THE WEB OF LIFE 55 



from their birth-place. This is curious enough, 

 but the idea of linkages becomes incandescent in 

 the mind when we note that, just as the fresh- water 

 mussel has young temporarily parasitic on fishes, so 

 a fresh-water fish, the bitterling (Rhodeus amarus), 

 has its young temporarily parasitic in the gills 

 of the mussel. 



LIFE-HISTORIES OF PARASITES. When we pass 

 to parasites in a stricter sense we find the most 

 extraordinary interconnections, the most widely 

 separated animals often sharing a parasite between 

 them. Liver-rot, which has repeatedly killed a 

 million sheep in a year in Britain alone, is due 

 to a parasite which passes from sheep to water, 

 from water to water-snail, from water-snail to 

 grass, from grass to sheep. The tapeworm of the 

 cat has its bladder-worm stage in the mouse, the 

 sturdie-worm of the sheep's brain has its tapeworm 

 stage in the dog, and similar relations hold for 

 hundreds of species. The troublesome thread- 

 worm of human blood (Filaria sanguinis hominis) 

 is transferred from man to man by the mosquito, 

 and the guinea-worm, which was probably the 

 fiery serpent that vexed the Israelites in the 

 desert, which passes into man in drinking-water, 

 spends its youth in a minute water-flea, called 

 by the giant's name of Cyclops. The import- 

 ance of tse-tse flies in transmitting the minute 

 animals which cause sleeping-sickness and allied 

 diseases is known to all. We have spoken of the 

 connection between cats and clover, and there 

 is a not less striking connection between cats 

 and plague. For it seems to have been shown 

 in India that the more cats the fewer rats, 

 and the fewer rats the fewer rat-fleas, which 



