LIFE HISTORY OF STOMACH WORMS 



297 



ing to the temperature. Either dryness or freezing temperature 

 will kill the eggs and the very young larvae. The newly hatched 

 larva feeds upon the fecal matter in which it lives until it de- 

 velops into the final stage outside the sheep or host. This is called 

 the final free living stage, and the time from hatching until this 

 stage is reached is also a matter of temperature, ranging from a few 

 days to a few weeks. On attaining the final free living stage the 



FIG. 189. 



FIG. 190. 



A B C 



FIG. 189. Stomach worm (Hcemonchus contortus). A, female larva X 5; B, male; 

 C, anterior end of body showing: n.r., nerve ring; c.p., cervical papilla; es, oesophagus. (From 

 Bureau of Animal Industry Bui. 127.) 



FIG. 190. Ensheathed stomach worm. In this stage the worm is highly resistant to 

 hot, cold and dry weather, and hence is hard to destroy. (From 22d Annual Report South 

 Carolina Station.) 



larva becomes enveloped in a thin, horny-like coat, called a chitinous 

 sheath, is no longer able to take in food, and hence must obtain its 

 nourishment from material stored up in its own intestine (Fig. 190). 

 At temperatures above 40 degrees F. the ensheathed larva or 

 embryo can* move about, very slowly of course, and it becomes more 

 active as the temperature rises. It is very unlike the newly hatched 

 larvae and eggs in that it is not killed even by long periods of 

 freezing or drying. When the vegetation is wet from rain or dew, 



