FINS : THEIR USES. 61 



Because the fish swims, generally, parallel with 

 the surface of the water. There is no need to 

 come to the surface for air periodically, since the 

 breathing is performed by gills. But the whale 

 and his kind swim by alternately rising and 

 diving in a sort of undulating course. This is 

 necessary, because the whale breathes by lungs, 

 and must accordingly rise frequently to the 

 surface. The vertical tail drives the fish 

 forwards ; the horizontal drives the whale 

 upwards or downwards as the case may be. 



The last of the median fins to be considered is 

 that which lies between the caudal and paired 

 pelvic fins. Like the dorsal, this may be divisible 

 into two or more portions. Sometimes the rays of 

 the first anal are spiny, whilst those of the second 

 anal are " soft," and branched like those of the 

 second dorsal. Spiny rays, then, associated with 

 " soft " rays, are always confined either to the 

 first anal, or the first few supports of the first 

 anal. But they are never preceded by "soft" 

 rays; in other words, soft rays always foUoiv 

 spiny rays. Sometimes the anal fin is wanting 

 altogether. 



So much for the median, we turn now to the 

 paired fins. These, as we have already remarked, 

 correspond to the arms and legs of the higher 

 animals. In our typical fish — the salmon or 

 perch — the pair which correspond to the arms, 

 the pectoral fins, as they are called, and will be 

 called here henceforth, are situated one on either 

 side of the body, just behind the gill opening. 

 The pair which correspond to the legs, the pelvic 

 fins — or ventral fins as they are sometimes called 



