GENERAL SHAPE OF THE BODY.—The majority of the Polychaeta have an elongated and very mobile body, like that of _Nereis_, consisting of an indefinite and usually of a considerable number of segments; a few, however, have a shorter body, with fewer segments, definite in number, for instance _Aphrodite_ and _Polynoë_, which have thirty to forty segments; and some Hesionids, with only some seventeen to twenty segments. {259}In Aphroditidae and certain Amphinomidae the body is more or less oval in shape. In _Lipobranchius_ and _Sternaspis_ it is grub-like, short, and cylindrical, with rounded ends; in the former it is difficult to distinguish head and tail, or dorsal and ventral surfaces. The segments composing the trunk may be all alike, or may constitute two more or less sharply marked regions, the _thorax_ and _abdomen_, differing in the character of the chaetae, or in their arrangement, or in some other way, as in the Sabelliformia and the Capitelliformia. As peculiar cuticular structures, the curious shields of _Sternaspis_, and of certain of the Maldanidae may be mentioned. The posterior extremity is generally more or less narrowed, and most of the Nereidiformia are provided with special elongated cirri, borne by the anal segment. In the Maldanidae and others the body terminates in a funnel, at the bottom of which is placed the anus. Only in a few cases is the anus not terminal; in _Notopygos_ and other Amphinomidae, as well as in some species of _Polynoë_, it is dorsal.[313] In _Sabellaria_ and _Pectinaria_ the hinder end of the body undergoes great degeneration; in the former it is achaetous, but cylindrical and bent forwards alongside the body (Fig. 131). In _Pectinaria_ (Fig. 177), this region, which is called the "scapha," is leaf-like, and serves to close the narrower end of the tube in which the worm lives. _Arenicola marina_, and some Terebellids have no chaetae in the hinder, narrower part of the body. [Illustration: FIG. 131.—_Sabellaria alveolata_ L. × 3. (After Malmgren.) _a_, Anus.] The HEAD.—The prostomium is, in the majority of cases, rounded or conical, though it may be square (_Nephthys_) or elongated and jointed (_Glycera_), or even hammer-shaped (_Tomopteris_); or it may be fused with the peristomium, and apparently absent (_Arenicola_). In the great group Cryptocephala, the peristomium grows forwards so as to hide the prostomium entirely. In a few of the Nereidiformia the prostomium is compressed, {260}and in the Amphinomidae it is provided with a dorsal ridge or "caruncle," which is a leaf-like process overlapping three or more segments. In many Aphroditidae (as well as in _Polydora_) there is a peculiar "frontal" ridge passing forwards from the prostomial tentacle, and downwards into the mouth (Figs. 132, _c_, and 133, A, _x_).