CHAETAE.—The passive organs of locomotion in these animals are the chaetae, which are absent in only one family, Discodrilidae, and in one other genus, _Anachaeta_. In this latter worm the chaetae are represented by large glandular cells, which seem to correspond to the cells from which the chaetae arise in other forms. They are in this case, as in the others, cells of the epidermis. The chaetae of the Oligochaeta are not quite so variable in form as in the marine Polychaeta (see Fig. 138, p. 267). Figs. 187 and 188 illustrate some of the principal shapes which these bristles assume. The most prevalent form is an elongated S, which has been aptly compared to the mathematical sign ∫. This kind of chaeta is found in all earthworms, and in not a few aquatic genera such as the Lumbriculidae. In some of the latter and in the Tubificidae and Naids there is the same form of chaeta, which is cleft at the free end, and possibly enables the worm to grasp the leaves of aquatic plants, and otherwise facilitates progression in a laxer medium than the stiff soil frequented by the earthworms. Even earthworms, at any rate the genus _Pontoscolex_, have chaetae of this kind; some of the aquatic Oligochaeta have elongated and hair-like bristles, such as that of _Tubifex_. {351}[Illustration: FIG. 188.—Chaetae. × 10. 1, _Onychochaeta_; 2, _Pontoscolex_; 3, _Trichochaeta_; 3_b_, the same, more highly magnified.] In the Tubificid _Lophochaeta_ (Fig. 187, 5) the chaetae are ornamented on both sides with delicate processes, which give them the appearance of Crustacean hairs. Among earthworms the simple S-like form is sometimes complicated by the development of sinuous ridges upon the distal end. No doubt these bristles enable their possessor to get a firmer grip of adjacent objects; they are very commonly found, in the family Geoscolicidae, upon the segments of the clitellum, and permit of a firmer union during sexual congress. In no Oligochaeta are the chaetae borne upon parapodia, as is the case with the Polychaeta; but in many of the aquatic forms there are a considerable number to each bundle. In earthworms the number of chaetae varies greatly. The common earthworms of this country, belonging to the genera _Lumbricus_, _Allolobophora_, and _Allurus_, have only eight chaetae upon each segment of the body, and these are then, as a rule, arranged in pairs or rather couples, two of each on each side of the body. The genus _Perichaeta_ and some of its allies have a much larger number of chaetae to each segment, disposed in a continuous row round the middle of the segment. The intermediate condition is to be seen in the genus _Deinodrilus_, where there are twelve in each segment, and in certain members of the genus _Megascolex_, where there are eight in each segment in the anterior region of the body, the number increasing in the posterior segments. The four bundles of chaetae in the Naids and Tubificids have been