414 



FISHES. 



When it is exhibited it is usually by the males, witness the 

 sea-horse {Hippocampus) and the pipe-fish {Syngnathus), 

 which hatch the eggs in external pouches, and "the male of 

 some species of Arius, who carries the ova about with him 

 in his capacious pharynx." The female of Aspredo carries 

 the eggs on the under surface of the body until they are 

 hatched, much in the same way as the Surinam toad bears 

 her progeny on her back, while in Solenostoma a pouch for 

 the eggs is formed by the ventral fins and skin. At least a 

 dozen kinds of fishes make nests, of which the most familiat 

 illustration is that of the male stickleback, who twines grass 

 stems and water-weeds together, glueing them by mucous 

 threads exuded as semi-pathological products from the 

 kidneys, which are compressed by the enlarged male organs. 



Fishes have a less definite limit of growth than most other 

 Vertebrates. Surroundings and nutrition affect their size 

 and colour very markedly. Some marine forms, such as 

 flounders, may survive being shifted to fresh water, while 

 others, such as salmon and sturgeon, pass from sea to rivers 

 at spawning time. But most 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 Silu- 

 roids and others, encyst themselves in the mud and remain 

 for a long time in a state of " latent life." 



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 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 bladderworm or 

 tapeworm 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. 



There are about 2300 fresh-water species of Fishes — three 

 or four of these, are Dipnoi, about thirty are Ganoids, and the 

 rest Teleosteans. Among marine Fishes, about 3500 species 

 frequent the coasts, rarely descending below 300 fathoms ; a 



