The Nodular Worm. 



49 



had been under observation without having- liad a chance of reinfec- 

 tion, this period must be of considerable length. It is during this 

 period that the female is fertilized by the male and then begins to 

 lay eggs. The life-history has partly been worked out by Dr. Veglia 

 of this Division, and the data given here are taken from his observa- 

 tions made as a corollary to his investigation into the life-history of 

 the wireworm {Haemonchus contortus). The eggs leave the sheep 

 with the faeces and can easily be brought to hatch when put up under 

 suitable conditions in a glass jar. In the life-history of a parasitic 

 nematode, such as the one under discussion, we can distinguish four 

 different larval stages through which the worm has to go before it 

 reaches maturity, two of these stages are passed outside the final 

 hosts and two inside. The larva which hatches out of the egg soon 

 surrounds itself with an envelope which it casts and a second one is 

 formed. It is now called a mature larva. Its destiny is to reach the 

 sheep, and since it has to wait for its chance to be eaten in by the 

 sheep, it possesses certain qualities that achieve it. The larva is 



Plate II. 



Eekziekte. — Small intestines (ileum) cut in half ; 

 the arrow indicates a nodule. 



endowed with longevity. It may safely be stated that it can live at 

 least one year under favourable outside conditions. It is able to crawl 

 about and does so mainly in two directions ; one is the upward one by 

 which it reaches the tufts and grasses, the other one is the downward 

 one, which enables it to return to the ground. The larvae can only 

 crawl on a moist surface; this is, of course, the case during rain and 

 at a time of heavy dew when the blades of grass are wet. Further- 

 more, the larvae cannot stand the action of the direct sunlight and try 

 to escape from it without delay. Hence the migration of the larvae on 

 the grass can only take place under certain conditions, viz., when 

 moisture enables them to do so, in the summer and in the absence of 

 sunlight, during night, the early morning, and when the sky is over- 

 cast, particularly on rainy days. Moisture, of course, is retained in 

 "the portion of grass tufts near the soil, and sheep feeding very close to 

 the ground are thus exposed all through the summer to infection 



