Chap. 26 ROUNDWORMS — THE TUBULAR PLAN 525 



more dangerous to the host and more difficult to eHminate. The fertilized eggs 

 are extruded in the intestine and, as early embryos, pass out of it with the 

 feces. On moist warm soil, the larvae hatch within 24 to 48 hours. They bore 

 downward a little way into' the soil but never travel far in any other direction. 

 Their very presence on the ground or in water means that human excrement, 

 known as night soil, has been deposited in the immediate vicinity. This insures 

 an abundance of bacteria on which the larvae feed. At the end of about five 

 days they molt a second time although the loosened cuticle is not cast off but 

 stays on until it is worn away by the worm's movements against the soil. 



They are now in the infective stage, with bodies that are slender, sharply 

 pointed, and of microscopic size. They become not only different in shape but 

 their appetites change. They forsake the bacteria on which they have fed, are 

 restless and go without food. Instead of boring downward as they did earlier, 

 they now squirm upward and lie as close to the surface of the soil as possible 

 and still keep moist. They are now prepared to bore into human skin, usually 

 on the feet. The country may be one in which night soil is used as a fertilizer 

 as is common in Asia. In that case, the larvae wander over the vegetables and 

 so have a good chance at the human mouth and a direct route to the intestine. 

 If they enter through the skin, they burrow until they reach a lymph or blood 

 vessel, and in the circulation they are ultimately taken to the lungs. There they 

 are caught in the capillaries and this particular environment stimulates them 

 to burrow out into the air chambers. This is nicety of discrimination at its 

 height. In the lungs, the upward movement of the cilia acts as an escalator 

 that carries them to the throat from which they are swallowed. They are then 

 on the way to their final stop in the intestine. There they bury themselves for 

 a short time between the villi, go through a third molt and develop a mouth 

 by which they grasp the intestinal wall (Fig. 26.4). They grow rapidly until 

 they are about one-quarter of an inch long and then molt for the fourth and 

 last time. With this molt, the mouth is changed to its final form and the worms 

 become mature. They are now able to clamp their mouths to the intestinal 

 lining, to wound the capillaries and to suck blood. Eggs begin to appear in the 

 feces about six weeks after a known infection, a sign that the parasites consti- 

 tute a growing population and are steadily drawing blood from their host. By 

 ingenious calculations upon the number of the female population it is figured 

 that each female sucks one cc. of blood from the host per day. In doing so 

 they are provisioning a metabolism that according to careful estimates enables 

 a female of Necator americaniis to produce from 5000 to 10,000 eggs per day. 

 Each one is fertilized internally and the embryo leaves the female body in the 

 four-celled stage of development. 



Fortunately, this multiplicity is reduced by circumstances. The embryos will 

 not develop beyond four cells unless they are exposed to air. This hinders the 

 succession of one generation after another within the intestine. Whatever sub- 



