22 A HISTORY OF FISHES 



compared the body of this swift fish to the man-made and 

 screw-propelled submarine, but here the resemblance ends, 

 since no revolving motion of one part on another is possible in 

 the living animal. It has been stated that the Mackerel is able 

 to twist its tail to such an extent as to be "very much after the 

 manner of a screw in a steamship, and thus to drill the water," 

 but this is, to say the least, very unlikely, and is in no way 

 supported by actual observation. Most standard text-books on 

 fishes contain the statement that they swim by means of lateral 

 flexions of the hinder part of the body, aided by movements 

 of the caudal fin. This is, in the main, correct, but recent 

 investigations tend to show that the part played by the tail 

 in producing a forward thrust is far less important than was 

 previously supposed. In this connection the following experi- 

 ment carried out by Mr. Breder is of some interest. Two Rudd 

 (Scardinius) of equal size were selected, and, the tail of one of 

 them having been carefully amputated, the two fishes were 

 placed in a tank eight feet long, where they rested side by side 

 at one of its ends. The investigator carefully approached this 

 end and gave the glass a smart blow with his hand. Imme- 

 diately the two Rudd scurried away to the opposite end of the 

 tank, where they came to rest in similar positions. This experi- 

 ment was carried out several times, and each time it was 

 repeated both fish arrived at the opposite end at the same time, 

 having traversed the intervening space side by side. The only 

 noticeable difTerence between the two specimens was that the 

 fish without a tail-fin "waggled" the hinder part of its body 

 faster and through a wider arc. 



When the Mackerel wishes to move forward the first action 

 which takes place is the contraction of the first few myomeres 

 at the front end of the body on one side only, resulting in the 

 throwing of the head sharply to one side. The successive 

 segments then alternately contract and relax from the head 

 towards the tail, and the curve or flexure of the body is, so 

 to speak, passed backwards (Fig. 9). The effort culminates in 

 bringing the tail to the axis of the head with a powerful sweep. 

 Since, at the commencement of the stroke the pivotal point of 

 the swing lies just where the backbone joins the skull, a com- 

 paratively short swing of the head is all that is necessary to 

 bring the tail into position for a long and strong sweep; as this 

 is carried out the pivot must necessarily move backwards, 

 until at the end of the stroke it lies nearer to the tail than to 

 the head. The accompanying illustrations of a swimming fish 



