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 
i 
devour the conidia, take them 
into their interior and digest 
them. If a conidium be too 
much for one cell others join 
Fic. 114. — White blood- cells it, form a giant-cell, and thus 
(leucocytes) attacking bacilli. stryoole with the invader. 
(After Metschnikoff.) oe 
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 
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 
