THE NEMATODA. PARASITISM 



3°5 



blood-sucking. They may cause no ill effects, but the adults may block 

 lymphatics, or unripe eggs, freed by injury to the mother, being 

 broader than the larvae, may choke capillaries. Thus dropsy, and it 

 is believed elephantiasis, are caused. Three kinds of Filaria have 

 similar habits : F. ferstans, whose larvae remain in the surface vessels 

 of the host day and night ; F. bancrofti [ot.F. nocturna), whose larvae 

 retreat to deeper vessels, out of reach of mosquitoes, by day ; and F. loa 



Fig. 215. — The 

 female of Filaria 

 bancrofti. 



Fig. 216. — Filaria bancrofti. — From 

 the Transactions of the Society of 

 Tropical Medicine, after Leiper. 



A , Head ; B, tail of male — showing the arrange- 

 ment of papillae characteristic of the species. 



(or F. diurna), whose larvae retreat at night. If the patient be made 

 to sleep by day, the larvae reverse their habit. The parasites cannot 

 be destroyed, but may be avoided by measures against mosquitoes. 

 They are widespread in warm countries. 



7. Larvm and adult parasitic, without free stage, in hosts of the same 

 or related species. — The cause of trichinosis, Trichinella spiralis, lives, 

 when it is adult, in the small intestine of rat, pig, or man, and there 

 pairs (male I mm. long, female 3—4 mm.). The larvae migrate through 

 the gut- wall to blood vessels, and are carried to muscles, where they 

 encyst. The tissues of the host enclose the cyst in a capsule, and the 

 larvae remain till the host be eaten by another individual of the same 

 or another species, when they become adult in the intestine and young 

 are produced viviparously, the mother lying in the intestinal wall. 

 The symptoms produced in the host are intestinal catarrh during the 

 piercing of the gut-wall, rheumatic pains and fever during migration, 

 and general cachexia during encystment. The disease may or may not 

 prove fatal. It can be avoided by meat inspection and thorough cook- 

 ing. It is found in all parts of the world. 



8. No larval stage. Adult parasitic in the gut of a vertebrate : 

 eggs pass out in faces and are swallowed by another individual of the same 



