676 TEE POPULAR SCIENCE MONTHLY. 



True parasites are beings entirely dependent on their neighbors 

 for support ; unable to provide for themselves, they are fed wholly at 

 the expense of others. It is generally believed that they are an ex- 

 ceptional class of organisms, constituting a group by themselves, and 

 knowing nothing of the world outside the organ which shelters them. 

 This is an error. Representatives of all the principal divisions of the 

 animal kingdom below the vertebrate are found pursuing this mode 

 of life. There are few jjarasites that are not wanderers at some period 

 of their lives ; and it is not uncommon to find some which live alter- 

 nately as noblemen and as beggars. Many are paupers only during 

 infancy, or at the approach of adult age, living at other times a com- 

 paratively free and independent life. ISTor are all the members of a 

 species necessarily parasitic ; sometimes it is only the female that 

 takes the relation of a dependent, the male continuing his nomad life. 

 Again, there are cases where, the female being provided for, the male 

 relies on her for support, and thus the charitable animal which comes 

 to her help is laid under contribution by the whole family. 



Parasites present an extraordinary variety of forms, and differ 

 very widely in size and aj)pearance, these differences being often re- 

 markable between the sexes of the same species. The male of the 

 urubu of Brazil has the usual form of a round long worm, while the 

 female resembles more than any thing else a ball of cotton, not hav- 

 ing the slightest analogy with the other worms of the order. As to 

 the enormous proportions parasites may attain, Boerhaave mentions a 

 bothriocephalus 300 ells * in length ; and, at the Academy of Copen- 

 hagen, it was reported that a solitary tapeworm i^Tcenia solium) had 

 been found 800 ells long. Parasites are found in every region of the 

 globe, but, like other animals, they observe the laws of geographical 

 distribution. Some, like the leeches, take their food, and then detach 

 themselves until the demand for food returns, never becoming identi- 

 fied for any length of time with their host. Others, like the lernjBans, 

 commence their parasitic existence when approaching maturity, and 

 thereafter are permanent dependents ; others, again, like the ichneu- 

 mons, begin life as parasites, and on reaching maturity assume and 

 maintain an independent existence ; while still others, like the taenia, 

 are parasitic from first to last, although changing their abode at a 

 certain stage of development. 



All animals, man included, have their parasites, which usually come 

 from without, those entering the body being generally introduced 

 with the food or drink. No organ is exempt from their incursions, 

 as they have been found in the brain, the ear, the eye, the heart, the 

 blood, the lungs, the spinal cord, the nerves, the muscles, and even 

 the bones, Cysticerci have been seen in nearly all these situations, 

 and worms of various kinds are common in the cavities of the body, 

 as well as in many of the solid organs, such as the muscles, liver, and 



' The Flemish ell is probably meant : this is 27 inches long. 



