74 



LIFE-mSTOliY OF PAKASITES. 



with an intercalated free-living stage) accomplished hy a passive trans- 

 fcTcncc, — a process which we shall have specially to notice when the 

 final fate of an asexual internal parasite comes to be treated of. This 

 transference is, however, by no means always the result of a change of 

 hosts. 



In the mesenteric artery of the horse there is commonly to be found 

 a more or less conspicuous aneurismal swelling. This is caused by 



parasitic Nematodes, belonging to 

 the life - cycle of Sclcrostomwm. 

 equinum {Strongylus equinus), 

 which originate from the above- 

 mentioned (p. 61) Ehabclitis-Yik.Q 

 embryos. The worms live in the 

 fibrous lining of the aneurism (Fig. 

 52), and grow to an inch in length ; 

 they then, after casting their skin, 

 change into the adult condition, 

 which is characterised not merely 

 by the development of the sexual 

 organs, but by the possession of a 

 conspicuous horny mouth-armature 

 with a serrated margin.^ Eipe 

 sexual products are indeed some- 

 times absentjhaving been developed 

 directly after the animal has aban- 

 doned its first habitation for the 

 intestinal canal. This wandering, 

 then, as has been pointed out, 

 takes place without the parasite having to leave the body of its 

 host. Subsequently the worm becomes detached from the lining of 

 the aneurism, and is carried by the blood stream into the branches 

 of the arterial system of the intestine, until their decreasing size 

 puts a stop to further progress. Here the parasite begins to bore 

 through the wall of the intestine, which it accomplishes by the trephine- 

 like action of its mouth-cavity, and reaches its ultimate destination. 



But such instances are particularly rare. Whenever we have had 

 the opportunity of observing, under similar circumstances, the trans- 

 ference of an Entozoon to its definitive condition, it is always ac- 

 complished by the worm— and its host— being devoured by the 

 definitive host.^ The importance of this for the distribution of 



' For a detailed account see Vol. II. 



* Ocoasicnally the reverse is the case, as in certain tape-worms {Ligida, Schisto- 

 cephalus), which are often taken up by water-fowls directly from the water (see p. 26). 



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Fig. 52. — Worm aneurism of the horse. 



