Preliminary remarks on immunity in animal kingdom 49 



cells in the process of resorption, a striking analogy may be observed. 

 To establish this satisfactorily it is essential to begin with a study 

 of intracellular digestion properly so called, especially as in this 

 phenomenon we have the fundamental basis of the whole of the theory 

 developed in this work. 



In our first two chapters we have already cited examples of this 

 intracellular digestion in the Protozoa (Amoebae, Infusoria, etc.) and 

 in the plasmodium stage of the Myxomycetes. In all these cases it 

 goes on in the organism, in a distinctly acid medium, by the aid of 

 ferments which could be demonstrated in the Amoebae and Myxo- 

 mycetes, and which are analogous sometimes with trypsin, sometimes 

 with pepsin. 



In the lower Invertebrata we find the principal source of our 

 knowledge of intracellular digestion in the digestive organs. This 

 form of digestion is met with in Sponges, in the whole of the Coelen- 

 terates (Medusae, Siphonophora, Ctenophora, etc.), in the great 

 majority of the Turbellaria (Planarians, Rhabdocoela), and in certain 

 of the Mollusca (the lower Gasteropods). In the Invertebrata higher 

 in the animal scale, intracellular digestion in the digestive organs 

 becomes more and more rare, and sometimes it manifests itself only 

 in the larval condition (Phoronis) ; ultimately it gives place perma- 

 nently to digestion by juices secreted into the gastro-intestinal canal. 



In his sketch of the comparative physiology of digestion, 

 Krukenberg 1 sought to establish two types : protoplasmic or cellular 

 digestion and secretory digestion. The former is effected, according 

 to this observer, by a vital action independently of any production of 

 soluble ferments. Secretory digestion alone, characteristic of the 

 Vertebrates and of almost all the higher Invertebrates, is effected by 

 means of these ferments (diastases or enzymes). Many observers, 

 adopting this view, maintain that intracellular digestion presents [53] 

 a purely vital phenomenon essentially different from that of chemical 

 digestion due to juices containing soluble ferments secreted in the 

 gastro-intestinal canal. That this theory is absolutely erroneous the 

 succeeding pages of this work will furnish ample proof. 



The Protozoa, from their small size, are unsuitable for researches 



on the essential phenomena of intracellular digestion. Amongst animals 



higher in the scale the Planarians lend themselves most readily to the 



observation of this process. These flat worms are very common in 



both fresh and sea water and are easily fed in captivity. They are 



1 " Grundzuge einer vergl. Physiologic der Verdauung," Heidelberg, 1882. 



B. 4 



