LIFE-HISTORIES 



145 



6. LarvcB and adults parasitic in different animals, with no free 

 stage. — The best-known of these is Wuchereria [Filaria) hancrofti, 

 the cause of elephantiasis, which is found in all tropical countries 

 and as far north as southern Spain. The females are about four 

 inches long, the males half this size, and both live in the lymphatics 

 of man. The female is ' ovo viviparous ', that is, the egg-case 

 contains a completely formed larva. This is set free and goes to 

 the blood, where it lives for some time and is known as a Micro- 

 filaria. The larvae congregate in the peripheral blood vessels at 

 night, retreating to the capillaries of the lungs and to neigh- 

 bouring vessels during the day. In due course the host is bitten by 

 a gnat, and some of the microfilariae 



are sucked up. They develop for about 

 a fortnight, and may then be passed 

 to a new host when the gnat next 

 bites. They are carried by several species 

 and genera of gnats, including the com- 

 mon Culex pipiens. Other species of 

 Filaria have somewhat similar life- 

 histories, with different invertebrate 

 vectors ; F. ( =Loa) loa is carried by the 

 tabanid fly Chrysops, and F. perstans 

 by a leech. 



7. Adult and larvcB parasitic, without 

 free stage, in the same or a closely 

 related species of host. — Trichinella 

 spiralis lives as an adult in the small 

 intestine of man, rat, and pig, and some 



other mammals , and experimentally even salamanders may be 

 infected. The males are a millimetre long, the females three or four 

 millimetres. The male dies soon after copulation, but the female 

 bores into the lymphatics and produces many larvae, which are 

 carried in the general circulation to the muscles and there 

 encyst (Fig. 107). A new host is infected if it eats meat containing 

 the cyst. The commonest natural cycles are rat/rat, rat/pig, and 

 pig/rat, but man may be infected if he eats raw pork (as in German 

 sausages) or even that which is underdone. The worms in the 

 intestine cause general intestinal symptoms resembling those of 

 typhoid, and the encysted larvae produce general weakness and 

 often death. 



8. Parasitic in vertebrates, no larval stage. — Trichocephalus 



Fig. 107. — Trichinella spiralis, 

 young encysted in muscle 

 of host. — From Thomson, 

 after Leuckart. 



