360 
Journal of Agricultural Research 
Vol. XXV, No. 8 
A prominent symptom connected with the penetration of hookworm 
larvae into the human skin is an itching sensation lasting several days. 
It seems quite as likely that this irritation is caused by a secretion of 
the larva as by its motions. Reflecting that the “bite” of many insects 
often, at the time, is accompanied by very little sensation of any kind, 
while the wound after the withdrawal of the insect’s mouth parts becomes 
inflamed, it is an obvious inference that some substance injected into the 
wound causes the inflammation rather than the puncture itself. This 
line of observation and reasoning is in harmony with the supposition 
that Necator americanus works its way through the human cuticle partly 
by the aid of a solvent, which, incidentally, may account for the accom¬ 
panying symptoms. 
The fact that this onchium of Necator larvae has never been seen in 
action, that is, has never been seen to move in living larvae under the 
microscope, throws little, if any, doubt on its being protrusile, for it is 
well established that the examination even of thousands of specimens of 
nemas of various kinds possessing a protrusile spear may not enable one 
to make such a direct observation. It is only on rare occasions that the 
protrusile onchium of a nema has been seen in action, the commonest 
occasion being its use by the larva in escaping from the egg. Here the 
larva, even under the blaze and other inhibiting conditions of the micro¬ 
scope, sometimes remains in condition to proceed with its operations. 
However, many thousands of observations made by numerous competent 
observers, using hundreds of different species of nemas possessing a pro¬ 
trusile onchium, has resulted in accumulating circumstantial evidence so 
strong as practically to prove, in this way alone, even unsupported by 
other evidence, that the ondiium is protrusile. This circumstantial evi¬ 
dence, as in the case of Necator , relates very largely to differences in the 
observed position of the onchium, but also relates to its form. On such 
evidence alone belief that the onchium is protrusile is practically un¬ 
avoidable, since no other explanation of its form or function can reason¬ 
ably be offered. 
These various considerations appear to me to make it at the very least 
a reasonable working hypothesis that the pharynx of Necator americanus , 
at the time it reaches the condition of the “full grown larva,” is modified 
into what may fairly be regarded as a protrusile organ. 
Interesting morphological speculations with regard to nema anatomy 
are associated with such an hypothesis. The young larva of Necator 
americanus is said to be rhabitdiform. Now there are certain nema 
genera in which the rhabditoid form of pharynx, when the species are as¬ 
sembled and suitably arranged, may be “ traced ” step by step to a pharynx 
similar to that of Tylenchus , a well known genus containing both free- 
living and parasitic nemas and possessing a protrusile onchium used for 
piercing, and this fact has led to the promulgation by Marcinowski of a 
theory that the tylenchoid pharynx may have been evolved from the 
rhabditoid pharynx. 
THE AMPHIDS OF NECATOR 
The writer's observations on the amphids on Necator americanus 
(amph) are not subject to the qualifications necessary in the case of 
the pharynx. That these organs on the front of the head of Necator 
fall into the group of organs known as amphids seems to me beyond 
question. Their number, form, exact symmetry, position, and structure 
are those typical of well known amphids. The nature of the amphidial 
