THREAD- WORMS. 



453 



enlarged ; the lower illustration depicting the upper (a) and the lower (6) side of 

 the head, with the prominent lips. Other mammals, besides man, are the hosts of 

 different species of Ascaris. For instance, A. mystax is found in dogs and cats, 

 and sometimes even in man ; while A. Iwmbricoides 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 lumbricoides). 

 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). 

 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). 



The gullet, at least in individuals 



