45 2 



WORMS. 



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 contains 

 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 (SplKerularia bombi), 

 which infests humble-bees, closely re- 

 sembles the first in development ; but the 

 excrescence, or brood-pouch, of the mother- 

 worm is changed into a tube, and ultim- 

 ately 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-living male, B represents the 

 free-living female, and G is the parasitic 

 female (w), with her brood-pouch. 



Of the thread -worms 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 6 inches or more, and 

 the females produce about sixty millions 

 of eggs annually. These are naturally 

 dispersed abroad everywhere, and as the 

 young worm retains its power of growth 

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

 most unfavourable circumstances imagin- 

 able, and is, moreover, far too small to be noticed, we need not feel surprised at 

 the prevalence of the pest. The form and some of the structural characters of 

 this worm are shown in the two illustrations on p. 453. In the uppermost, 1 is 

 the male, and 2 the female, of the natural size, 3 being the Q^g, enormously 



HUMBLE-BEE THREAD-WORM. 



A , Male enlarged ; a, nat. size. 



B, Female enlarged : b, nat. size. 



C, Brood-pouch of female (w) ; c, nat. size. 



