onsiderable velocity, and with all the
appearances of spontaneity, as locomotive bodies, which, from their
similarity to animals of simple organisation, were called "zoospores."
Even as late as 1845, however, a botanist of Schleiden's eminence dealt
very sceptically with these statements; and his scepticism was the more
justified, since Ehrenberg, in his elaborate and comprehensive work on
the _Infusoria_, had declared the greater number of what are now
recognised as locomotive plants to be animals.

At the present day, innumerable plants and free plant cells are known to
pass the whole or part of their lives in an actively locomotive
condition, in no wise distinguishable from that of one of the simpler
animals; and, while in this condition, their movements are, to all
appearance, as spontaneous--as much the product of volition--as those of
such animals.

Hence the teleological argument for Cuvier's first diagnostic character--
the presence in animals of an alimentary cavity, or internal pocket, in
which they can carry about their nutriment--has broken down, so far, at
least, as his mode of stating it goes. And, with the advance of
microscopic anatomy, the universality of the fact itself among animals
has ceased to be predicable. Many animals of even complex structure,
which live parasitically within others, are wholly devoid of an
alimentary cavity. Their food is provided for them, not only ready
cooked, but ready digested, and the alimentary canal, become superfluous,
has disappeared. Again, the males of most Rotifers have no digestive
apparatus; as a German naturalist has remarked, they devote themselves
entirely to the "Minnedienst," and are to be reckoned among the few
realisations of the Byronic ideal of a lover. Finally, amidst the lowest
forms of animal life, the speck of gelatinous protoplasm, which
constitutes the whole body, has no permanent digestive cavity or mouth,
but takes in its food anywhere; and digests, so to speak, all over its
body. But although Cuvier's leading diagnosis of the animal from the
plant will not stand a strict test, it remains one of the most constant
of the distinctive characters of animals. And, if we substitute for the
possession of an alimentary cavity, the power of taking solid nutriment
into the body and there digesting it, the definition so changed will
cover all animals except certain parasites, and the few and exceptional
cases of non-parasitic animals which do not feed at a