NATUKE AND ORGANIZATION OF PAEASITES. 



It lias by this time, I hope, been made clear that the stationary- 

 parasite differs much more from ordinary free-living animals, both in 

 the outward form of the body and in its armature, than the tem- 

 porary parasite. How great the difference really is between 

 these two forms of life, is most distinctly seen in those para- 

 sites which are free at one period of their existence, and parasitic 

 at another ; the free stage may perhaps be entirely unlike the parasitic, 

 especially in those cases where the conditions of life enjoyed by the 

 animal during its parasitic and free stages 

 differ markedly from each other. The larva of 

 Gastrus, which inhabits the stomach of the horse, 

 has all the characters of a stationary parasite ; a 

 simple cylindrical body without eyes and sense 

 organs, and, instead of organs of locomotion, 

 organs of attachment in the shape of powerful 

 hooks at both sides of the mouth, and numerous 

 variously sized setse upon the surface of the body.' 

 How different is the form of the adult free-living 

 animal, with its segmented body, eyes, ten- 

 tacles, legs, and wings ! Who would believe 

 that these two creatures were merely stages in 

 the development of the same animal, had not 

 actual observation demonstrated the fact, and 

 shown that the worm-like larva was produced 

 from the eggs laid by the fly. 



But this striking difference, we cannot doubt, 

 corresponds less to the needs of parasitism as such, than to the differ- 

 ences which usually obtain between a stationary mode of life and a 

 free existence. In this way we can understand the fact, already men- 

 tioned, that metamorphoses quite similar to that of Gastrus are com- 

 monly met with in other insects, where the young are not parasitic, 

 but only live a stationary life like parasites. 



Conversely, there are periodic parasites, whose structure, during 

 both stages of their life-history, is quite the same. This is the case 

 with the Gordiacese, which pass their young stage in the body-cavity 

 of snails and insects, and afterwards live in water or damp earth 

 without any further ingestion of nutriment. In this instance, how- 

 ever, there is no great difference in the manifestations of life between 

 the free and parasitic stages ; in both, the animal leads a stationary 

 existence, and it is only the medium in which it lives that changes. 



It has been already pointed out in this chapter that the characters 

 of parasites cannot be said to have the value of specific peculiarities, 



and this is well shown in certain remarkable cases of parasitism, to 

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Fig. 4. — Cephalic end 

 of Tania solium. 



