NO. 2131. NEMATODE PARASIITE8 OF RODENTS— HALL. 5 



sists of a nerve ring surrounding the esophagus and of the nerve 

 cords directly or indirectly connecting Avith this ring. The sexes are 

 usually separate, though there are a number of genera, especially in 

 free-living forms, which are hermaphroditic, or what Cobb terms 

 syngonic, a condition which perhaps grades into parthenogenesis in 

 some forms. In syngonism the same gonad produces sperm cells 

 and, later, eggs. The genital glands of both sexes are tubular struc- 

 tures lying in the body cavity. The males are usually smaller than the 

 females and are usually equipped with chitinous copulatory organs 

 known as spicules. Frequently the males have a membranous struc- 

 ture at the posterior extremity of the body known as a bursa. This 

 structure reaches its highest development in the strongyliform nem- 

 atodes. The vulva may be located anywhere on the ventral sur- 

 face from near the mouth to near the anus. The ovary and uterus 

 form a continuous structure. The ovary has the double function of 

 an ovary {s. str.) and a vitellarium. There is no vitellarium distinct 

 from the ovary. Usually there are two ovaries and uteri, but there 

 may be more than two or only one. The worms may be oviparous 

 (in which case the egg may or may not contain a developed embryo 

 when oviposited) , ovoviviparous, or viviparous. The word " ovovivip- 

 arous" is commonly used in two senses, the correct use referring 

 to an egg in which the embryo develops and from which it later 

 escapes while still in the uterus. The condition in which an figg con- 

 taining a developed embryo is oviposited is here referred to as ovip- 

 arous, with a specification as to the presence or absence of a de- 

 veloped embryo. The eggs are simple, not compound, and are usually 

 ovoid or elliptical in outline. 



ANALYTICAL KEY TO SXIPEKFAMILTES OF NEMATODA.^ 



1. No parasitic males. The para.sitic females very small, not over G mm. long, 



and with not over 30 eggs in uterus; eggs ellipsoidal and with very thin 



shells Angiostomoidea, p. 6. 



Parasitic males and females, the latter commonly with more than 30 eggs In 

 uterus 2. 



2. Forms with the esophagus consisting of a chitinous tube which is embedded 



along the greater part of its length in a single row of cells. Male with 

 a single spicule or without spicules. Female with a single uterus and 

 ovary. Eggs, in oviparous forms, lemon-shaped, with rather thick shells, 

 with an opening at each end closed by an opercular plug. 



Trichinelloidea, p. 9. 

 Forms with esophagus composed of several cells arranged about a lumen. 

 Male with 1 or 2 spicules. Female with 1 or more ovaries. Eggs with- 

 out opercular plug, or rarely with an inconspicuous operculum 3. 



3. Males provided with a well -developed membranous bursa supported by a 



system of rays typically consisting of G paired rays and 1 median dorsal 



1 In this paper the term anterior means toward the head, and posterior means toward 

 the tail. 



