Page:Tropical Diseases.djvu/829

XLII] As many as fifteen or twenty young guinea-worms may be counted in each of the little crustaceans, which, unless the infection is excessive, seem in no way inconvenienced. After a time the embryos so transferred undergo a metamorphosis. They cast their skins two or three times, get rid of their long swimming tails, acquire a cylindrical shape and, ultimately, along with increased size, develop a tripartite arrangement of the extreme posterior end, which recalls a similar arrangement of the tail of F. banecrofti and of F. recondita towards the termination of the stay of the former nematode in the mosquito, and of the latter in the dog-flea (Grassi). Mode of infection.— The metamorphosis of D. medinensis in cyclops was discovered by Fedschenko in Turkestan. His observations I have been able to confirm in England; but, owing to the colder climate of the latter country, in English cyclops the metamorphosis takes somewhat longer to complete— eight or nine weeks, instead of five weeks as in Turkestan. Fedschenko supposed that the cyclops containing the larvae of the guinea-worm, on being swallowed by man in drinking-water, was digested; and that the parasite, being then set free, worked its way into the tissues of its new and definitive host; but all the experiments he made to bring about infection by means of ingestion of cyclops containing dracunculus larvæ gave a negative result. These failures led to the belief that the larvæ might have to undergo further changes before it was fitted for life in man. Considering the peculiar geographical limitations of this helminthiasis, and the very general distribution of cyclops, such an arrangement seemed likely enough.*

Recently, Leiper has shown that when an infected cyclops is transferred to a 0.2 per cent, solution of hydrochloric acid it is immediately killed, but the larvæ, so far from being destroyed, are aroused to great activity, and eventually escape into the fluid, in which they swim freely. From this he conjectured that under natural conditions man becomes infected through the ingestion of infected cyclops, the gastric juice acting on cyclops and larva in the same way as the hydrochloric acid in his experiment. In