80 M. J. Frenzel on the Mesozoon Salinella. 



already im])lies, of a single 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 dcvclojnnent. In tlie 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 emhryological foundations are insufficient ; and we 

 are eveu forced to exclude them, at any rate in general, from 

 Hackel's fundamental principle of biogenesis, which is equally 

 unsatisfactory. 



The multicellular animals, on the other hand, are not mere 

 aggregates 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 internal 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 ])arasites, 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 into itself^ digests it in its inferior, and 

 absorbs what is suitable. This is a so-called intra-cellular 

 digestion, which in Metazoa, on the contrary, is only met 

 with in isolated and exceptional cases ; for in the \ixttQi- 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 //«<'(/ substances in the shape of peptone, 

 sugar, fat, tJcc. 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, flagcUa, cilia, &c., arc no longer necessary. VVe may 



