324 BIOLOGY: GENERAL AND MEDICAL 



takes them up with the rootlets of the plants. In such 

 cases it may be that the animal from which the ova 

 come or one of its fellows that can act as host, or 

 it may be an entirely different kind of animal by which 

 the egg must be swallowed, in order that development 

 may occur. The intestinal parasites of man furnish 

 interesting examples of direct and indirect infection. 

 The pin-worm or seat-worm (Oxyuris vermicularis) so 

 common in children, occasions considerable local irrita- 

 tion about the anus and causes the host to scratch the 

 part, thus taking up the eggs with the nails and carrying 

 them later on to the mouth, directly infecting himself 

 and continually adding to the number of his parasites. 



The round worm, Ascaris lumbricoides, discharges 

 eggs surrounded with a thick albuminous coating that 

 enables them to resist drying for a long time. Such 

 eggs find their way to foods in water, in dust, or by 

 being carried by flies, or adhering to vegetables fertilized 

 by human excrement, may be swallowed and reach the 

 stomach where the albuminous capsule is dissolved by 

 the digestive juices and the embryo set free to mature 

 in the intestine. The hook worms, Anchylostoma duode- 

 nale and Necator americana, produce abundant eggs 

 which, after having been discharged in the excrement, 

 develop in moist soil into diminutive embryos which 

 attach themselves to the skin of the feet or hands, bore 

 through, enter the capillaries, and are carried by the 

 blood to the intestine where they develop into the adult 

 parasites. The eggs of Schistosoma hematobium, falling 

 into water, develop into a ciliated merecidium or embryo. 

 Whether this reaches new hosts by directly perforating 

 the skin during bathing or must be swallowed is not 

 yet known. 



In all of the examples thus far given the transmission 

 of the parasite is said to be direct; that is, from host to 

 host of the same kind, the independent embryonal life 

 being quite short. 



But in many cases the embryonal life of the parasite 



