578 - Heredity and Evolution 



Fig. 30-6. Enlarged model of the body louse, Pediculus vestimenti. Note the 

 adaptations that enable this ectoparasite to cling to its host. (Courtesy of the 

 American Museum of Natural History, New York.) 



hosts as a means of regaining access to a 

 primary host. 



Many parasitic species display enormous 

 powers of reproduction — which assures a rea- 

 sonable chance that some of the offspring 

 may gain access to a proper host. In an adult 

 tapeworm, for example, each of the many 

 body sections usually bears two ovaries and 

 many testes, and the number of self-fertilized 

 eggs produced by a single tapeworm amounts 

 to many millions. Also many parasites multi- 

 ply asexually during one or more stages of 

 the life cycle. Thus, large numbers of off- 

 spring may be produced by fission, budding, 

 or sporulation during one or more asexual 

 stages (Fig. 30-7). 



In regaining access to a primary host, a 

 parasite may develop the capacity to infest 

 intermediary hosts; and such an intermediary 

 host may become a vector, which facilitates 

 the transfer of the parasite back to the 

 primary host (Fig. 30-7). This tendency, in- 

 deed, accounts for the complexity that is 

 characteristic of the life cycles of many para- 

 sites. The parasite must adapt itself to a 

 series of intermediary hosts, and perhaps also 

 to one or more free-living periods between 

 successive hosts before it comes back to its 

 primary host. 



Effects of Parasitism on the Host: Para- 

 sitic Diseases. Although a parasite may not 

 harm its host very seriously, or at all, para- 

 sites generally are responsible for infectious 

 diseases. This is true among plants, as well 

 as animals, including man (see Table 30-1). 

 When a virus, fungus, protozoan, or other 

 parasitic species invades the tissues, fluids, or 

 hollow organs of a host, the parasitized indi- 

 vidual often suffers damage, and often the 

 host displays symptoms of a definite disease. 



A parasite may harm the host in various 

 ways: (1) by digesting and destroying the 

 living tissues (Amoeba hystolytica, in amoebic 

 dysentery); (2) by producing toxic products 

 (fever-producing organisms); (3) by causing 

 internal bleeding (hookworm); (4) by appro- 

 priating organic food from the digestive 

 tract of the host (tapeworms); or (5) by com- 

 binations of these and other effects. 



Disease Resistance: Xati ral Defenses. 

 Natural defenses against infective organisms 

 have been evolved in every species. In man, 

 for example, the skin, because of its tough- 

 ness and dryness, offers an effective barrier 

 against the penetration of potential parasites, 

 which often ma)' gain access to the more vul- 

 nerable deeper tissues only when the skin is 

 broken. Moreover, a parasite, once past the 



