Immunity against pathogenic micro-orgaiiisms 135 



be accepted that bacteria, living in the blood plasma, become the 

 prey of phagocytes Avhich render them inoffensive and kill them. 

 This example demonstrates once again that the organism gets rid of 

 bacteria by the same mechanism as that which serves for the re- 

 sorption of any of the formed elements. The snail reacts to bacteria 

 as it does to the red corpuscles of the goose. 



It is imnecessary to insist further on the natural immunity of the 

 Invertebrata, and it is useless to multiply examples which always 

 point in the same direction : to the importance of phagocytic reaction 

 and of intracellular digestion in resorption and immunity. We must 

 pass on to the examination of the reaction phenomena of the 

 vertebrate organism toAvards pathogenic micro-organisms, following, 

 as hitherto, the comparative method. We will commence with the 

 study of the natural immunity of fishes as lower representatives of 

 the great group of the Vertebrata. 



It is well known that fishes are liable to infective diseases and 

 pisciculture has often to deplore considerable losses brought about 

 sometimes by certain of the lower Fungi {e.g. Saprolegniae), some- 

 times by Bacteria. The pathogenic microbes which produce epi- 

 demics in fishes are still little understood ; but among the bacteria 

 which kill many of the higher animals are some which cause fatal 

 maladies in certain fishes. Thus the anthrax bacillus so virulent for 

 many mammals, is capable also, as we have seen, of producing an 

 infection in the cricket, and may cause the death of small marine 

 osseous fishes, the Hippocampi. Sabrazes and Colombot^ who have 

 studied this question, have demonstrated that the anthrax bacillus, 

 which is virulent for the rabbit, when inoculated into these fishes 

 first produces swellings at the seat of inoculation and ultimately 

 becomes generalised throughout the body, producing a fatal septi- 

 caemia. As these experiments have given this result at a temperature 

 of 14'' — 16° C, it is quite evident that the bacillus, in order to 

 manifest its pathogenic efiect, in no way needs the high temperature [144] 

 of the mammalian body for its action. 



Now among fishes there are not wanting species which resist the 

 anthrax bacillus. MesniP has, in our laboratory, thoroughly studied 

 the mechanism of this immunity. He has shown that several fresh- 

 water fishes, e.g. the perch {Perca fluviatilis), the gudgeon (Gobio 

 fluviatilis), and the gold-fish (Carassius auratus), will resist an 



1 Ann. de VInst. Pasteur, Paris, 1894, t. viii, p. 696. 

 * Ann. de VInst. Pasteur , Paris, 1895, t. ix, p. 301. 



