= Solution
\b[<Ascaris lumbricoides> provides an example of a human <parasite>: the adult is an intestinal roundworm, while its infective stage is an embryonated egg acquired from a contaminated environment.] The account below gives the established modern life cycle; its full migratory detail should not be assumed to have been demonstrated by December 1864.
The adult is elongated, cylindrical and unsegmented, with a tapering body, a tough <cuticle> and a mouth bordered by three prominent lips. Sexes are separate. The female is generally larger; the male has a characteristically curved posterior end. Their form distinguishes them from segmented <tapeworms> and from an earthworm, despite superficial resemblance at a distance. Adults occupy the lumen of the <small intestine>, obtaining nourishment in the host's intestinal environment.
Females pass large numbers of eggs into the intestinal contents, and the eggs leave in <faeces>. Fertilized eggs have a thick shell, commonly with an outer mammillated coat; the coat may be absent in a decorticated egg. Unfertilized eggs have a different appearance and do not develop into the infective stage. Warmth, moisture, shade and time permit larval development inside suitable fertilized eggs. Freshly passed eggs are therefore not equivalent to fully developed infective eggs.<DPDx: Ascariasis>[Causal Agents; Life Cycle; fertilized, unfertilized and decorticated egg descriptions]{ref}
After an infective egg is swallowed, a larva hatches, crosses the intestinal mucosa and reaches the <liver> and then the <lungs> through the circulation. It enters the pulmonary air spaces, ascends the bronchial tree to the throat, and is swallowed. Back in the <small intestine>, it develops into an adult and completes the cycle. No intermediate insect, fish or mammal is required for this human roundworm cycle.<Ascariasis in MedlinePlus>[Causes: intestinal hatching, pulmonary migration, swallowing and adult development]{ref} Development in the outside environment helps explain why ordinary immediate contact with newly passed <faeces> is not the same exposure as swallowing mature eggs from contaminated soil.
The migrating larva and the adult affect different tissues. Many light intestinal infections are asymptomatic, whereas heavy burdens can impair <nutrition> or obstruct the intestine; wandering adults can enter biliary passages. Larval migration can produce respiratory <inflammation>. These effects arise from particular stages and burdens, not from venom injected by a biting organ. Eggs in a suitable stool preparation and the morphology of an expelled adult provide different kinds of identifying evidence; an early migratory infection need not yet supply eggs in stool.<About Ascariasis by the Centers for Disease Control and Prevention>[Overview; Symptoms; Causes: intestinal adults, environmental transmission and pulmonary manifestations]{ref}
The natural history explains prevention: separating human waste from <food> and <water> and limiting contaminated-soil ingestion interrupt transfer of eggs. It also explains why a household observation of a worm does not establish spontaneous generation inside the intestine. The absence of an intermediate host is compatible with a complex larval migration within the definitive host.
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