ECOLOGY AND ZOOGEOGRAPHY 



641 



Parasitism in the 

 animal kingdom 



Almost every large group in the animal 

 kingdom contains parasitic species. There 

 are very few of these, however, in certain 

 phyla and large numbers in other phyla. A 

 survev of the animal kingdom shows clearlv 

 that the parasites of principal importance 

 are included in the phylum Protozoa, in 

 several phyla of worms, and in the phylum 

 Arthropoda. Among the Sarcodina are both 

 free-living and parasitic species, including 

 the amoebas of man and other animals. 

 The flagellates also contain free-living and 

 parasitic representatives, such as the intes- 

 tinal and blood-inhabiting parasites of man 

 and other animals. The sporozoans are all 

 parasitic; among them are the coccidia and 

 malarial parasites of man, and the parasites 

 of the intestine, blood, and tissues of other 

 animals. The ciliates are free-living or para- 

 sitic; Balantidium coli is the only species 

 that is parasitic in man; various species in- 

 habit other animals. 



Very few parasites occur among the 

 sponges and coelenterates. The flatworms 

 include many free-living and parasitic spe- 

 cies. The Turbellaria are mostly free-living, 

 but the Trematoda and Cestoda are all 

 parasitic. The roundworms likewise include 

 free-living species and parasitic species that 

 live in man and other animals; e.g., hook- 

 worms, ascarids, and others. 



The Acanthocephala (spine-headed) 

 worms are mostly parasitic in lower animals, 

 and the Nematomorpha (hairworms) are 

 parasitic in insects. Echinoderms are almost 

 entirely free-living; annelids may be either 

 free-living, such as the earthworm, or para- 

 sitic as the leeches. The mollusks are mostly 

 free-living. 



The arthropods are free-living or parasitic; 

 many species act as intermediate hosts of 

 parasitic protozoans and worms. Many crus- 

 taceans are parasites of fish; many insects are 

 parasitic or serve as intermediate hosts; and 

 a large number of arachnids are parasitic. 

 The vertebrates are mostly free-living, but 



there are species that may be classed as 

 parasites. 



Origin and evolution 

 of parasitism 



We really know nothing definite about 

 the origin and evolution of parasitism, since 

 no one has ever observed a free-living species 

 become parasitic, but many known facts 

 are of value in any attempt to work out lines 

 of descent. In the first place, the parasitic 

 habit must have been more recently evolved 

 than the free-living habit, since free-living 

 forms must have existed before the parasites 

 could obtain hosts on which to live. Ecto- 

 parasites probably evolved before endopara- 

 sites, because the change from a free-living 

 existence to that of ectoparasitism does not 

 appear to be so difficult as to that of endo- 

 parasitism. Inasmuch as there are free-living 

 as well as parasitic species in many large 

 groups of animals (e.g., the Sarcodina, 

 Mastigophora, and Ciliata among the Pro- 

 tozoa), it is evident that the parasitic habit 

 has arisen independently in each of these 

 groups, and this type of evolution may there- 

 fore be considered of rather common occur- 

 rence. 



PRINCIPLES ASSOCIATED WITH 

 BIOTIC COMMUNITIES 



It has been stated that a biotic community 

 is essentially the group of populations in an 

 ecosystem. They may be named after some 

 conspicuous feature such as the most con- 

 spicuous or numerous animal or plant spe- 

 cies (beech-maple) or after some physical 

 feature (tropical rain forest). There is no 

 set rule for the naming of communities. A 

 biotic community (Fig. 450) is a more or 

 less complex group of plants and animals 

 that occupies a particular area and influences 

 the life of each member. In this area, the 

 environmental factors are such that certain 

 types of plants and animals are able to live 

 there. Other areas, where a similar environ- 



