NO. 2131. NEMATODE PARASITES OP RODENTS— HALL. 3 



of the host than that of the parasite. It commonly happens that the 

 host is a known animal and that it is easy to look up the parasites of 

 the host group if they have been brought together in some paper 

 such as this. On the other hand, the parasite is the unlaiown thing, 

 and without literature based on the host group it must be studied 

 apart from its literature before any writings can be consulted. 



The reason for working on rodents and allied forms as a host group 

 instead of on some other group is partly casual. The w^riter had the 

 good fortune to have considerable material turned over to him by 

 Mr. E. It. Warren, of Colorado Springs, Colorado, including some 

 nematode material from rodents, and its possession- was a deciding 

 factor. The collection of the Bureau of Animal Industry is also 

 rich in nematodes from rodents as a result of the work of Dr. Albert 

 Hassall, an indefatigable collector, and of the policies of the present 

 and former chiefs of the zoological division. Doctor Ransom and 

 Doctor Stiles. Incidentally the increasing recognition of the im- 

 portance of rodents, as reservoirs of disease and as destroyers of 

 property of various sorts, lends additional value to a study of their 

 parasites. 



The inclusion of the Tlyracoidea is quite casual. It is a small, 

 rather isolated group that might be ignored in general w^orks on the 

 larger groups unless included, as here, without any particular reason. 



Much of the material collected in this paper is in unsatisfactory 

 shape. The older diagnoses are frequently worthless, and most 

 modern authors have found it easier to describe a new Strong ylus or 

 Ascaris than to examine the literature for genera, old or new, that 

 would fit the case and give readers some inkling as to what place the 

 new parasite occupied, llecently the writer counted the species listed 

 in the index catalogue of the Zoological Division of the United 

 States Bureau of Animal Industry under the genus Ascaris. In 

 round numbers there were a thousand species, many, of course, 

 synonyms or errors. Under such conditions it is obvious that a new 

 Ascaris will probably prove to be an ascarid in the most extended 

 meaning of that general term, but it is unlikely that it will be con- 

 generic with Ascaris lumhricoides. The same general rule holds for 

 such a genus as Stron,gylus. In dealing with descriptions of this 

 sort the writer has often been able to determine that whatever it w^as 

 that an author had before him when he described his new Ascaris or 

 Strong ijlus it was certainly not Ascaris or Strong ylus. Unfortu- 

 nately, such descriptions are almost ahvays inadequate to show any- 

 thing more than this. They have been worked into this paper as 

 w^ell as possible. The writer has taken the position that it is not in- 

 cumbent on him or other workers to send to Europe or around the 

 world for specimens of species where he is unable to obtain the essen- 

 tial or significant facts from the author's description. Such a pro- 



