484 



FRESH-WATER BIOLOGY 



Representative species. 



8 (3) Spear without a bulbous base 9 



9 (10) Esophagus with a median bulb; males with bursa. 



Dolichodorus Cobb. 



This genus is distinguished from Tylenchus by the peculiar lobed bursa without ribs, by the 



relatively long and slender oral spear and peculiar lip region, and by the presence of a double 



sexual organ in the female. There are few Tylenchi the females of which possess two ovaries. 



. . Dolichodorus heterocephalus Cobb 19 14. 



The transverse striie are resolvable with high 

 powers under favorable conditions into rows of 

 exxeedingly minute, somewhat irregular elements. 

 The flaps of the bursa are striated in much the same 

 manner as the cuticula, and the margins of the 

 flaps are distinctly thickened. The sper- 

 matozoa are small and numerous and it ap- 

 pears that the reduction divisions take place 

 in a short segment of the testis not far from 

 the blind end. The organs obscurely figured 

 in connection with the head appear to be the 

 outlets of glands located in the neck. 



The "cardiac swelling" h appears to have 

 the same structure as in some species of Ty- 

 lenchus, in which it is known to be caused by 

 the presence of glands exterior to the esoph- 

 agus, and therefore not properly to be re- 

 garded as a cardiac swelling of the ordinary 

 kind. In the Tylenchi mentioned, these 

 glands empty through a minute duct which 

 enters the esophagus, passes through the 

 median bulb on the dorsal side of its valvular 

 apparatus, and, continuing, empties into the 

 pharynx at the base of the spear. These so- 

 called salivary glands are designated at g in 

 I, under Tylenchus dipsaci (Fig. 776). Similar 

 structures may occur in the present species. 



Inequality of the ovaries is characteristic 

 of a vast number of species of nematodes 

 and may have a deep morphological signifi- 

 cance. It is nearly always the posterior 

 ovary which is the smaller. Every degree 

 of inequality exists even to the extinction 

 of one ovary. The smaller branch may pro- 

 duce smaller and what appear to be inferior 

 eggs, and may even cease to function as a 

 reproductive organ and function merely as a 

 minor part of the other branch, serving, for 

 instance, either as an extension of the uterus, 

 or as a seminal receptacle. 



Habitat: Douglas Lake, Michigan; Silver 

 Spring, Florida. 



Fig. 778. Dolichodorus heterocephalus. 

 I, nearly side view of a female; II, lateral view 

 of surface of head, more highly enlarged; III, 

 sagittal section of head; IV, dorso-ventral view 

 of head; V, front view of head; VI, side view, 

 posterior extremity of male; VII, ventral view of 

 posterior rxtremity of female; VIII, ventral view 

 of posterior extremity of male. 



a, papilla; b, cephalic organ of unknown signiQ- 

 cance; c, spear; d, base of spear; e, median bulb; 

 /, nerve-ring; g, excretory pore; //, cardiac swell- 

 ing; i, intestine; j, anus; k, lateral caudal pores; 

 /, terminus; m, blind end of posterior ovary; 

 n, ovary; o, left spiculum; p, accessory piece; 

 q, distal end of accessor^' piece; r, left flap of 

 bursa; s, terminus oE male; t, ovum; u, sperma- 

 tozoa; ;•, vaginal muscles; w, uterus; x, vulva; 

 i', anus. (After Cobb.) 



10(9) Esophagus with only an elongated posterior swelling; no bursa. . 11 



