Chap. 6 MUTUAL RELATIONSHIPS OF ANIMALS 99 



species, in which case it has a direct life history. Such parasites may live 

 through their entire lives in one host, producing eggs and larvae which in turn 

 live and reproduce in the sarne place. Many of them are usually carried out of 

 the body with waste from the intestine. They then await the chance of getting 

 into the mouth of another individual; this is the usual history of pinworms 

 {Enterobius vermicularis) , common parasites of children (Chap. 26). In con- 

 trast to such direct life cycles are the indirect ones of parasites with hosts that 

 belong to two or more different species. Larvae of these parasites develop to 

 a certain stage in one host, such as a sheep. But they cannot develop further 

 unless they are cast out of the sheep's intestine at the edge of a pond where 

 they can enter certain pond snails, their intermediate hosts. In the snails they 

 develop to a particular stage in which they leave the snails, swim about in the 

 water, and finally onto the wet grass around it. In this stage and in no other are 

 they able to infect another sheep when swallowed (Fig. 25.11). These para- 

 sites, called flukes, prove that gambling is a very ancient and enduring practice. 



Certain important variations apply to both direct and indirect life histories 

 of parasites. Some species with direct life histories can live parasitically in 

 several related animals, such as sheep, cattle, and others that chew the cud; 

 while other species, such as the human hookworm, can live only in one type 

 of host. Parasites with indirect life histories spend part of their lives in an 

 intermediate host before they can pass to the definitive host in which they re- 

 produce. An example of indirect life history is that of the liver fluke of sheep; 

 the intermediate host is a snail in which the parasite is immature; the definitive 

 hosts are sheep in which the flukes reproduce. 



Effects of Parasitic Life on Parasites. Parasitic animals have to contend with 

 many difficulties and risks. Such gamblers stake their lives on finding their 

 hosts and maintaining themselves upon or within them. They accomplish this 

 by an enormous production of sex cells, by the development, in many species, 

 of male and female organs in the same individual, making fertilization of the 

 eggs more certain, and by parthenogenesis, the production of young from un- 

 fertilized eggs. It has been calculated that the beef tapeworm of man produces 

 between 50 and 150 millions of eggs a year. American hookworms probably 

 release about 6 to 20 thousand eggs per day. Numbers are also increased by 

 asexual reproduction. In certain parasitic wasps one egg divides so as to produce 

 several embryos. The single-celled malarial parasite produces many new in- 

 dividuals by division. It has been estimated that these parasites (Plasmodium 

 vivax) can produce about 40 thousand parasites to every cubic millimeter of 

 the host's blood. Eventually parasites kill their host and destroy their own 

 welfare by overpopulation, just as too many gasoline stations kill a business. 



Some Important Parasites of Man. Parasites occur in all the main groups 

 (phyla) of animals. Parasitic members of the phylum Chordata are extremely 

 rare, e.g., hagfishes and a few blood-sucking bats. Of the invertebrates, the 



