THb: AllT OF PAllASITISM 



229 



the parasite, the plant is weakened and Httle or no grain is produced. 

 A few of the wheat rusts do not require two hosts but complete their 

 life cycle on wheat alone. Such rusts pass the winter by means of 

 thick-walled spores which may remain in the stubble or in the ground 

 until the young wheat 

 plant appears the follow- 

 ing year, or the spores are 

 carried by the wind from 

 other regions. 



Parasites Requiring More 

 Than Two Hosts 



Tapeworms show a va- 

 riety of adaptations and 

 exhibit a unique and deli- 

 cate balance that permits 

 the completion of their 

 various cycles. Roughly 

 they may be divided into 

 two groups, one in which 

 the eggs reach water, sub- 

 sequently passing through 

 some aquatic organism, 

 and a second in which ova 

 are scattered in the soil 

 and reach the intermedi- 

 ate host by means of food 

 or drink. In the first 

 group are the broad tape- 

 worm of man, the bass 

 tapeworm, and many 

 others, while the second 

 includes the various tae- 

 nioid worms and their 

 relatives. All of these par- 

 asites show a remarkable 

 degree of specialization. 





N. Y. State Conservaiion Depi. 

 The life cycle of the bass tapeworm {P. arnblo- 

 plilis). (1) The mature tapeworm occurs in the 

 intestines of the large- and small-mouthed hass. 

 (2) Contact with water causes the proglottids 

 to liberate the eggs which are eaten, (.3) by 

 various copepods. \\ hen infected copepods are 

 eaten by many species of plankton-feeding fish 

 (1) a larval tapeworm (plerocercoid) develops in 

 the mesenteries, liver, spleen, or gonads of these 

 fish. Heavy infections in the small-mouthed 

 bass affect reproduction. The tapeworm reaches 

 maturity when fish infected with the larval stage 

 are eaten by larger ones. How could this cycle 

 be controlled in fish hatcheries!' 



The broad tapeworm of man, Diphyllobothrium latum, was brought 

 to this country sometime during the last century by immigrants from 

 the shores of the Baltic Sea. The worm matures in 



the digesti\e 



