shall also refer at intervals to conditions in other common 

 freshwater fish such as the dace, Leuciscus, and perch (Perca fluvi- 

 atilis). There is considerable confusion about the various types of 

 trout and their close relatives the salmon. The brown trout is abundant 

 in rivers and streams throughout Europe and is commonly about 

 20 cm long at maturity, though it may grow larger. It is grey above 

 and yellowish below, with a number of dark spots scattered down 

 the sides of the body (Fig. 116). 



The body form is typical of that of teleostean fishes in being short, 

 narrow in the lateral plane but deep dorso-ventrally, in fact more ob- 

 viously streamlined than the shape of elasmobranchs. The movements 

 of a trout do not at first sight obviously involve the bending of the body 

 into an S; nevertheless, the method of swimming is essentially by the 

 propagation of waves along the body by the serial contraction of the 

 longitudinally directed fibres of the myotomes (p. 133). 



