_Filariæ_ are also commonly found in birds, and in this country this is the most available source of laboratory material. We have found them locally (Ithaca, N. Y.) in the blood of over sixty per cent of all the crows examined, at any season of the year, and have also found them in English sparrows. In the crows, they often occur in enormous numbers, as many as two thousand having been found in a single drop of the blood of the most heavily infested specimen examined. For study, a small drop of blood should be mounted on a clean slide and the coverglass rung with vaseline or oil to prevent evaporation. In this way they can be kept for hours. Permanent preparations may be made by spreading out the blood in a film on a perfectly clean slide and staining. This is easiest done by touching the fresh drop of blood with the end of a second slide which is then held at an angle of about 45° to the first slide and drawn over it without pressure. Allow the smear to dry in the air and stain in the usual way with hæmatoxylin. OTHER NEMATODE PARASITES OF MAN AND ANIMALS DEVELOPING IN ARTHROPODS _Dracunculus medinensis_ (fig. 121), the so-called guinea-worm, is a nematode parasite of man which is widely distributed in tropical Africa, Asia, certain parts of Brazil and is occasionally imported into North America. The female worm is excessively long and slender, measuring nearly three feet in length and not more than one-fifteenth of an inch in diameter. It is found in the subcutaneous connective tissue and when mature usually migrates to some part of the leg. Here it pierces the skin and there is formed a small superficial ulcer through which the larvæ reach the exterior after bursting the body of the mother. [Illustration: 122. Cyclops, the intermediate host of Dracunculus.] Fedtschenko (1879) found that when these larvæ reach the water they penetrate the carapace of the little crustacean, _Cyclops_ (fig. 122). Here they molt several times and undergo a metamorphosis. Fedtschenko, in Turkestan, found that these stages required about five weeks, while Manson who confirmed these general results, found that eight or nine weeks were required in the cooler climate of England. Infection of the vertebrate host probably occurs through swallowing infested cyclops in drinking water. Fedtschenko was unable to demonstrate this experimentally and objection has been raised against the theory, but Leiper (1907), and Strassen (1907) succeeded in infesting monkeys by feeding them on cyclops containing the larvæ.