Acanthocephala 75 



Even in species for which intermediate hosts .have been discovered it 

 seems fairly clearly demonstrated that infestation of the definitive 

 host may occur directly through the primary host. Adaptation to 

 an intermediate host seems in these forms to be more strictly a 

 facultative adaption to prevent extermination of the individual para- 

 site. If a primary host carried very young larvae these insuffi- 

 ciently developed worms would not be able to establish themselves 

 as intestinal parasites even if the primary host were eaten by a 

 vertebrate which is suitable as a definitive host. Such insufficiently 

 developed larvae, freed in the digestive tract of the vertebrate, must 

 either pass out of the body because of their inability to maintain 

 their position or must penetrate the wall of the digestive tract of the 

 new host. The early appearance of the proboscis hooks would 

 make this latter possibility entirely plausible. Once outside the lumen 

 of the digestive tract the larvae find conditions where they might 

 become encysted and continue the larval development temporarily 

 interrupted by the destruction of the sheltering primary host. It 

 accordingly happens occasionally that a fish serves in the capacities 

 of both definitive and intermediate hosts to the same species of 

 acanthocephalan. 



In some instances invertebrates have been reported as intermediate 

 hosts of some species of Acanthocephala. If a primary host bearing 

 larval Acanthocephala were eaten by an invertebrate, the larvae of 

 any stage which escaped destruction might be able to continue their 

 interrupted course of development in their new environment, having 

 acquired thereby a new, though non-essential, link in their develop- 

 mental cycle. 



FACTORS IN DISTRIBUTION 



Numerous factors influence the geographical distribution of para- 

 sitic organisms having an alternation of hosts. I have previously 

 called attention ('19, p. 229) to the absurdity of assuming that the 

 distribution of such parasites is coextensive with the range of their 

 hosts. Individuals of a given host species may be heavily infested 

 with a given parasite in one part of their range, and entirely free 

 from the same species in other localities. Correlated with this is 

 the fact that a given endoparasitic species may be the dominant 

 parasite of entirely different host species in different regions within 

 its range. Factors controlling these changes are but little under- 

 stood. Dissimilarity in the food habits of the hosts in various parts 

 of their range might result in the hosts of one locality being heavily 

 infested with parasites while those of another locality would be 

 relatively, if not entirely, free from the same parasites. 



It is known that the distribution of some parasitic worms is 

 broader than that of any single species that serves it as host during 

 its larval period. Thus it is possible that in part of its range a 

 species of endoparasite and one of its normal final hosts might occur 

 side by side in the same locality without having their usual intimate 

 interrelationship. Such a condition would be possible if the parasite 



