1H4 PHYSIOLOGY 



those belonging to the natural order Leguminosae. In herbivorous 

 animals the presence of specific bacteria in the paunch or caecum 

 causes the breakdown of the cellulose walls of the food and may 

 indeed lead to a building up of protein from amino-acids or even from 

 salts of ammonia. It is probable that in these cases the animal is 

 decidedly benefited from the presence of these bacteria in its 

 alimentary canal, so that here also we may speak of a symbiosis. In 

 most cases invasion of a higher animal or plant by some lower organism 

 is fraught with danger to the host, so that special mechanisms have 

 to be provided for the protection of the tissues from infection. The 



^ FIG. 480. A, amoeba, infected by Microsphcera ; a, early stage. B, a dying 



amoeba, full of parasitic Microsphcerce. (METCHNIKOFF.) 



i 



most primitive means of defence, and one which is found through- 

 out the whole animal kingdom, is exactly analogous to the process 

 by which the amoeba destroys and utilises any bacteria present 

 in its environment. The prevention of infection is of course the 

 function of the external layers of the organism, i.e. the epithelial 

 covering, either of the skin or of the surface of the gut. Protection 

 here may be of a physical or chemical character. The cells may 

 secrete a horny or chitinous layer which presents a mechanical 

 obstruction to the entry of bacteria. They may secrete mucin, which 

 entangles and hinders the movements of invading micro-organisms, or 

 they may secrete substances which actually destroy the life of such 

 organisms. When, however, a micro-organism has obtained entrance 

 to the interior of the body, e.g. through a wound of the surface epithe- 

 lium, the task of dealing with the invader becomes the office of a 

 special type of cells belonging to the mesoblast. These cells are 



