FORM AND LOCOMOTION 13 



The fins, which form so characteristic a part of the fish, may 

 well be considered here. Within recent years a French scientist 

 has carried out some very interesting experiments, during which 

 bodies composed of more or less plastic material were drawn 

 through the water at varying speeds. As this took place these 

 artificial bodies tended to become more or less closely moulded 

 to the typical fish-like form described above. At the same 

 time, however, they were inclined to roll over to one side when 

 poised motionless in the water, and to wobble to an alarming 

 extent when moving at any speed, and it was only by the 

 appropriate placing of artificial keels that he was able to 

 stabilise the flight of his models. There can be little doubt 



Spinous Dorsal ^ ^ , 



"'"'" ttitT/Ty^ Soft Dorsal 



J ^lililtif illl^ .v^^^K'' Caudal 



Pectoral 



Petvics 



Fig. 7. — TOPOGRAPHY OF FINS. 



that it was in order to obtain stability that wedge-shaped body 

 keels have been evolved along the back and belly of a fish, as 

 well as paired balancing and steering organs projecting from 

 the sides. These keels and balancers are the fins, and, although 

 they are discussed in greater detail in another chapter, it will 

 be necessary to study their arrangement and distinctive names 

 in the accompanying figure (Fig. 7) in order to understand the 

 part which they play in locomotion. The fins are of two kinds, 

 median or unpaired, and paired. The median fins or keels 

 consist of a dorsal in the middle line of the back, an anal on the 

 belly behind the vent, and a caudal or tail-fin at the hinder end of 

 the fish, which, in addition to assisting stability, also acts as a 

 rudder and plays an important part in forward movements. 

 In the fast-swimming Mackerel the dorsal and anal fins form 



