[320] From the number of eggs present in a given quantity of fæces, the number of female Ancylostomes present in the gut can be reckoned by a formula of Leichtenstern’s (x = a/47, in which a signifies the number of eggs counted in a single gramme of fæces).
[321] [In most Arthropoda the skin is hardened by a deposit of chitin (Hexapoda, etc.).—F. V. T.]
[322] Parasitic or free-living Crustaceans may now and then invade man abnormally. Thus, according to Betten, Caligus curtus invade the cornea (Betten, R. A., “Par. Crust. as a Foreign Body on the Cornea,” Lancet, 1900, i, p. 1002; and Centralbl. f. Bakt. u. Par., xxix, p. 506). According to Laboulbène, also Gammarus pulex (Laboulbène, A., “Obs. d’accid. caus. par le G. pul. apport. avec l’eau de boison dans l’estomac d’un homme,” Bull. Acad. méd., 1898, p. 21).
[323] R. Blanchard has compiled thirty-five cases in which Myriapoda have been observed in the intestine as well as in the nose of human beings (“Sur le pseudopar. d. myriap. chez l’homme,” Arch. de Par., 1898, i, p. 452). E. Munoz Ramos reports an additional case (ibid., p. 491). A few years ago a doctor in East Prussia sent me a rain worm out of a lady’s nose (cf. Hanau, A., “Wahrsch. Pseudo-paras. v. Schmeissfliegenlarv. u. angebl. Paras. v. Regenwürmern b. einer Hysterischen,” Arch. de Par., 1899, ii, p. 23).
[324] [This is only so in the Acarina or mites, not in the Araneida or spiders.—F. V. T.]
[325] [The true character of the Arachnoidea is the presence of four pairs of ambulatory appendages. This number is reduced to two pairs in the gall-making Phytoptidæ, and they differ from all other Arthropoda in having no antennæ.—F. V. T.]
[326] Twelve orders are now recognized, as follows: Pentastomida or Linguatulids; Tardigrada or bear-animalcules; Phalangidæ or harvest-men; Acarina or ticks and mites; Palpigradi; Solifugæ; Pseudoscorpionidea or book mites; Pedipalpi or false scorpions; Scorpionidea or true scorpions; Araneida or spiders; Xiphosura or king crabs; and Pycnogonida, marine Arachnoids.
[327] Chelifer cancroides has also been observed as a pseudoparasite in man (Arnault de Very, S., “Pseudopar. du. Chel. cancr. chez l’homme,” Compt. rend. Soc. de Biol., 1901, liii, p. 105).
[328] [The cheliceræ are sometimes regarded as modified antennæ, but it is more natural to regard them as the morphological equivalent of the mandibles of Hexapoda.—F. V. T.]
[329] [The pedipalpi, or second pair of jaws, consist of a stout basal segment and a palp, which may have the appearance of a leg in Arachnida; this may end with or without a claw, or with a chela (scorpions); they may also form a tube enclosing the styliform cheliceræ (mites).—F. V. T.]