ANIMAL PARASITES AND MESSMATES. 685 



turns out by the million. A description of Tmnia solium^ the most 

 common tapeworm of man, will enable us to understand all the others. 

 Under its first, or vesicular form, this parasite comes from the flesh 

 of the pig, where it is often found in large numbers, when the pig is 

 said to be " measly." This condition of the pig has been attributed 

 to damp, to feeding on acorns, to hereditary causes, to contagion, and 

 various other influences, but none of these notions are correct. The 



Fig. 25. Cysticercus. 



<i, Upper Part of the Vesicle; h. Place where the Vesicle is about to separate: c, NecU of the 



Worm ; d. The Head, showing the Suckers and the Crown of Hooks. 



only true cause is the introduction of the eggs of Tmnia solium into 

 the intestines of the pig* These eggs, or fragments of taenia contain- 

 ing them, are swallowed by the animal. In the gastric juice of its 

 stomach the eggs are set at liberty, lose their shells, and there issues 

 an embryo singularly armed. It carries in front two stylets, in the 

 axis of the body, and on the right and left sides two other stylets, 

 which act like fins. These embryos bore into the tissues as the mole 

 burrows in the soil. The middle stylets are pushed forward like the 

 snout of the insectivore, and the two lateral stylets act like the limbs, 

 taking hold of the tissues and forcing the head forward. In this man- 

 ner the embryos perforate the walls of the digestive tube, and find 

 their way, by means of the blood or otherwise, to the organ or tissue 

 which is to become their temporary home. When arrived at this 

 point they surround themselves with a sheath ; their stylets, no longer 

 of use, decay ; and at one of the extremities appears a crown of new 

 hooks, quite different from the former ones, which will serve to anchor 

 their progeny in the new host to which they are ultimately destined. 

 This vesicular worm, or cysticercus, fully formed, and without under- 

 going any change, waits till its host, the pig, or that part of him which 

 it inhabits, is eaten, and, if its life has not been destroyed on its way 

 through the frying-pan, it wakes up in some human stomach. Once 

 there, it instantly quits its torpid state, gets rid of its useless envel- 



