ON PARASITES. 13 



the beacon light of hope to guide him, — the mens conscia recti to in- 

 vigorate him. Although great additions have been made within the 

 present century to the list of known parasites, it is still far from com- 

 plete : vide RudolpM Synop. Ent. The migratory habits of animal 

 parasites are a source of much difficulty in tracing their history. In 

 fact it is the greatest impediment in the way of investigation. 

 The condition of their existence being so much specialized, so dis- 

 similar and often so far asunder from one another. Innumerable 

 abortive experiments require to be made before those conditions are 

 all examined, and the entire history of the particular organism which 

 is the subject of investigation ascertained. Impelled by instinct they 

 traverse the organisms which they infest, or leave them for the outer 

 world. They also are transferred passively from place to place, from 

 organism to organism. Por example the six-hooked embryo of the ces- 

 toid entozoa having been set free in the interior of the alimentary canal 

 of various animals, migrates actively into a portal vessel, then passively 

 floating in the circulating blood it lodges in some remote capillary 

 and renews the active migration, passing into the interior of the tis- 

 sues which its instinct leads it to select for its dwelling place, prece- 

 ding its development into a cystic worm. 



How many observations required to be made to determine this single 

 fact ? A six-hooked animal was seen in the intestine of a cystic worm 

 in a distinct tissue, a strong imagination would hardly have suggested 

 any relation between them. 



A cercaria without sexual organs and two thirds tail, swims freely 

 in the water among little mollusks like the paludina. In the interior 

 of one of these mollusks is found a distoma with several organs, but 

 without tail, and in no respect resembling the cercaria. It would not 

 be expected that these two animals had any relation, yet the cercaria 

 is developed into the distoma. In this connection, mention need but 

 be made of Kuchenmeister who for four years was vainly on the look 

 out for a tsenia belonging to the cysticercus of the meal-worm ; of 

 Pilippi, who opened hundreds of animals to trace the development 

 of the eggs of distoma into cercaria. Simple in apparent structure 

 though they be, rude and loathsome to the eye, not from any special 

 deformity but from association, these entozoa afford a deep insight 

 into the mysteries of vitality. The higher organisms that inhabit 

 the outer world, are opaque, and consequently present insuperable 

 obstacles to the ocular examinations of their vital functions, no light 



