INTER-RELATIONS. 525 



time. 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." 



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 medusse. On the 

 outside or about the gills of Fishes parasitic Crustaceans, 

 fish lice, are often found ; various Flukes are also common 

 external parasites, and many Cestodes in bladderworm or 

 tapeworm stage infest the viscera. The immature stages of 

 Bothriocephalus laius occur in pike and burbot ; a remark- 

 able hydroid (Polypodiuni) is parasitic on the eggs of a 

 sturgeon ; the young of the fresh water mussel are tem- 

 porarily parasitic on the stickleback ; and the young of the 

 Bitterling {Rhodeus amarus) live for a time within the gills 

 of fresh water mussels. 



Distribtition 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 

 (Cyprinidce) and cat fishes (Silurid^). 



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 So-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 seem to follow the 

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

 blind fishes occur with rudimentary organs. Many of these abyssal 

 fishes are phosphorescent ; the colouring is usually simple, mostly 

 blackish or silvery ; the skin exudes much mucus ; the skeleton tends 

 to be light and brittle ; the forms are often very quaint ; the diet is 

 necessarily carnivorous. 



[Table. 



