GENERAL NOTES ON FISHES. 



563 



Iransparent larvse (Leptocephali] rise to the surface and are for a year 

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

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

 parently going farther inland than the males. 



Inter-relations. Commensal ism is illustrated by some small 

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

 goes in and out of sea-cucumbers and medusae. 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 bladder-worm or tape-worm stage infest the viscera. The immature 

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



FIG. 300. Development of eel. After Smit. 

 Change from Leptocephalus shape (I.) to " Elver " shape (V.). 



hydroid (Poly podium] 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 Bitterling (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 

 (Cyprinidse) and cat-fishes (Siluridea). 



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. Glinther has shown that in forms living at 



