THE PHARYNX AND ALIMENTARY CANAL OF THE 
HOOKWORM LARVA—NEGATOR AMERICANUS 1 
By N. A. Cobb 
Agricultural Technologist in Charge , Bureau of Plant Industry , United States Department 
of Agriculture 
The following facts, based on observations recently made by the writer 
(PI. i), may be given as reasons for regarding the pharynx of the full- 
grown larva of Necator americanus as somewhat in the nature of a pro- 
trusile onchium: 
(1) The apex of the pharynx {on) in different specimens, fixed and 
living, varies in position from considerably behind the amphids to a 
slight protrusion beyond the lips. 
(2) The form of the pharynx reminds one of the protrusile onchium 
of Mermis. 
(3) The pharyngeal wall is of considerable thickness, such as would 
impart to it the rigidity necessary for puncturing, while the lumen is 
very narrow, as in many Tylonchs, “spear-bearing” nemas characterized 
by the possession of an onchium presumably evolved through conversion 
of a thin-walled cylindroid or prismoid pharynx into a relatively thick 
walled, tubular, protrusile spear. 
(4) Round the front portion of the pharynx is a refractive ringlike 
element {dir on) similar to that found in nemas armed with a protrusile 
spear. This encircling element in such cases serves as a guide for the 
spear when in action. The location, form, and size of this element in 
Necator larvae harmonizes well with the supposition that the associated 
“onchium” is protrusile; its position accords with the limits of longitud¬ 
inal motion apparently justly attributable to the “onchium” on the basis 
of observations made on a considerable number of specimens. 
(5) The front part of the pharynx is surrounded (?) by tissue readily 
explainable as contractile, ( msc ?) similar to that seen in nemas having 
a protrusile onchium. Such muscles are usually attached to the posterior 
part of the onchium and to the labial cutin. 
(6) The oesophagus of the Necator larva contains “salivary” glands 
emptying precisely as in a large number of well known nemas possessing 
a protrusile onchium, namely, three unicellular oesophageal glands having 
their nuclei located in the posterior oesophageal swelling and emptying 
forward through three separate ducts, two emptying into the lumen of 
the oesophagus near its middle, and the third extending farther forward in 
the dorsal sector of the oesophagus and emptying at the base of the 
onchium (g/ sal dsl). 
(7) Considered in the light of the known entrance of hookworm larvae 
into the human host through the skin, these pharyngeal and oesophageal 
elements fall into a harmonious group in accord with the proposal that 
the pharynx when acting as a mechanical puncturing organ is assisted by 
the oesophageal fluid, acting as a solvent, in forcing a passage through 
the skin. 
1 Accepted for publication May 26, 1923. 
journal of Agricultural Research, 
Washington, D. C. 
age 
Vol. XXV, No. 8 
Aug. 25, 1923 
Key No. G-326 
54486—23-3 
(359) 
