Cobb[174] has recently devised an ingenious formula in which measurements of different parts of the body appear as percentages of the whole length of the body. The nature of this will be understood by reference to Fig. 68. Such a formula should, however, be used with caution, since it rests on the assumption that the proportions of the various parts of the body are constant in different individuals, and it is by no means certain that this is the case. [Illustration: FIG. 68.—Diagram to explain the descriptive formula used for Nematodes. (From Cobb.) 6, 7, 8, 10, 6 are the transverse measurements, while 7, 14, 28, 50, 88 are the corresponding longitudinal measurements. The formula in this case is 7 14 28 50 88 ------------- 6 7 8 10 6 The unit of measurement is the one-hundredth part of the length of the worm. The measurements are therefore percentages of the length. The measurements are taken with the animal viewed in profile; the first is taken at the base of the oesophagus, the second at the nerve-ring, the third at the cardiac constriction, the fourth at the vulva in females and at the middle in males, the fifth at the anus.] Taking everything into consideration, it has seemed advisable in the following systematic account of the Nematoda to abandon the larger groups, and to deal directly with the families. Claus distinguishes seven of these, and the diagnoses given at the head of each are mainly taken from his _Grundzüge der Zoologie_.[175] I. FAMILY ASCARIDAE. Body rather stout. A dorsal and two ventro-lateral lips, bearing papillae. Buccal cavity distinct, seldom provided with chitinous armature. The oesophagus often has two dilatations. {139}The tail of the male is ventrally curved, and usually there are two horny spicules. The Ascaridae are found in the intestines of their respective hosts. Genera: _Ascaris_, _Heterakis_, _Oxyuris_, _Nematoxys_, _Oxysoma_, and many others. Von Linstow[176] enumerates over 250 species of _Ascaris_, of which it will only be possible to mention here one or two. They are all parasitic in Vertebrata.