74 Roosevelt Wild Life Bulletin 



parasites and their hosts, it becomes a matter of considerable inter- 

 est to compare the lists of species of parasites found in similar 

 habitats. Only after this has been done carefully for a considerable 

 number of regions will it be possible to offer any very reliable data 

 concerning the distribution of individual species. 



HABITS OF ACANTHOCEPHALA 



The Acanthocephala constitute a group of parasitic worms the 

 individuals of which have become so thoroughly adapted to the para- 

 sitic life that they spend their entire existence within the bodies of 

 other animals. They render no known service to the organisms 

 which they parasitize, but on the contrary inflict damage by appro- 

 priating food materials from the host for their growth and develop- 

 ment, and by doing violence to the tissues of the host through 

 the action of the spine-covered proboscis or hold-fast organ. Beyond 

 the simple observation that animals heavily infested with these 

 parasites are usually much thinner than those free from infestation, 

 nothing has been done to determine the actual effect of Acanthoce- 

 phala upon their hosts. As has been pointed out in an earlier paper 

 (Van Cleave, '19, p. 227), it is probable that because of lacerations 

 and punctures of the wall of the intestine, due to the action of the 

 proboscis, these parasites facilitate the entrance of disease-producing 

 organisms into the body of a host. 



THE LIFE CYCLE 



Relatively little is known concerning the development of North 

 American species of Acanthocephala, but they belong for the most 

 part to the genera that include other species for which the life cycle 

 has been determined wholly or in part. In every species that has 

 been studied two or more hosts have been found essential for the 

 completion of the life cycle. Of these the animal that shelters the 

 sexually mature parasite is called the definitive host, while the first 

 organism that harbors the parasite during its larval existence is 

 termed the primary host. The definitive host of an acanthocephalan 

 is invariably a vertebrate, while the primary host is usually a crusta- 

 cean, though insects and insect larvae frequently serve some species 

 in that capacity. Intermediate hosts, though apparently not essen- 

 tial to the development of these worms, are interposed between the 

 primary and definitive hosts in some species. So far as known the 

 acanthocephalans enter the final or definitive host only through the 

 introduction of organisms sheltering the larval worms into the diges- 

 tive tract of the definitive host. Since these parasites are thus 

 acquired in feeding, their study becomes a matter of considerable 

 importance if we are to understand the full significance of the rela- 

 tionship between hosts of economic importance and their food supply. 



The significance of intermediate hosts in the life cycle of the 

 Acanthocephala is apparently not great, for, at least in some instances, 

 they do not seem to be essential links in the chain of development. 



