PERIODIC PARASITES. 77 



are developed from younger stages that are also internal parasites. 

 Filaria Medinensis has been shown by Fedschenko 1 to make its way 

 into its host as a larva concealed in the body of a Cyclops, which is 

 swallowed along with the water in which it lives. The young worms 

 then reach the intestine, where they remain but a short time, and 

 then bore their way out. The analogy of Filaria sanguinolenta, which 

 is often met with in a larval condition in the so-called "worm- 

 knots/' suggests this, and also the consideration that the difficulties 

 of further internal wandering increase with the growth of the worm. 

 It is no doubt a fact that large, full-grown thread-worms and Tcenicc 

 do bore through the alimentary canal, and even the body-wall of 

 their host ; but this is rare, and when it does occur, the progress of 

 the worms is no doubt assisted by pathological conditions set up in 

 the tissues by their boring. These facts have no special importance 

 in the life-history of parasites, and are rather to be looked upon as 

 accidental, often indeed seriously affecting the life of the host. 



It does not at all follow that every Entozoon that lives outside the 

 alimentary canal must necessarily pass through the latter to reach its 

 desired locality ; Nature has many ways of achieving her ends. An 

 instance of this is afforded by Pentastomum tcenioides, which has a 

 life-history like that of a typical Entozoon. The young form (for- 

 merly described as a distinct species, Pentastomum denticulatum 

 Fig. 56) inhabits cysts in the liver and lung (Fig. 55) of herbivorous 

 mammals ; presently the young animal breaks through its cyst, and 

 makes its way into the body-cavity, after causing considerable injury 

 to the tissues during its transit, and occasionally even causing the 

 death of its host. Sometimes it wanders again into the viscera, 

 most frequently the lymphatic glands. If the body of its host be 

 devoured by a dog or some carnivorous animal, the young Penta- 

 stomum, if not already encysted, finds its way directly through the 

 nostrils (and perhaps also the posterior nares) into the olfactory cavity, 

 where it attains sexual maturity. 



This habit of active migration accounts for the presence of special 

 organs of locomotion, hooks and spines (Fig. 56), which are developed 

 towards the close of the resting stage, and finally laid aside after they 

 have served their purpose. If the young Pentastomum left of its 

 own accord the body of its host, and sought out no fresh host, 

 it would be an example of a periodic parasite attaining sexual 

 maturity while leading a free life. That there are parasites with a 

 life-history of this kind, was briefly stated at the commencement of this 

 chapter ; they are mainly insects, especially flies and wasps. The 



* See Vol. II. 



