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. For this reason in
Selachia the branchial orifices themselves effect their own closure,
and thus there is no need for an operculum to ensure its taking
place with due rapidity. In some fishes the gills are numerous, in
others few in number; in some again they are double, in others single.
The last gill in most cases is single. For a detailed account of all
this, reference must be made to the treatises on Anatomy, and to the
book of Researches concerning Animals.
It is the abundance or the deficiency of the cardiac heat which
determines the numerical abundance or deficiency of the gills. For,
the greater an animal's heat, the more rapid and the more forcible
does it require the branchial movement to be; and numerous and
double gills act with more force and rapidity than such as are few and
single. Thus, too, it is that some fishes that have but few gills, and
those of comparatively small efficacy, can live out of water for a
considerable time; for in them there is no great demand for
refrigeration. Such, for example, are the eel and all other fishes
of serpent-like form.
Fishes also present diversities as regards the mouth. For in some
this is placed in front, at the very extremity of the body, while in
others, as the dolphin and the Selachia, it is placed on the under
surface; so that these fishes turn on the back in order to take
their food. The purpose of Nature in this was apparently not merely to
provide a means of salvation for other animals, by allowing them
opportunity of escape during the time lost in the act of turning-for
all the fishes with this kind of mouth prey on living animals-but also
to prevent these fishes from giving way too much to their gluttonous
ravening after food
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