ANIMAL PARASITISM 737 



nematodes mentioned above. "Vinegar eels," nematodes in vinegar, 

 sometimes establish themselves as harmless parasites in the human 

 urinary bladder. Facultative parasites are able to live almost equally 

 well as free-living animals or as parasites; many leeches are faculta- 

 tive 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 the horsehair worms and some ticks and 

 mites, are called temporary parasites, while animals like Acantho- 

 cephala 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. Endo- 

 parasites, which live inside their hosts, also require special adapta- 

 tions. 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 endo- 

 parasite; usually all sense organs are either absent or very poorly 

 developed. There is little or no need for rapid locomotion, so most 

 endoparasites have locomotor structures much reduced or even en- 

 tirely 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 



