GENERAL NOTES ON FISHES. 563 
transparent larvee (Leptocephalz) 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.—Commensalism is illustrated by some small 
fishes which shelter inside large sea-anemones, and by Fverasfer, which 
goes in and out of sea-cucumbers and medusz. 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 Bothriocephalus latus occur in pike and burbot ; a remarkable 
TY 
PT PUN UL LTO 
TT ANNA} AIN DION 
Fic. 300.—Development of eel.—After Smit. 
Change from Leptocephalus shape (I.) to “‘ Elver” shape (V.). 
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 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 
(Cyprinidee) and cat-fishes (Siluridz), 
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 
