(3) The uncini are very short chaetae, which are simply embedded in the skin, and do not extend beyond the body-wall into the body-cavity. An uncinus is a sharply curved hook, which may have more or less numerous secondary teeth on it. They are characteristic of the Sabelliformia and the Terebelliformia. The chaetae appear as solid, usually fibrillated structures, of a yellow or golden tint, transparent and refringent. Chemically {267}they consist of _chitin_, and each chaeta is the product of a single cell. The chaetae of _Euphrosyne_ are hollow and calcareous, being peculiar in both characters. [Illustration: FIG. 138.—Chaetae of various Polychaetes (the magnification is not the same in all cases). A, Doubly-fringed capillary, from Terebellid; B, hooded crotchet, from _Polydora_; C, a fork, from _Euphrosyne_; D, jointed chaeta, from _Phyllodoce_; E, simple chaeta, with serrated ridges or frills, from a Polynoid; F, jointed chaeta, from _Eunice_; G, uncinus, from _Pomatoceros_ (Serpulid); H, one of the outer series of paleae from the hood of _Sabellaria spinulosa_; I, jointed chaeta, from a Syllid; J, multidenticulate crotchet, from a Maldanid; K, comb-shaped chaeta, from _Eunice_; L, uncinus of a Sabellid; M, uncinus of Terebellid (_Amphitrite Johnstoni_); _a_, edgewise; _b_, side view; _m_, attachments of muscles into _ba_, basal plate; _x_, accessory teeth. N, Sheathed spear of _Hermione_; _a_, the spear-shaped capillary removed from its sheath; _b_, the same, with sheath.] Certain modifications of the chaetae presented by various worms deserve mention. In _Polydora_ (Fig. 133, A) and in _Chaetopterus_ (Fig. 173, p. 324) those of one segment are especially strong, but their significance is uncertain. In _Capitella_ those {268}of the notopodium of the eighth and ninth segments are specially modified; they are analogous to the copulatory chaetae of Oligochaeta. In _Aphrodite_, in addition to the ordinary locomotor chaetae, there are brilliant, iridescent bristles and peculiar felting threads arising from the indistinct notopodium; these latter, however, are not true "chaetae," but are separate chitinous filaments similar to the constituent fibres of an ordinary chaeta.[322] While the chaetae in the Nereidiformia and others are grouped in bundles, those of many other families are in vertical, transverse rows, as in Maldanidae and in _Arenicola_. The uncini are always embedded in such rows, usually slightly raised from the general level of the body surface, each being termed a "torus uncinigerus." These tori are usually limited to the sides of the body, but in _Myxicola_ and in _Notomastus_ they encroach upon the dorsal surface, and in _Chaetozone_, also upon the ventral, so as nearly to encircle the body, recalling the "perichaetous" condition of some earth-worms.