NEMATODA. 773 



first skin in about three weeks, at whicli tirae, as happens with 

 many other larval nematodes, they part with their tails. 

 , The case is still more complicated when the nematode re- 

 quires two hosts to complete its metamorphosis. In some cases 

 it is the egg which is taken into the first or intermediate host, 

 in other cases it is the larva. "We have a good example of a 

 parasite requiring two hosts in the case of the Spiroptera oUusa 

 of the mouse. The eggs pass out of the alimentary canal of 

 the mouse, and are eaten by the meal-worm. The eggs are 

 hatched, and the larva, after living in the meal-worm for about 

 five weeks, forms round itself a capsule of connective tissue. 

 If meal-worms containing these encysted larvae are eaten by 

 mice, the larvae leave their capsules and become the sexually 

 mature Spiroptera ohtitsa of the mouse. The Filavia sanguinis 

 hominis, or Filaria Bancrofti, is another good example of a 

 parasite which requires two hosts. The sexually mature worm 

 is found in the human tissues, and is viviparous, producing 

 numerous larvae, which make their way into the blood. The 

 blood is sucked by mosquitos, and thus the larvae get into the 

 mosquito, where they increase in size, and undergo various 

 changes. When the mosquito dies the larvae pass into the 

 water and live for a time a free life. Ultimately they gain 

 access to the intestines with the water, and, boring into the 

 tissues, produce again the larvae on ^ttainipg sexual maturity in 

 their human bearer. 



A third example of a nematode requiring two hosts is the 

 Trichina spiralis. The sexually mature worm lives in the 

 alimentary canal, and is viviparous, producing numerous larvae, 

 which make their way out of the alimentary canal into the 

 muscles, where they encyst. When flesh containing the sexually 

 immature larvae is eaten, the cystic stage comes to a close, and 

 sexual maturity is attained in the alimentary canal, where the 

 larvae are again produced. 



The eminent Eussian traveller Fedtschenko observed the 

 primary larval changes to take place in the young of the guinea- 

 worm during their sojourn in the alimentary canal of Cyclopes, 

 these entomostracous crustaceans' being destined to play the 

 part of intermediary bearers. In like manner, there can be 

 little doubt that the young of the strongyles, which occasion 

 husk and lamb disease, undergo their primary changes of de- 



