80 M. J. Frenzel on the Mesozoon Salinella. 
already implies, of a s¢ngle cell which unites in itself all the 
various functions of an animal organism, but also assume 
quite a peculiar position in many other respects, especially 
with regard to development. In the systematic arrangement 
of the group we are even obliged, hard though it will be for 
every modern zoologist, to allow ourselves to be swayed by 
physiological considerations, since here the purely morpho- 
logical and embryological foundations are insufficient ; and we 
are even forced to exclude them, at any rate in general, from 
Hiickel’s fundamental principle of biogenesis, which is equally 
unsatisfactory. 
The multicellular animals, on the other hand, are not mere 
ageregates of cells, such as, moreover, are not unknown 
among the Protista, but they permit us to distinguish, albeit 
frequently with difficulty, a structure consisting of three 
layers, in that in the simplest case they possess an external 
layer of cells, which provides for sensory perceptions &c., next 
a median supporting tissue, and finally an ¢nternal one, which 
discharges the function of nutrition, since it clothes a cavity 
which is known as the gastral chamber, alimentary canal, &c. 
There is yet another by no means ‘unimportant difference 
between unicellular and multicellular animals to which unfor- 
tunately far too little attention is paid, perhaps in conse- 
quence of the fact that it arises in the first place from physio- 
logical conditions only. 
For if we disregard forms which exhibit holophytic nutri- 
tion, and therefore live like a lower form of plant, and further 
neglect the intestinal parasites, which in many cases, but not 
always, are able to absorb that which has already been 
digested by other animals, we find that the Protozoon cell 
receives its food ¢nto ctself, digests it in its cnterior, and 
absorbs what is suitable. ‘This is a so-called tntra-cellular 
digestion, which in Metazoa, on the contrary, is only met 
with in isolated and exceptional cases ; for in the latter extra- 
cellular digestion prevails, which is accomplished on the 
principle of “ one for all and all for one,” since all the partici- 
pating cells to a certain extent throw their digestive ferments 
into a common pot, in which digestion proceeds, exactly as 
cooking is done in a kitchen for a large number of persons. 
It follows that solid, in part absolutely indigestible bodies, are 
no longer taken up by the cells, as we found to be the case 
in Protozoa, but only fludd substances in the shape of peptone, 
sugar, fat, &c. In consequence of this, those morphologi- 
cally specially constructed organs for the acquisition of food, 
such as we meet with in the Protozoa in the form of pseudo- 
podia, flagella, cilia, &c., are no longer necessary. We may 
