562 PISCES—FISHES. 
laid at once. In Elasmobranchs the egg is large, and in the oviparous 
genera it is enclosed in a ‘‘ mermaid’s purse.” 
Most sharks and a few Teleosteans, ¢.g. Sebastes marinus, Zoarces 
viviparus, are viviparous, the eggs being hatched in the lower part of 
the oviduct in sharks, in the ovary or oviduct in Teleosteans. In two 
viviparous sharks (A/ustelus levis and Carcharias glaucus) there is a 
union between the yolk-sac and the wall of the oviduct, to be com- 
pared with a similar occurrence in two lizards, and with the yolk-sac 
placenta of some Mammals. 
As to fertilisation, the usual process is that the male deposits 
spermatozoa or ‘‘ milt” upon the laid eggs or ‘‘ spawn,” but fertilisation 
is of course internal when the eggs are enveloped in a firm sheath, or 
when they are hatched within the mother. 
Most fishes have a great number of offspring, and parental care is 
proportionately little. Moreover, the conditions of their life are not 
suited for the development of that virtue. When it is exhibited, it is 
usually by the males,—e.g. by the sea-horse (Azpfocampus) and the 
pipe-fish (Syzgnathws), which hatch the eggs in external pouches, and 
‘*the male of some species of Arcus, 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 
familiar illustration is that of the male stickleback, who twines grass 
stems and water-weeds together, glueing them by mucus 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, 
and it is rare for a fish to exhibit any of the senile changes associated 
with old age in other Vertebrates. But surroundings and nutrition 
affect their size and colour very markedly. Some, such as the flounder, 
seem almost equally at home in fresh or salt water, 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,’ 
Life histories.—The life histories of fishes form the subject of an 
endless chapter, of which we can only give a few illustrations. We 
know how the lusty salmon return from the sea to the possibly safer 
rivers, and after a period of fasting deposit their eggs and milt on the 
gravelly bed of the stream. A similar migration is true of the 
sturgeon. 
In great contrast to these cases is the life history of the eel, the 
mystery of which has been at least partially removed. From the 
inland ponds and river-stretches the female eels migrate on autumn 
nights 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 
