CHAP. XXV. 



How Fish Swim. 



355 



in the need for exceptional rest in the absence of food, and this I should 

 call sleep. 



" In my humble opinion, therefore, fish do sleep. But it is as difficult to 

 prove as to ' catch a weasel asleep.' 



" And now it is my turn to propound a conundrum to ' Laird.' Do fish 

 yawn? My belief is that I saw one yawn to-day. I have a stream of 

 ornamental water running through my grounds, and as I stood, motionless, 

 watching a trout keeping his position, with 

 scarcely perceptible motion, in a languid part 

 of the clear stream, now and again rising to a 

 black gnat,. he opened his mouth very slowly 

 below the surface, and more widely than for 

 ordinary respiration ; and it seemed to me 

 that he was longer in closing it than he ordin- 

 arily would be when taking food. But I have 

 never seen the like movement in a fish before, 

 and it is idle to draw any conclusions from a 

 single, and perhaps imperfect, observation." 



A fish's means of progression has in- 

 terested me much, and may similarly interest 

 some of my readers. I, therefore, make 

 the following extract, and copy the drawing 

 from the English Cyclopaedia : 



"The whole of the fins are more or less 

 employed in certain kinds of movements. In 

 order to ascertain the true use of the fins in 

 swimming, Borelli, having cut off the ventral 

 fins of a living fish, put it back again into the 

 pond. It then rolled from side to side like a 

 drunken man, and could not keep an upright 

 position. When the fish move with great 

 velocity the pectoral fins are laid close to the 

 body, in order that they may not retard its 

 motion ; and in rapid motion the tail becomes 

 the great propelling organ. We shall there- 

 fore now investigate its mode of action. The first movement of a fish 

 from a state of rest is produced by a flexion of the tail (as seen in 

 Fig. at a] ; during this movement the centre of gravity (c) is drawn slightly 

 backwards. When the tail has arrived at a, it is forcibly extended by its 

 muscles in the direction a z, perpendicular to its plane ; the force of its 

 action upon the water in a i is translated to the fish in the direction of i a 

 causing the centre of gravity (c] to move obliquely forwards, in the direction 

 c h parallel to i a. The tail having reached the centre line c d. its power 



2 A 2 



