THREAD- WORMS. 
453 
enlarged; the lower illustration depicting the upper (a) and the lower (b) side of 
the head, with the prominent lips. Other mammals, besides man, are the hosts of 
different species of Ascaris. For instance, A. viystax is found in dogs and cats, 
and sometimes even in man; while A. lumbricoides also occurs in swine. A large 
species, A. megalocephalus, the female of which reaches a length of over a foot, lives 
in horses and cows. A second common parasite of mankind is Oxyuris vermi- 
cularis, a small, white, sharp-tailed worm, which measures about one-quarter of 
human round worm [Ascaris lunibricoides). 
1, Male ; 2, Female. (Nat. size.) 3, Eggs (enlarged). 
an inch in length. It occurs abundantly in children and growing people. As, in 
the case of Ascaris, it seems that before development can take place, the egg must 
pass out of the host, and again make its entry into the alimentary canal through 
the mouth. These worms are so small and 
light that, when dried, every current of air 
will scatter them, and they may make their 
way into the alimentary canal of their host 
in connection with almost any kind of food. 
To the family Strongylidce belongs a danger¬ 
ous parasite, Dochmius duodenalis, occurring 
in the intestine. It is about half an inch in 
length. A peculiarity of this species and 
others of the genus is that the posterior end 
of the male is furnished with a curious bowl- 
or fan-shaped ruff, which is often supported 
by thick ribs (a and c of the figure on p. 454). The gullet, at least in individuals 
that are still growing, is furnished with strong teeth (b). When this worm appears 
in masses, it produces the disease known as Egyptian cholera. 
The accompanying illustration is an enlarged view of the head of Cucullanus 
elegans, a parasite in fresh-water fishes like the perch, and having for its inter¬ 
mediate host the Crustacean Cyclops. The worm is about half an inch long, and 
the aperture of its mouth forms an elliptical case with thick brown walls. The 
female bears living young, which creep forth from their egg-cases while still within 
their mother’s body, where they may be counted in thousands. Protected by a 
tough skin, the worms, which have reached the outside world, frequently live for 
several weeks in the water on the look out for a favourable host. Having come 
across, and made their way into a Cyclops , they undergo various changes, but only 
head OF human round worm (enlarged). 
