_B. With fifteen pairs of elytra._—_P. imbricata_ L. is probably the commonest species of the genus, occurring nearly everywhere under stones at low tide. It is about an inch in length; the elytra are deciduous, and are very variously coloured and marked; sometimes uniformly grey or even black, sometimes {311}mottled with brown: in other specimens each elytron has its outer half pale or white, while its inner half is darker, usually some tint of brown or olive green, so that the worm appears to have a dark band along the middle of its back. Other patterns occur. The body is entirely covered by the elytra. The chaetae project considerably, and are nearly as long as half the width of the body; those of the notopodium are brown and are directed upwards, being nearly as long as the golden neuropodial chaetae. This species has a very wide range, occurring on both sides of the Atlantic, even on the shores of Nova Zembla, and reappearing again at Japan. _P. semisculptus_ Leach is rather larger than the foregoing. The elytra are very readily detached: they are light in colour, without a fringe, but with large papillae near the margin. The notopodial chaetae are thicker than those of the neuropodium. Several other species are also common, but _P. johnstoni_ v. Marenz.[372] differs from the rest in having an elongated body of some seventy segments, so that the posterior half is uncovered by the elytra, which are small, greenish-grey, speckled, and have no fringe. It is common and widely distributed, but appears to be only found in the tubes of _Terebella nebulosa_. [Illustration: FIG. 163.—Elytron of _Polynoë imbricata_. _a_, Area of attachment to body; _e_, outer border; _i_, inner border.] _C. With eighteen pairs of elytra._—_P. gelatinosa_ Sars, may attain a length of 2 inches. The elytra are very faintly coloured, transparent and soft, attached by rather long peduncles. In spirit they become swollen and folded, giving the worm a very untidy appearance. The prostomium is partly overlapped by a peculiar collar-like fold of the peristomium. _D. With numerous pairs of elytra._—_Lepidasthenia_ has a very long body, consisting of more than eighty segments. The elytra are quite small, and occur throughout the body on the usual segments. There are no notopodial chaetae. _L. elegans_ Gr. is a very elegantly marked worm, which, however, has not been recorded from the British area; it occurs in the Mediterranean (see Fig. 156, p. 293). Sub-Fam. 2. _Hermionina._—The body is short, oval and {312}depressed; the particularly strong notopodial chaetae are directed upwards and backwards so as to protect the elytra. The neuropodial chaetae are also strong. The prostomium carries a single tentacle and two long palps; the prostomial ridge may be well developed. The peristomium is chaetigerous, with long cirri. The jaws are represented merely by thickened prominences.