INTERMEDIATE AND DEFINITE HOSTS. 79 



the interior of the infected animal (sometimes even in the intestine, 

 e.g., Gastrus equi) a parasite which may be compared to the second 

 developmental stage of a Helminth. Though there is but rarely an 

 actual encystation (even in the Helminths this condition is sometimes 

 absent), the parasite usually remains quiescent for a time, which it 

 spends in growth and preparation for its future metamorphosis. At 

 the end of this period, it instinctively begins to travel, and leaves its 

 place either by the natura] passages (the gad-fly of the horse, for 

 instance, through the anus, that of the sheep through the nasal 

 cavities), or if this be impossible, by boring through the tissues ; the 

 parasite thus arrives at sexual maturity at large, and often differs 

 markedly in form from the preceding larval stage. 



This wandering often causes the death of the host when it is only 

 a small animal, which is hardly surprising, considering the relative 

 size of the parasite and the injuries it must cause by making its way 

 out. In Gordius the life-history is more complicated, inasmuch as 

 this parasite passes into a second host before commencing its meta- 

 morphosis. There are some facts which show that this is not peculiar 

 to Gordius, and that certain other Nematodes have in all probability a 

 similar life-history. 1 It is evident that, in spite of apparent differ- 

 ences, the parasitism of Gordius is fundamentally similar to the cases 

 already mentioned, and may without any difficulty be classed with 

 them. In both cases there are three life periods, generally distinguished 

 by a difference in form the embryo, the sexually mature adult, and 

 an intermediate stage, which, in view of its outward characters, may 

 be termed a " pupal " stage, if the use of this word will not bring us into 

 a hazardous conflict with the customary terminology when we come 

 to treat of the larvae of parasitic insects. Each of these three stages 

 represents in its biological relations a special department of life. The 

 embryo is destined to commence the parasitism ; it migrates, while the 

 " pupa " resumes the prematurely broken development, and carries it 

 on so far that, after passing to the third stage, sexual maturity appears. 

 The migration, which is the cause of this transitional condition, is 

 usually passive, requires no special advances in structure, and is not 

 effected by any particular developmental conditions. 



This is, of course, merely a rough sketch of the life-history of para- 

 sites, and may be regarded as a generalised description, subject, there- 

 fore, to manifold variations in the way of either greater complexity or 

 greater simplification. Complications arise, for example, by the intro- 

 duction of an intermediate generation with independent migrations. 

 On the other hand, the life-history may be simplified by the inter- 



1 For details see Vol. II. 



