218 EVOLUTION AND DISEASE. 



amoebic warfare may be described from attacks actually 

 witnessed by Metschnikoff in the water-flea Daphnia. 

 This observer kept many of these interesting trans- 

 parent creatures in a tank, and noticed that they became 

 affected with spores which gained an entrance into the 

 body of the crustacean, germinated, and were dispersed 

 by the blood over the body (in Daphnia the blood 

 circulates in lacunar spaces) and deposited in those 

 parts where the blood moves slowest, viz., in the cephalic 

 and hinder portions of the mantle cavity : in these 



places heaps of conidia collect. 

 In the meantime the leuco- 

 cytes do not remain idle against 

 the invasion, but attack and 

 devour the conidia, take them 

 into .their interior and digest 

 them. If a conidium be too 



(-4 much for one cell others join 

 \P\f\ 



FIG. n 4 . -White blood-ceils it, form a giant-cell, and thus 

 (leucocytes) attacking bacilli, struggle with the invader. 



(After Metschnikoff.) 



Should the leucocytes over- 

 power the spores, the daphnia lives ; if not, the conidia 

 overrun the crustacean and death is the result. 



A similar process takes place in animals more highly 

 I organized, and as no disease illustrates more thoroughly 

 /the defending power exercised by leucocytes than that 

 known as avian tuberculosis, the leading points in this 

 widespread affection will be briefly considered. Tuber- 

 culosis in man is unfortunately very prevalent, but in 

 birds, especially those which live on grain, it is more 

 common than in human beings. On examining a bird 



