25 
swollen portion, foll owed by a thjn middle portion, the latter bnin^^ 
fol lowed by more or less globular esophageal bulb whicli ])osses^(‘s a 
triradiate chitinous armature. This kind of esophagus i^common to 
the early stage of al l nieml)ers of t he family Strongylida* and also to 
numerous other free-liying or parasitic* nematodes. It is eyidently a 
worm with an esophagus of this sort 
which was recently found in the earth 
taken from the New York tunnel ex- 
cayations, and upon which was based 
the report that uncinariasis was 
present. 
The embryo of the hookworm liyes 
in water or moist ground. In its 
eyolution the worm casts its skin 
f our time s, thus passing through 
Iiye_stag(‘s. and changes its structure 
so as to assume more and more the 
characters of the adult. During 
these changes the sexes become dif- 
ferentiated. Some of these changes 
occur in water or moist ground, and 
the rest after infection takes place. 
DKCELOPMENT OUTSIDE Til?: HODY. 
Stijmentathnu — The eggs deyelop 
best in the unaltered fecal mattm-. 
esjx'cially when this is well formed: 
not so well when it is more fluid in 
character. The additu^n of water 
retards the deyelopment. and if con- 
siderable water is added the eggs 
perish. Air is necessary to deyelo})- 
ment, and the eggs nearer the sur- 
face of the feces segment more rap- 
idly than those in the center. At a 
temperature of about 27^ C. the em- 
l)ryo may form and esc-apefrom the 
shell in twenty-four hours. Lower 
temperatures retard deyelopment, so 
that at 21^ or 22^ C. the eml)ryo may not escape for from thirty-six 
to forty hours; 1° C. kills the eggs in twenty-four to foi‘ty-(*ight 
hours, so that freezing weather may be look(*d upon as disinfecting 
areas exposed to the cold. 
Kinhry<i . — Upon escaping^ from the slndl, the embryt> (tig. 27) mejis- 
Figs. 30-31. — Two larva- of tin* oM WorM 
hookworm at tin* vml of llu* wooihI stai;o 
(“cneyste<l larva* rfprvs«.*nlin>s' tiu* 
yomiK worms n-tnu-tiMl from thvir skin. 
(.\ftcr IVrroiicilo, ls*v2, p. l js a-h. ■ 
