The male hookworms are a little more than ⅓ of an inch (9 mm.) long and the females a little more than ½ inch (13 mm.) in length. The males can readily be distinguished by their posterior, umbrella-like expansion or copulatory bursa. The tail of the female is pointed. The vulva of A. duodenale is located in lower half of the ventral surface; that of N. americanus in upper half. The large, oval mouth of the Old World hookworm has four claw-like teeth on the ventral side of the buccal cavity and two knob-like teeth on the dorsal aspect. It also has a pair of ventral lancets below the four ventral teeth. One cannot make out a dorso-median tooth. In N. americanus the buccal capsule is round, smaller and the ventral teeth are replaced by chitinous plates. Dorsally there are two similar but only slightly developed lips or plates. A very prominent, conical dorso-median tooth projects into the buccal cavity. Through it passes the duct of the dorsal oesophageal gland. There are also 4 buccal lancets. The copulatory bursa of the Necator americanus is also different, being terminally bipartite and deeply cleft in the division of the dorsal ray, rather than tripartite and shallow, as with A. duodenale.
The anterior extremity of Ancylostoma bends in the same direction as the general body curve while that of Necator hooks back in an opposite direction to the body curve.
By dropping the worms, while still alive, into hot 70% alcohol they readily assume the attitudes noted above.
In general, Ancylostoma is larger and thicker than Necator.
The name hookworm was given to these nematodes from the hook-like processes of the ribs of the rays of the copulatory bursa. Dubini called the Old World parasite Agchylostoma, properly Ancylostoma, on account of the 4 formidable hook- or claw-like ventral teeth of the buccal capsule. (αγχὶλος, hook, and στομα, mouth.)
A. ceylanicum is somewhat smaller than A. duodenale and in the copulatory bursa of the male we have a deeper cleft in the dorsal ray and 2 rather long tips to each branch instead of the shallow cleft and 3 stumpy processes of the 2 branches as in A. duodenale.
Life History.—The delicate-shelled eggs pass out in the faeces, and in one or two days a rhabditiform embryo (200 × 14 microns) is produced. The mouth cavity of the embryo is about as deep as the diameter of the embryo at the posterior end of the mouth cavity; that of Strongyloides is only about one-half as deep as the diameter.
Fig. 89.—1a, Copulatory bursa of Necator americanus, showing the deep cleft dividing the branches of the dorsal ray and the bipartite tips of the branches; also showing the fusion of the spicules to terminate in a single barb. Scale 1/10 mm. 1b, Branches of dorsal ray magnified. 2a, The buccal capsule of N. americanus. 2b, The same magnified. 3a, Copulatory bursa of Ancylostoma duodenale, showing shallow clefts between branches of the dorsal ray and the tridigitate termination. Spicules hair-like. 3b, The dorsal ray magnified. 4a, The buccal capsule of A. duodenale, showing the much larger mouth opening and the prominent hook-like ventral teeth. 4b, The same magnified. 5a, Egg of N. americanus. 5b, Egg of A. duodenale. 6a, Rhabditiform larva of Strongyloides as seen in fresh faeces. 6b, Rhabditiform larva of hookworm in faeces eight to twelve hours after passage of stool. From Stitt.