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Ci\ iC JUOLOGY 



forms may be direct (that is, worm, egg, worm in the same 

 host) or it may require two or even three hosts for its com- 

 pletion. The sexes axe usually distinct. Typical nematode 

 parasites are 



Roundworm — Ascaris lumbricoides. This is the most coninion para- 

 site of iiiau; it is reddish-yellow, resembles the earthworm in general 



shape, and may attain a length of from 

 12 to 15 inches. It lives in the small in- 

 testine, and, being unattached, is easily 

 dislodyed. The life cycle is direct, the 

 eggs being taken in with polluted water 

 or dirty foods. Since they have been 

 found to pass through flies uninjured, 

 these insects are thought to be one of 

 the active means of distributing the 

 euus to human foods. 



Pinworm — Oxyuris vermicularis. This 

 is a minute nematode, from 4 to 12 mil- 

 limeters long, that affects children the 

 world over. Its life cycle is direct. 



Gape worm — Syngamus trachealis. The 

 habitat of this parasite is tlie trachea 

 or bronchi of birds. The male is smaller 

 than and permanently attached to the 

 female, and the eggs are not laid, but 

 are set free by the disintegration of the 

 mother in the soil. Here they hatch in 

 about a week, in warm weather, the 

 eml>ryos are picked up by the l)irds, and the direct life cycle is repeated. 

 Trichina worm — Trichinella spiralis. The presence of this parasite in 

 man causes the well-known disease trichinosis', and our constantly re- 

 curring epidemics prove that, with all the publicity given to the matter, 

 we have not reached a solution of the problem. Has proper attention been 

 directed to extermination of rats from premises where swine are raised? 

 The course of infection is indicated in Fig. 119. The adult worm, 

 the size of a very fine hair 2-4 millimeters long, lives in the wall of 

 the small intestine, where the female gives birth to from 1000 to 1500 ^ 



Fig. 118. Gapeworms, female 

 with small male attached 



1 The Cambridge Natural History (Vol. II, p. 146) estimates 12,000. 



