Morph. 0:1 



the median bulb (Plato III, J'lg. 9). 



Esophagi may be quite diversified due to fe'Jdinf; habit. Certain thin.f^s, 

 however, arc constant: (1) triradiate lumen; (2) cuticular linint^, and 

 (3) slands. 



The esophagus may be connected with the intestine through a short struc- 

 ture termed the esophago-intestinal valve^ which often projects some 

 distance into the Ivunen of the intestine. This functions as a valve 

 which irapedes the flowiiig back into the esophagus of food contained in 

 the intestine. Like the esophagus, it is lined with cuticle. 



The intestine is composed of a single layer of epithelial cells (Plate 

 II, Fig. 6, a) . It is usually a straight tube in contrast to the 

 reproductive organs which may be reflexed or coiled. 



Posteriorly, the intestine leads into the rect um, which is lined by an 

 invagination of the l^ody cuticle, and opens at the anus. The rectal 

 glands, generally three in number, open into the rectum, one dorsal 

 and two subventral. In the male, the spenri duct enters the ventral 

 wall of the rectum. The rectum is thus in whole, or in part, a cloaca 

 in male nematodes. In both sexes, the rectum empties posterdad on the 

 ventral surface of the body. 



The itenroductive System 



Nematodes, as a rule, are dioecious, existing as separate males and 

 females. Males are readily distinguished externally from females by 

 the presence of copulatory spicules (Plate I, Fig. 2, W). Other dis- 

 tinguishing features which may or may not be present are smaller size, 

 curvature of the posterior end, and presence of bursae, genital papillae, 

 arid other accessory copulatory structures. In marine and most parasitic 

 nematodes, the sexes occur in about equal proportions, but this is not 

 the case with terrestrial and fresh-water forms. In the terrestrial and 

 fresh-water forms, the female predominates. The scarcity or absence of 

 males indicates a tendency toward hermaphroditism or parthenogensis in 

 nematodes from these types of habitats. The hermaphroditism is usually 

 of the protandric type; the gonad first produces sperm that are stored 

 and later fertilize the eggs subsequently develooed by the same gonad. 

 The occurrence of parthenogenesis has been proved for several terres- 

 trial nematode , e.g. Mermis subnigrescens (Christie, 1929), and in the 

 root-knot nematode Heloidogyne sp. (Tyler, 1933), although males are 

 knox-m in both groups. 



Intersexes are known in some nematode genera ( Mermithid , Meloidogyne , 

 Ditylonchus ) . An intersex is an individual which exhibits a blending 

 of male and female characters. In mo^t cases, the intersexes are 

 females which show secondary male characters. They may copulate vjith 

 ifiales ami lay viable eggs. The cause of intersex fonnation is not Quite 

 understood. 



