20 A HISTORY OF FISHES 



adaptations, cxolvcd in response to a change in habits or 

 environment, and, since they are often accompanied by a 

 modification of certain organs for the particular end, most of 

 them may be conveniently considered in later pages. Flying, 

 for example, which involves the modification of the pectoral 

 fins, may be discussed in the chapter devoted to those organs. 



The three classes of vertebrates here grouped together as 

 fishes include a very diversified assemblage of forms, but there 

 is, nevertheless, a basic similarity in their swimming movements, 

 however diflferent these may appear to be at first sight. The 

 earliest fishes probably swam by means of simple rhythmic 

 contractions of the muscles of the trunk and tail, designed to 

 produce certain definite contortions of the body; by the pressure 

 of diflferent parts of the body in succession against the sur- 

 rounding water the animal was driven forward. Most fishes 

 have retained the primitive arrangement of the great body 

 muscles, the myomeres, as they are called {cf. p. 167), which 

 form a series of blocks or segments, arranged in pairs one 

 behind the other and separated by partitions. In this respect 

 fishes diflfer from all land vertebrates, in which the main muscle 

 masses are more or less concentrated on the fore and hind 

 limbs, these being the normal organs of locomotion, whereas 

 the corresponding pectoral and pelvic fins of fishes more often 

 than not perform quite subsidiary functions, such as balancing 

 and steering. We have already noticed the essential similarity 

 in the shape of the body in the cetaceans and fishes, but, 

 owing to its diflferent ancestry, the arrangement of the body 

 muscles in a whale is quite unlike that found in a fish, and the 

 swimming movements themselves are in a diflferent plane, 

 being up and down instead of from side to side. This explains 

 the dissimilarity in the position of the tail-fin in the two groups 

 {cf. p. 4) ; the fish generally swims more or less parallel with 

 the surface of the water, and the vertical caudal fin is designed 

 to assist in driving it forward and to act as a rudder; the whale 

 is under the necessity of coming to the surface from time to 

 time, and swims by alternately rising and diving in a sort of wave- 

 like curve, the horizontal tail assisting to drive the animal 

 upwards or downwards as the case may be. 



Three primary methods are employed by fishes to produce 

 forward movements while suspended in a fluid medium: 

 (i) body movements due to alternate expansion and con- 

 traction of the myomeres; (2) movements of the appendages 

 (fins) ; and (3) movements caused by the action of jets of water 



