PAPERS ON BIOLOGY AND AGRICULTURE 139 



ISOLATION 



Among non-migratory animals, spread of parasitic in- 

 festations may be prevented by a natural quarantine re- 

 sulting from isolation. Lack of communication between 

 groups of individuals in different localities may be a fac- 

 tor helping to explain why individuals in one locality may 

 be heavily and generally infested by certain parasites 

 while those in another region are exempt. 



CONCLUSION 



In view of the numerous limiting factors, only part of 

 which have been touched upon here, it becomes evident 

 that there are many obstacles to the attainment of cos- 

 mopolitan distribution by any parasite involving two or 

 more essential hosts. These limiting factors on the one 

 side determine that a parasite may not extend its range 

 to the full range of its normal host and on the other hand 

 permit some forms to exceed the range of any one host 

 species through utilization of several host species. Man, 

 who has extended his distribution over much of the sur- 

 face of the globe, is subject to attack by numerous 

 different species of endoparasitic animals, but many of 

 these are limited to rather sharply defined areas, while 

 in other instances he shares them with the animals of 

 the native fauna. 



Among the Acanthocephala or thorny-headed worms, 

 there is a genus, Moniliformis, which presents unique 

 conditions favoring cosmopolitan distribution. Mem- 

 bers of this genus live as adults chiefly in rodents. The 

 rat is one of the commonest hosts. Embryos, covered by 

 highly resistant shells, are deposited by the worms but 

 never leave the confining- membranes until they are 

 swallowed by a suitable host. Insects, of which the cock- 

 roach seems to be one of the most important, serve the 

 larvae as hosts. Thus, with rats as common adult hosts 

 and cockroaches as larval hosts, these parasites have 

 both of the essential stages in their development provided 

 for by practically cosmopolitan hosts, and have been dis- 

 tributed to practically every quarter of the globe. 



