CLASSIFICATION.—The classification of the Nematodes is a matter of very considerable difficulty; their structure is unusually monotonous, and, owing perhaps to their largely parasitic mode of life, they show practically none of those external features which are so useful to the systematist in other groups. Schneider in his Monograph divides the group into three subdivisions—(i.) {137}the POLYMYARII, in which numerous muscle cells are seen in a transverse section; (ii.) the MEROMYARII, in which only eight are seen, two in each quadrant; and (iii.) the HOLOMYARII, in which the muscles are either not divided, or only divided by longitudinal lines. This grouping has, however, to some extent broken down, since Bütschli[171] and others have shown that the third subdivision is founded on insufficient observation, whilst the first two include, in different subdivisions, Nematodes which are closely allied in all respects except as regards their muscle cells. The details of the life-history have been used by other writers as a basis of classification. Linstow[172] enumerates fourteen distinct modifications of the post-embryonic development (_vide_ p. 159), and Örley[173] has grouped these under three headings. The animals which fall under each group to some extent resemble one another in structure. Örley's groups are:— (i.) NEMATOZOA.—Thread-worms with free larval life, the mature forms being parasitic in animals. Enormous numbers of eggs are produced, and the development is indirect. The genital organs are complicated by many convolutions. (ii.) RHABDITIFORMAE.—Small, as a rule microscopic, thread-worms, usually living free, but rarely parasitic. They become sexually mature only in decomposing organic substances, or in earth saturated with such substances. They live gregariously and do not produce immense numbers of ova. The metamorphosis is slight, or is complicated by sexual metamorphosis. The oesophagus has two dilatations. The genital tubes are simple and not coiled. (iii.) ANGUILLULIDAE.—Small microscopic thread-worms, with a free existence in mould or water, throughout all stages. They produce large eggs. They are provided with a caudal sucker and bristles, sometimes with eyes and other structures characteristic of a free life. Genital tube simple and not coiled. The disadvantage of such a system is, that to accurately place a specimen in its proper class we must be acquainted with its life-history, and this is known in but few cases. The determination of the species to which a Nematode belongs is a matter of considerable difficulty. Amongst the more important features for purposes of classification are the arrangement of the {138}muscles, the character of the tail in the male, especially when papillae are present, the number and the size of the spicules, and the arrangement of the lips and mouth-parts generally.