GENERAL NOTES ON STRUCTURE OF FISHES. 511 



seawards, meet their mates lower down the rivers, and descend to very 

 deep water in the sea (250 fathoms or more). There the eggs are laid, 

 and there in all probability the parents die. Thence the transparent 

 larvte (Leptocefhali) 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 apparently going 

 further inland than the males. 



Inter-relations. — Commensalism is illustrated by some small fishes 

 which shelter inside laige sea-anemones, and by Fierasfer, which goes 

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

 gills of Fishes, parastic 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 Bothriocephalas lotus occur in pike and burbot ; a remarkable 

 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 (Siluridse). 



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. Gunther 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 

 blind fishes occur with rudimentary eyes. 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. 



General Notes on Structure of Fishes. 



Fins. — Along the median line of the dorsal and ventral surfaces of 

 some fishes, e.g. flounder, there is a continuous fin — a fold of skin 

 with fin-rays and underlying skeletal supports. 



In the embryos of many fishes the same continuous fringe is seen, 

 while the adults have only isolated median fins. There is no doubt 

 that these isolated median fins — of which there may be two dorsals, 

 a caudal, and an anal or ventral — arise, or have arisen from a modifica- 

 tion of a once continuous fin, which is suppressed at one part and 

 increased at another. 



Now, the paired fins, which correspond to limbs, often resemble 



