SPONGES. 
29 
The necessity of a stomachal cavity to an animal is a 
more precise distinction, and appears to be the only one. 
Yet even this is not -without, obscurity. The Hydra, when 
turned inside out, like a glove, absorbs its nutriment as 
well as before, though the surface which is its stomach 
now was before external, and vice versA. And Dr Lindley 
remarks, in speaking of vegetable organisms, “ that it 
is impossible to say that the whole interior ol a living 
independent cell is not a stomach.” “ 
It will now be readily admitted that the limits between 
the animal and vegetable kingdoms are exceedingly indis- 
tinct and subtile, and that these two grand divisions of 
organised being merge into each other by shadowy and 
almost imperceptible gradations. In fact, it is more than 
doubtful whether there are any boundaries at all. 
In a former chapter we described beings of excessive 
minuteness, but of energetic motions, most of which have 
been universally allowed to be animals ; yet a considerable 
number of those which were included by the illustrious 
Ehrenberg in the same class, are now pi-etty generally 
* “As is well known, all the older criteria by which animals were separated 
from plants have long since been regarded invalid; and some of those which in 
late years have been regarded among the most constant, have, quite recently, 
been declared aa equally unsound. Cellulose bus been shewn to bo a component 
of animal as well as of vegetable struct urns, and KSlliker has insisted that some 
forms which have neither mouth nor stomach, but consist of a homogeneous 
mass, are true animals. If these promises :u*e correct, nothing will remain, as 
1 conceive, for a distinctive characteristic, but •ruluntary motion. This when 
positive, ia indubitable evidence of any given form being of an animal character ; 
and it must remain for each individual observer to determine what is, and 
what is not, voluntary action, in each particular case. Moreover, even should 
Kollilxr'a view of a stomachless animal prove correct, the inverse condition of 
a true stomachal cavity being present must, T think, be regarded as positive 
evidence of the animal nature of the form in question; for this must always 
be a distinctive characteristic of the two kingdoms when present.” — {Dr 
Burw.it , in Subold's “Comparative Anatomy” p. 18.) 
