IMMUNITY AGAINST MICROBES. 103 



living in the animal's tissues, and may form normal blastulse. 

 Contrariwise, the living spermatozoa of Sphserechinus 

 granularis introduced under the skin of Phyllirrhoe soon 

 fall a prey to mesodermic cells. 



The mesodermic cells of Bipinnaria asterigera also eat 

 and destroy the necrosed parts of the animal. 



The function of amoeboid cells ^ is not limited to the absorp- 

 tion of weakened or dead tissues ; but the same structures 

 take an active part in the fight between the animal organism 

 and the surrounding microbes. When a solution crowded 

 with micro-organisms is injected under the skin of Bipin- 

 naria or Phyllirrhoe, the parasites, whether mobile or not, 

 are soon taken into the interior of the protoplasm and 

 vacuoles of mesodermic cells, and gradually digested by the 

 latter. The tunica of Botryllus, for instance, even in an 

 animal taken fresh from the sea, always contains colonies of 

 micro-organisms. The latter are actively pursued by the wan- 

 dering cells of the tunica, which digest them after swallowing 

 them up. The struggle is not a one-sided one, however, for 

 the victory often remains with the micro-organisms, as is 

 proved by the presence of dead amoeboid cells containing a 

 number of dead bacterial filaments radiating out of them. 



Similar facts have been observed in Daphnia by Metsch- 

 nikoff.2 These small fresh-water crustaceans are frequently 

 invaded by a fungus belonging to the yeast family (Mono- 

 spora bicuspidata). The long needle-like spores of the 

 parasite penetrate with the food into the alimentary canal, pass 

 into the body-cavity after perforating the walls of the intestine, 

 quickly invade the entire body, and often kill the animal. 

 Whilst the disease is in active progress the leucocytes still 

 strive to fight, and absorb numbers of conidia. The latter, 

 however, rapidly multiply, and destroy the amoeboid cells, the 

 victory ultimately falling to the parasites. 



The study of the development and of the life-history of 

 Vertebrata afi'ords many examples showing that the meso- 



» ' Aimal. de I'Institut Pasteur,' 1887, p. 323. 

 2 Ibid., p. 325. 



