354 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 oc- 

 cur. The intestinal parasites of man furnish interesting ex- 

 amples of direct and indirect infestation. The pin-worm 

 or seat-worm (Oxyuris vermicularis) so common in children, 

 occasions considerable local irritation 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, 

 thus continually adding to the number of his parasites. 



The round worm of the intestine, Ascaris lumbricoides, 

 lives in the intestines of men and hogs. It discharges 

 eggs surrounded with a thick albuminous coating that 

 enables them to resist drying for a long time. These 

 eggs in the evacuations of the host, which are frequently 

 used as fertilizer, may adhere to green vegetables, be 

 swallowed, and so reach the stomach, where the albuminous 

 capsule is dissolved by the digestive juices and the embryo 

 set free. The embryo works its way into the circulation, 

 is carried to the lungs, where it seems to undergo an addi- 

 tional development, "is coughed up and again swallowed, 

 thus finally reaching the intestine ready to mature. 



The hook worms, Anchylostoma duodenale and Neca- 

 tor americana, produce abundant eggs which, after 

 having been discharged in the excrement, develop in 

 moist soil into diminutive embryos which attach them- 

 selves to the 'skin of the feet or hands, bore through, 

 enter the capillaries, and are carried by the blood to 

 the lungs, where they undergo a* further developmental 

 stage, are later coughed up, and some being swallowed 

 in the mucus find their way to the intestine where they 

 develop into the adult parasites. The eggs of Schisto- 

 soma 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 



