28 



ROOT-KNOT AND ITS CONTROL. 



if 



9 -..^'f^^^^'^xm I 



thickness. The average length is 420 to 475 /j.. The structure of the 

 larva is comparatively simple, consisting essentially of a tube (the 

 aUmentar}^ canal) wdthin a tube (the body wall), the space between 

 (the body cavity) being filled with a liquid and minor structures 

 (fig. 1). The body cavity has no opening to the exterior. The ali- 

 mentary canal opens anteriorly at the end of the body, but posteri- 

 orly it opens in the median ventral line about one-eighth of the dis- 

 tance forward from the tip of the tail 

 (i. e., 50 to 65 ,«). The body wall con- 

 sists of an external cuticle and a dermal 

 layer of cells beneath which are the 

 four "fields" of obliquely longitudinal 

 muscle cells. Longitudinal tissue 

 masses springing inward from the der- 

 mal layer at the median dorsal, ventral, 

 and lateral lines separate the muscles 

 into the four "muscle fields" men- 

 tioned. Only occasionally the opening 

 of the excretory canal can be made out 

 in the larva, but it is quite distinct in 

 the mature male. It is in the ventral 

 median line, opposite or sHghtl}^ i:fos- 

 terior to the esophageal bulb. These 

 details of structure are clearly shown in 

 the accompanying text figures (figs. 1, 

 2, and 3), contributed by Dr. N. A. 

 Cobb. 



The alimentary canal consists first of 

 a buccal spear (PI. I, fig. 4) 10 to 15 ,« 

 long (usually about 12 jj.), a chitinous 

 organ, pointed at the anterior end and 

 wdth three small knobs at tlie posterior 

 extremity and pierced its whole length 

 by a fine canal. Connected with tho 

 basal knobs are retractile and exsertile 

 muscles. This spear is used by the nem- 

 atode in boring its way out of the egg 

 and through plant tissues, and through it the nourishment is apparently 

 drawn, for its canal is continuous with the lumen of the remainder of 

 the aUmentary canal. This spear hes in a cavity, the buccal cavity, 

 from wliich it may be exserted. At the base of the spear begins the 

 slender esophagus, 40 to 50 /i long, which expands then into the thick, 

 muscular-walled esophageal bulb (figs. 2 and 3). This is a stout, 

 muscular body, often nearly spherical, but more often a little longer 



217 



X250 



Fig. l.—Heterodera radicicola. Half-grown 

 female ( ?) Individual shortly before the 

 final molt: a, Anterior end; 6, speai; c, 

 esophagus; d, esophageal bulb; e, nerve 

 ring; /, excretory pore; g, gland; h, thick 

 wall of alimentary canal; i, body wall; 

 i, begiiming of reproductive organs; fc, 

 anus. Magnified 250 diameters. Drawn 

 by W. E. Chambers. 



