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. This increased consumption necessarily entails the
production of an increased amount of residue; which thus becomes too
abundant to be concocted by the stomach and excreted with its own
residual matter. The residual fluid must therefore of necessity have a
receptacle of its own; and thus it comes to pass that all animals
whose lung contains blood are provided with a bladder. Those
animals, on the other hand, that are without a lung of this character,
and that either drink but sparingly owing to their lung being of a
spongy texture, or never imbibe fluid at all for drinking's sake but
only as nutriment, insects for instance and fishes, and that are
moreover clad with feathers or scales or scaly plates-all these
animals, owing to the small amount of fluid which they imbibe, and
owing also to such residue as there may be being converted into
feathers and the like, are invariably without a bladder. The
Tortoises, which are comprised among animals with scaly plates, form
the only exception; and this is merely due to the imperfect
development of their natural conformation; the explanation of the
matter being that in the sea-tortoises the lung is flesh-like and
contains blood, resembling the lung of the ox, and that in the
land-tortoises it is of disproportionately large size. Moreover,
inasmuch as the covering which invests them is dense and shell-like,
so that the moisture cannot exhale through the porous flesh, as it
does in birds and in snakes and other animals with scaly plates,
such an amount of secretion is formed that some special part is
required to receive and hold it
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