4 BULLETIN 140, UNITED STATES NATIONAL MUSEUM 



but often two testes in free-living forms. Males frequently pro- 

 vided with a membranous structure at the posterior extremity of 

 the body known as a bursa, this structure reaching its highest de- 

 velopment in the strongyliform nematodes. Spermatozoa of vari- 

 able shape, being spherical, conical, elongated, or discoid, without 

 so-called tails but capable of ameboid motion. Vulva may be 

 located anywhere on or near (Arduenna) the ventral surface from 

 near the mouth to near the anus. Ovary and uterus forming a con- 

 tinuous structure. Ovary with double function of an ovary (s. str.) 

 and a vitellarium, there being no vitellarium distinct from the ovary. 

 Usually two ovaries and uteri, but there may be more than 2 (up to 

 10 or 11 (Turgida) or 15 (Physaloptera) or even more) or only one. 

 Worms oviparous (in which case the egg may or may not contain a 

 developed embryo when oviposited), ovoAiviparous, or viviparous. 

 The word " ovoviviparous " is commonly used in two senses, but 

 the correct use refers to an egg in which the embryo develops- 

 and from which it later escapes while still in the uterus; the con- 

 dition in which an egg containing a developed embryo is oviposited 

 is correctly referred to as oviparous, with a specification as to the 

 presence or absence of a developed embryo. Eggs simple, not com- 

 pound, usually ovoid or elliptical in outline. 



KEY TO ORDERS OF NEMATODA 



Forms with muscular esophagus of tripartite cross-section__ Myosyringata, p. 4- 



Forms with tubular capillary esophagus, the tube embedded iu or otherwise in 



relation to a single row of cells, the cells usually extending almost, but not 



quite, to the head end of the body and the tube continuing anteriorly to the- 



mouth Trichosyringata (not dealt with in this paper). 



The validity of the two orders proposed some years ago by Ward 

 has been generally concurred in by subsequent writers, such as 

 Rauther, Magath, etc. So far as the present writer is concerned it 

 is perhaps sufficient to state that the distinction between the two 

 forms of esophagus noted above seems decidedly important. 



Order MYOSYRINGATA Ward, 1917 



Order diagnosis. — Nematoda (p. 3) : Nematodes with a muscular 

 esophagus of tripartite cross-section, as opposed to the capillary 

 tubular esophagus of the Trichosyringata (whipworms and allied 

 forms). 



KEY TO St BORDERS OF MYOSYRINGATA 



1. Heterogamic forms, the parasitic generation consisting of females with no- 

 males, and the free-living generation consisting of males and females. 



Rhabdiasata (not dealt with in this paper) ^ 



Homogamic forms, the parasitic generation consisting of males and 



females 2. 



