4yO IxVFLUENCE OF CLIMATE ON AM.MAL i)lSl-:ASi:. 



its i)resence, except on microscopic examination of tlie fasces, 

 when the eggs of the worm may he fotnid, and a very large 

 nnmljcr of adnlt sheep may be found to harbour at least a few 

 incHvidnal worms. These, as meritioned before, may not pro- 

 duce any symptoms of their presence, but the imi)ortance of the 

 animal carrying them in acting as a constant source of infection 

 to the pastures over which it grazes will be ai)parent. 



Continuing, however, with the life-cycle of the ])arasite. we 

 note tliat, following the general rule, this does not occiu" vvitliin 

 the body of the host, but can only occur outside it. The eggs 

 must, therefore, reach the exterior, and this the\' do in the ffeces 

 of the host animal. Having done so. the next thing to occur is 

 that they hatch, and form them emerges a form which is known 

 as the larva ; this larva now proceeds to feed, and having grown 

 and cast its outer skin on tw^o occasions, we finally meet with it 

 in an ensheathed form in which the sheath possessed by it is 

 formed from the retained cuticle cast at the last moulting. Now 

 it is this stage which represents the infective form of the para- 

 site, the eggs and earlier larvse stages being incapable of produc- 

 ing infection, and for develo]:)ment to maturity to occur this 

 form must again find its way into the host animal. This is 

 accomplished, however, by th.e larva through climbing up the 

 blades of grass and herbage in its vicinity, with which herbage 

 it is consiuned by its host, or ])erhaps in rarer cases though being 

 consumed with drink'ing water, and now having gained the stom- 

 ach of the sheep, it loses its sheath, undergoes further develop- 

 ment, and within about three weeks' time is to be met with in the 

 stomach as the adult sexuallv mature worm. 



Tn coming now to consider the more detailed observations 

 made in connection with the rate of this development we shall, 

 therefore, speak of the different stages occurring outside the body 

 of the animal as the eg^:, or ovum stage, the immature larval 

 stage, and the mature larval stage, and in dealing with the sub- 

 ject reference may first be made to the work of Ransom on the 

 subject performed in America, the work of Veglia in this country 

 being mentioned later. 



The life-history of the ])arasite, as described by both of these 

 workers, is essentially th.e same, but in regard to the biology of 

 the worm some differences are to be observed in the results 

 obtained. These differences, however, will not be discussed here, 

 but the main facts observed by both workers are here set forth as 

 evidence of the marked effect of heat and moisture on the rate of 

 development of the worm, and some of Ransom's observations 

 are as follows : — 



According to this observer, the eggs do not hatch if the tem- 

 perature is below 40° F.. but tmder such conditions he says the 

 eggs remain in a dormant state. Provided they are not destroyed 

 "by freezing or drying, to both of which they are very susceptible, 

 he states, they ma}' survive in this dormant state for two to three 

 months and hatch, if a favourable opportunity occurrs. 



Above 40° F., however, hatching does occur, and from 40° 



