316 THE SEA SHORE 



Further, if we watch the development of one of the highest 

 fishes of the present day from its embryo, we find that similar 

 changes take place in the individual. At first its tail is a simple 

 fringe round the extremity of the backbone, the latter being 

 straight, or nearly so, to the end, so that the embryo fish, as yet 

 still in the egg, reflects a characteristic of its very early ancestors. 

 Then the end of the vertebral column turns upward, and strong 

 fin-rays are developed on its ventral side, so that the tail becomes 

 a heterocercal one like that of the less remote ancestors of a later 

 geological period. Next, the upward-bending portion of the vertebral 

 column is slowly absorbed, till nothing of it remains except a small 



FIG. 229. THE INTEIINAL ORGANS OP THE HERRING 



a, oesophagus ; be, stomach ; e, intestine ; /, duct of swimming bladder ; 

 t, air-bladder ; ft, ovary 



upturned bony spine, while, at the same time, the ventral lobe 

 expands on the upper side until the tail fin is once more of a sym- 

 metrical form. 



Following these interesting changes, it becomes evident that 

 the symmetry of the tail fin of the bony fishes is really a false one, 

 the whole of it having been formed from the ventral lobe of a 

 heterocercal tail ; and although the backbone seems to terminate 

 abruptly exactly opposite the middle of the fin, it still contains the 

 remnant of the raised extremity of the backbone that ran to the 

 tip of the dorsal lobe when the tail was of the heterocercal type. 



The flesh or muscle of fishes is usually white, but it often 

 assumes a pink colour in the case of those fishes that feed largely 

 on crustaceans. This is due to the presence of a substance in the 

 horny or calcareous skins of the crustaceans that is turned red 

 by the action of the digestive fluids the same substance that is 

 turned red when the crustaceans are boiled. This is notably the case 

 with the salmon ; but the red pigment thus derived originally 



