Fig. 122.—Nereis pelagica L., natural size. (From Ehlers.)
Fig. 123.—Chaetae of Nereis; enlarged. A, from neuropodium; B, from notopodium of N. diversicolor; C, swimming chaeta of Heteronereid stage of N. dumerilii.
The chaetae, or bristles, of each bundle project from the mouth of a great sac, the lips of which are particularly prominent in Nereis. Each chaeta arises from a single cell situated at the bottom of the sac. The chaetae of Nereis, as of many other Polychaetes, are of a kind usually termed compound or "jointed," each being composed of a long stalk and a small "appendix" articulated in a cup at its extremity (Fig. 123). The shape of the cup varies; it is in some cases of equal height all round, or it is higher on one side than on the other. Further, the appendix may be short and curved, or more elongate and spear-like; it is generally notched or finely toothed on one side.
Fig. 124.—Nereis. Somewhat diagrammatic transverse section through the body. On the left the chief constituents of the vascular system are represented; on the right side the chaetae and their muscles, as well as the distribution of the lateral nerve, etc., are shown. ac, aciculum; bv, network of blood-vessels; ch, chaetae (only a few are shown) in two bundles; ch.l, lips of the chaetigerous sac; cil.org, dorsal ciliated organ; cir, circular muscular coat; coe, coelom; d.cir, dorsal cirrus; d.long, dorsal bundle of longitudinal muscles; d.ve, dorsal blood trunk; ep, epidermis; INT, intestine; int.cap, blood capillaries in its wall; m.ch, muscles which move the chaetae; N.c, ventral nerve cord; ner, lateral sensory nerve, dividing into a ventral branch entering the ventral cirrus, and a dorsal branch (n.cir) for the dorsal cirrus; neph, nephridium, seen through the oblique muscle through which its funnel passes; nrp, neuropodium; nrp.lig, neuropodial lobe or ligule; ntp, notopodium; ntp.lig, notopodial ligule; obl, oblique transverse muscle (muscle of the parapodium); pv, peripheral blood-vessel; v.cir, ventral cirrus; v.long, ventral bundle of longitudinal muscles; v.ve, ventral blood trunk.
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.
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.
