4 I'ROCEEDIKGS OF TUE NATIONAL MUSEUM. vol. 50. 



cedure is proper, but it is a gratuitous task, not always feasible and 

 not always to be encouraged. It is understood, of course, that 

 generic concepts change, but this is all the more reason why modern 

 ■workers should abandon the use of the old extended generic concept. 



GENEllAI. REMARKS. 



The nematodes known from rodents include representatives of all 

 the large groups of parasitic nematodes and make up a considerable 

 total of species. A number of si)ecies parasitic in such animals as 

 the rat have a cosmopolitan distribution and have been recorded 

 from practically everywhere that they have been looked for. Others 

 are known only from obscure rodents in the heart of Africa or 

 America. Many of them are of no known importance, Avhile others, 

 such as Trichinella. spiralis^ have long been known as medically im- 

 portant parasites of man, and others, such as Gongylonemu neo- 

 plastlcuin.f are of considerable medical and scientific interest. 



The large groups of parasitic nematodes represented by rodent 

 species were formerly regarded as families, but the recent tendency 

 has been to bring together the genera Avhich are closely related to 

 form new families, and I have treated the old family groups as of 

 superfamily rank. 



I have not undertaken to give here any notes on the technique for 

 the collection or study of nematodes. That topic has been well 

 covered by other writers, one of the most recent studies being that of 

 Kansom (1911a). 



TAXONOMY AND MORPHOLOGY. 



Phylum NEMATHELMINTHES. 



Phylum diagnosis. — Elongated, cylindi-ical, unsegmented worms. 

 This phylum is a little difficult to characterize. It is commonly held 

 to include three classes — the Nematoda, the Acanthocephala, and the 

 Chaetognatha — but this grouping is based on convenience rather than 

 on demonstrable affinities. The Nematoda may be looked on as the 

 typical representatives of the phylum, as they constitute by far the 

 most important and largest group. 



Class NEMATODA. 



Class diagnosis. — Nemathelminthes (p. 4) : Body limited by a 

 chitinous cuticle which may be either plain, striate, or ornamented 

 with markings which may be simple or elaborate. A simple com- 

 plete digestive system is present in typical forms, consisting of a 

 terminal mouth at the anterior end of the body, followed by an 

 esophagus, and this in turn by an intestine which terminates in an 

 anus in the posterior portion of the body. The nervous system con- 



