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 of 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 tc* be animals ; yet a considerable 

 number of those which were included by the illustrious- 

 Ehrenberg in the same class, are now pretty 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 as equally unsound. Cellulose has been shewn to be a component 

 of animal as well as of vegetable structures, and Kolliker has insisted that some 

 forms which have neither month nor stomach, but consist of a homogeneous 

 muss, are true animals. If these premises are correct, nothing will remain, as 

 I conceive, for a distinctive characteristic, but voluntary motion. This when 

 positive, is 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 

 Kolliicer's view of a stomachless animal prove correct, the inverse condition of 

 a true stomachal cavity being present must, I 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."— (-Or 

 Burmtt, in Siebold's " Comparative Anatomy," p. 18.) 



