TYPICAL THREADWORMS 



3425 



worm reaching a length of nearly one quarter of an inch. Its shape is unusual, 

 on account of the presence of a hump projecting like an excrescence from the 

 surface of the abdomen, some distance from the tail end. When fully formed, 

 this excrescence amounts in size to half the length of the entire worm, and con- 

 tains the young, which, after making their escape, undergo a short development in 

 the body cavity of the midge, then reach the outside, where they are transformed 

 into mature males and females. After the pairing of the sexes, the males perish, but 

 the females again enter the larva and start another cycle of metamorphosis. 

 The second kind (Sphcerularia bombi}, which infests bumblebees, closely resembles 



HUMAN ROUND WORM (Ascarts liiMtbricoides) . 



i. Male; 2. Female. (Natural size.) 3. Eggs (enlarged). 



the first in development; but the excrescence, or brood pouch, of the mother worm 

 is changed into a tube, and ultimately reaches a size from fifteen to twenty thousand 

 times as great as the parent, which dwindles in size in proportion as the sac grows. 

 The life history of this worm is shown in the illustration, where A is the free-liv- 

 ing male, B represents the free-living female, and C is the parasitic female (a/), 

 with her brood pouch. 



Of the threadworms infesting the human body, one of the commonest is Ascaris 

 lumbricoides, which is found in numbers varying from one or two only to over two 

 thousand. These worms usually infest the 

 small intestine, but sometimes enter the 

 stomach, or even penetrate into the liver. 

 Large examples reach a length of six inches 

 or more, and the females produce about sixty 

 millions of eggs annually. These are natur- 

 ally dispersed abroad everywhere, and as the 

 young worm retains its power of growth in 

 spite of frost, drought, and, in fact, the most 

 unfavorable circumstances imaginable, and 

 is, moreover, far too small to be noticed, we HEAD OF HUMAN ROUNDWORM (enlarged), 

 need not feel surprised at the prevalence of 



the pest. The form and some of the structural characteristics of this worm are 

 shown in the two accompanying illustrations. In the uppermost, i is the male, and 

 2 the female, of the natural size, 3 being the egg, enormously enlarged; the lower 

 illustration depicting the upper (a) and the lower (b) side of the head, with the 



