In addition to these projecting locomotor chaetae, there is embedded in each of the two chaetigerous lobes a much stouter and dark-coloured, needle-shaped bristle known as an "aciculum," whose point only just projects beyond the surface. This aciculum extends into the interior of the body much farther than do the locomotor chaetae, and it is to it that the muscles serving to move the whole bundle of chaetae are attached. The acicula thus serve as an internal skeleton to the parapodium. The shape of the parapodium, the relative lengths of cirri and lobes, the shape and arrangement of the chaetae, are all employed as specific characters. {248}The HEAD consists of a preoral portion above the mouth, the "prostomium," and a postoral region surrounding the mouth, the "peristomium" (Fig. 125). The prostomium varies in shape in different species of _Nereis_; but it always carries on its dorsal surface two pairs of eyes. From its narrower anterior end there arises a pair of short, somewhat conical, sensory processes known as the "prostomial tentacles." A second pair of processes springs from the under surface, and rather to the side of the prostomium; these are known as the "palps," and in _Nereis_ are much more conspicuous than the tentacles; each is composed of two parts, a large basal piece and a smaller terminal joint, capable of being withdrawn into the former. The palps are highly muscular, and though they are sensory organs, act also as great lateral lips. [Illustration: FIG. 125.—_Nereis diversicolor_ Müll. × 4. Head, with buccal region everted. A, Dorsal view; B, ventral view. _a_, Prostomium; _B_, everted buccal region; _c_, _c'_, peristomial cirri, 1, 2, 3, 4; _d_, denticles or paragnaths; _e_, eyes; _E_, lower lip; _P_, palp in A, entrance to pharynx in B; _J_, jaw; T, tentacle; I, peristomium; II, foot of apparent second segment.] The peristomium is in many species of _Nereis_ (as in _N. pelagica_) considerably larger than the trunk segments; it carries at its anterior edge four filiform cirri on each side, which are directed forwards and used as feelers. They are arranged in couples; a more anterior couple of dorsal and ventral cirri, and a more posterior couple of dorsal and ventral cirri. The TAIL.—As the most anterior segment is perforated by the mouth, and is modified as described above, so the last or anal segment, which carries the anus, differs from the rest. It is more or less elongated, cylindrical, and without parapodia or chaetae. It retains, however, its pair of ventral cirri, which are very long.