452 SCIENCE PROGRESS 



of these experiments was g'6S grammes, that of the lead at the 

 end of the experiment was '188 grammes, or less than one-fiftieth 

 of the total weight of the fish. That would be to the fish what a 

 three pound weight would be to a man weighing ten stone. 

 Therefore, though the fish is normally in a state of stable equili- 

 brium, very little is needed to upset it. The mere removal of the 

 fins would do something towards this. It may safely be claimed 

 then that whatever the function of the paired fins may be they 

 are certainly not balancers. 



There are two occasions when the paired fins are thrown 

 vigorously into action. The one is when the fish is making a 

 sharp turn ; the other is when it is bringing itself to a standstill 

 after a rapid dart. In the former case (fig. 10) it extends the fin 

 on the side towards which it is turning to the full and keeps the 

 other flat against the side. In the latter case (fig. 9) it extends 

 both fins to the full. 



The purpose served is well indicated by another of Dr. Hele 

 Shaw's experiments. In this (fig. 4) a spindle-shaped body 

 having a projection from one side was used. The presence of 

 this led to the production of a swirl behind. 



Compare this with the moving fish. Suddenly both fins are 

 extended with their planes transverse to its axis and behind each 

 a swirl is produced. This causes a retardation which brings it 

 rapidly to a standstill. If only one fin is extended a more or 

 less fixed point comes into existence around which the body 

 swings by reason of its momentum. This is analogous to the 

 case of a man who runs along the street and, suddenly catching 

 hold of a lamppost, swings round it (fig. 10). 



In both the cases just dealt with the function of the paired 

 fins is that of a brake. Whilst this is the main purpose they 

 serve, they are also used for guiding the fish to lower or higher 

 levels in the water. In sticklebacks and their allies this function 

 is modified for the purpose of tilting the body into a vertical 

 attitude. This attitude is also assumed by other fishes. The 

 sticklebacks also use their pectoral fins for fanning a stream of 

 water through their nests. By a similar movement of these fins 

 they can swim backwards quite rapidly. Other fish, e.g. trout 

 and roach, can use them in the same way but not so effectively. 



Types of fish such as the sturgeon cannot use their paired 

 fins as brakes for the simple reason that they cannot place them 

 transversely to the axis of the body. But the necessity for 



