430 ESSENTIALS OF ZOOLOGY * 



almost equally well as free-living animals or as parasites; many 

 leeches are facultative parasites. Obligate parasites, on the other 

 hand, cannot live without the host. Parasites which are free-living 

 during part of the life cycle, as in the case of horsehair worms and 

 some ticks and mites, are called temporary parasites, while animals 

 like Acanthocephala and tapeworms which are parasitic during the 

 entire life cycle are called permanent parasites. 



The Successful Parasite 



Like all other ways of living, successful existence as a parasite 

 requires certain modifications or adaptations in structure and func- 

 tion. Parasites which live on the outside of the host's body are 

 called ectoparasites; they must have special organs for attachment 

 in order to maintain their hold on the host ; for example, lice have 

 hooklike feet with which they hold on to the skin, hair, or feathers 

 of the host, and ectoparasitic trematodes have either muscular 

 suckers or chitinous hooks for attachment to the outside skin or to 

 the gills of the fishes on which they live. On the other hand, ecto- 

 parasitic insects have no need for wings, so fleas and bedbugs con- 

 tinue to thrive without them. Many ectoparasites, such as fleas, lice, 

 bedbugs, mites, and ticks, also have specially constructed mouth 

 parts for piercing their host's skin and sucking blood. Endopara- 

 sites, which live inside their hosts, also require special adaptations. 

 For maintaining their positions in the intestine or other organs they 

 must have some sort of attachment organ, such as the muscular 

 suckers of trematodes and tapeworms and the hooks of thorny- 

 headed worms. On the other hand, they live in the dark so eyes 

 may be entirely lacking without inconveniencing the endoparasite ; 

 usually all sense organs are either absent or very poorly developed. 

 There is little or no need for rapid locomotion, so most endo- 

 parasites have locomotor structures much reduced or even entirely 

 lacking. Many endoparasites also have less of a digestive system 

 than their free-living relatives ; parasites in the liver, lungs, blood 

 vessels, etc., usually have some sort of digestive apparatus, but 

 many intestinal parasites, such as tapeworms and thorny-headed 

 worms, have no sign of digestive organs whatever, but depend on 

 the host to furnish them with food already digested and ready for 

 absorption. Most endoparasites have their reproductive organs 

 enormously developed, sometimes so much so that 90 per cent 



