gives rise to no serious pathological lesion; a fortunate circumstance, seeing that hitherto it has been found impossible, with any degree of certainty, to dislodge it by anthelmintics.
Ascaris lumbricoides (L.)
Though not quite so common in tropical countries as trichiurus, the ascaris is nevertheless very common indeed, especially in children, who often harbour these loathsome creatures in enormous numbers—in dozens, or even in hundreds. In those countries, at one time or another, nearly every child gets them; so much so that, when doubt exists about the nature of some obscure affection, a dose or two of santonin often produces results which will seem to justify a diagnosis of "worms," and, for the time being, perhaps satisfy an anxious mother.
Fig. 172.—Ova of cestoda.
(After Looss.)
a, Tænia solium; b, Tænia saginata; c, Hymenolepsis nana.
Mode of infection.—The reason for the great frequency of Ascaris lumbricoides in tropical countries is probably twofold—the warmth of the climate, and the habits of the people with regard to the disposal of night-soil. In the fæces the ova exhibit no trace of segmentation or of differentiated embryo; but if placed in water, or kept moist and in a warm place, in the course of one or more months—longer or shorter according to temperature—the larva is developed, and can be seen coiled up and moving about inside the egg-shell. If such an egg is accidentally or intentionally swallowed, on arrival in the stomach the shell is dissolved away and the contained larva is set free. In a month it grows into a sexually mature animal, and, if both sexes are present, eggs in countless numbers are soon produced and appear in the fæces. Desiccation of the egg at atmospheric temperature does not destroy the larva, which will quickly revive on becoming moistened. In many warm countries night-soil is the favourite fertilizer, and is regularly collected and spread upon the fields. In this way the ova of ascaris obtain an opportunity of maturing, and thus, too, they have an opportunity of being swallowed by man. They may also be washed into drinking-water; or, becoming desiccated on the drying up of the fields, be blown about as dust; or they may become attached to fruit or vegetables. In one of these, or in similar ways, they finally reach the human stomach and there attain maturity.
This view of the life-history of A. lumbricoides was based
