SWIMMING ANIMALS. 51 



front of the other ; the front one being often large and 

 spiny, and the hind one small and soft (Fig. 16). 



The tail and tail- fin form, as we have said, the chief 

 propeller of the fish ; and it will be particularly noticed 

 that the position of this fin is vertical. In swimming, 

 as we may observe in an aquarium where fish are kept, 

 the tail is rapidly and strongly bent from side to side, 

 while the two lobes of its fin have an undulating 

 motion, and thus act like the blades of a screw-pro- 

 peller. A difference between the structure of the tail- 

 fin in the two figured fishes recalls the one already 

 noticed in the pectoral fin. Thus in the Perch (Fig. 16) 

 the scaly part of the tail ends in an abrupt and almost 

 straight edge, from which the rays of the fin form a 

 nearly symmetrical fork. In Fig. 2, on the other hand, 

 the scaled part of the tail is produced to a point, 

 extending far back among the fin-rays, which are 

 arranged unsymmetrically along its two edges. It is 

 this latter mode of arrangement which is the older and 

 more primitive. 



In certain fishes which depart more or less widely 

 from the ordinary form, there is a corresponding modi- 

 fication in the shape and functions of the fins. For 

 instance, the Eays swim almost entirely by the aid of 

 the greatly expanded pectoral fins, which have an 

 undulating motion very similar to that of the median 

 fins of ordinary fishes. On the other hand, in the 

 Flying Fishes (see "Flying Animals," Fig. 10), the 

 pectoral fins are enormously elongated, so as to act as 

 organs of spurious flight. Again, snake-like fishes, as 



