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PHYLUM CHORDATA : CLASS PISCES — FISHES 



brates, and it is rare for a fish to exhibit any of the senile changes 

 associated with old age in other Vertebrates. But surroundings and 

 nutrition affect their size and colour very markedly. Some, such as the 

 flounder, seem almost equally at home in fresh or salt water, but many 

 are sensitive to changes of medium. Many can endure prolonged 

 fasting, and some may survive being frozen stiff. Lowered temperature 

 may induce torpor, as seen in the winter sleep of the pike, while in the 

 dry season of hot countries the mud-fishes, the Siluroids, and others, 

 encyst themselves in the mud, and remain for a long time in a state of 

 " latent life." 



Life-histories. — The life -histories of fishes form the subject of an 

 endless chapter, of which we can only give a few illustrations. We 

 know how the lusty salmon return from the sea to the possibly safer 

 rivers, and after a period of fasting deposit their eggs and milt on the 

 gravelly bed of the stream. A similar migration is true of the sturgeon. 



In great contrast to these cases is the Hfe-history of the eel, the 

 mystery of which has been at least partially removed. From the 

 inland ponds and river-stretches the female eels migrate on autumn 

 nights seawards, meet their mates lower down the rivers, and descend 

 to deep water in the distant Atlantic. There the eggs are laid, and 

 there in all probability the parents die. Thence the transparent 

 larvae {Leptocephali) rise to the surface and are for a couple of years 

 pelagic. From the open sea the young eels or elvers migrate up 

 the streams in a marvellous procession or eel -fare, the females 

 apparently going farther inland than the males. 



Inter-relations. — Commensalism is illustrated by some small 

 fishes which shelter inside large sea-anemones, and by Fierasfer, which 

 goes in and out of sea-cucumbers and medusa. On the outside or about 

 the gills of Fishes, parasitic Crustaceans (fish-hce) are often found ; 

 various Flukes are also common external parasites, and many Cestodes 

 in bladder-worm or tape-worm stage infest the viscera. The immature 

 stages of Bothriocephalus latus occur in pike and burbot ; a remarkable 

 hydroid {Polypodium) is parasitic on the eggs of a sturgeon ; the young 

 of the fresh-water mussel are temporarily parasitic on the stickleback ; 

 and the young of the Bitterhng (Rhodeus amarus) live for a time 

 within the gills of fresh-water mussels. 



Distribution in space. — There are about 2300 species of fresh-water 

 fishes — three or four Dipnoi, about thirty " Ganoids," and the rest 

 Teleosteans — over a half being included in the two families of carps 

 (Cyprinidae) and cat -fishes (Siluridae). 



Among marine fishes, about 3500 species frequent the coasts, rarely 

 descending below 300 fathoms. A much smaller number, including 

 many sharks, live and usually breed in the open sea. About 100 

 genera have been recorded from great depths. 



In regard to the last, Dr. Giinther has shown that in forms living at 

 depths from 80 to 200 fathoms, the eyes tend to be larger than usual, 

 as if to make the most of the scanty light ; beyond the 200-fathom line 

 small -eyed forms occur with highly developed organs of touch, and 

 large-eyed forms which have no such organs, but perhaps follow the 

 gleams of " phosphorescent " organs ; finally, in the greatest depths 

 some forms occur with rudimentary eyes. Many of these abyssal fishes 



