j.^) ROUNDWORMS. PHYLUM NEMATODA 



has the same general structure as the testis, a hollow region 

 which may be called the oviduct connects it with the wide uterus, 

 and the two uteri unite in a short, muscular vagina. The eggs 

 are produced in inunense numbers— up to 200,000 a day — fer- 

 tilised in the upper part of the uterus, enclosed in a chitinoid 

 shell, passed into the gut of the host, and by it voided with the 

 fiEces. Before they can bring about a new infection, they must 

 pass through a period of ripening, which normally lasts thirty 

 to forty days, and needs moisture, a temperature above 60° F., 

 abundant oxygen, and the absence of putrefaction, and therefore 

 cannot take place except in the outer world. The eggs survive 

 poorly in heat or on drying, and in sandy soils they are washed 

 to the surface and damaged. The most favourable situation for 

 development is therefore a clay soil, in which they are protected, 

 and, by being swallowed with food or water, may reach a new 

 host. Usually they hatch in his intestine, but in warm, damp 

 places may do so in soil. In the egg occurs the first of the four 

 moults which Ascaris, like other nematodes, undergoes in the 

 course of its hfe. The worms hatch as infective larvae which in 

 the new host do not at once become intestinal parasites but under- 

 take first a remarkable journey. Freeing themselves from the 

 remains of the second cuticle and piercing the wall of the intestine, 

 they enter venules and lymphatics (p. 454) and are carried through 

 the Uver and heart to the lungs, where they cause congestion and 

 haemorrhage and are thus discharged into the alveoli (p. 446). 

 Thence they travel along the bronchi and trachea into the gullet 

 and descend the alimentary canal to reach the intestine once 

 more. In the lungs they undergo two more moults, and acquire as 

 swimming-organs lateral membranes, which they afterwards lose, 

 and grow from 0.28 mm. to 2 mm. in length. The final moult 

 occurs in the duodenum, and a mature adult is formed four to 

 five weeks after infection. The adult may live for a year. 



Ascaris lumbricoides is found throughout the world, and can 

 only be avoided by care taken in regard to the cleanness of raw 

 foods and drinking water. It may cause little trouble to the host, 

 or be the source of diarrhoea, anaemia and other complaints, 

 the latter apparently through an enzyme formed b}^ it which 

 interferes with digestion. In severe infections with the eggs, 

 temporary bronchitis may occur during the passage of the larvae 

 through the lungs. Santonin, thymol, and other vermifuges are 

 1 against the worm. 



