[Illustration: FIG. 157.—_Chaetopterus variopedatus_ Ren. × ½. On the left the entire animal, with the three regions A, B, C. _c_, Peristomial cirrus; _d_, "sucker"; _e_, the great "wings"; _f_, "fan"; _m_, mouth. On the right the animal is represented in the dark, under stimulation, so as to exhibit the phosphorescent portions of the body. (From Panceri.)] PHOSPHORESCENCE.—Many worms of very different habits have the power of emitting a light from some parts of the body, and they are then said to be "phosphorescent."[354] Probably _Chaetopterus_ is most eminently photogenic; the base of the great "wings," the "fans," and other parts emit, on stimulation, an azure blue to greenish light, so bright that one may read one's watch by it. Several species of _Polynoë_ exhibit a similar phenomenon, each elytron, with the exception of the area of attachment, being brilliantly illuminated. In these species the phosphorescent elytra are frequently thrown off by the animal, so that possibly they deceive enemies. _Polycirrus aurantiacus_ produces a beautiful violet phosphorescence; usually its many tentacles alone show the light, but under strong stimulation the entire body takes {296}part in the display, and no doubt the phosphorescence has, like the colour, a "warning" purpose. The production of the light in these various forms is apparently due to two different processes. In some cases, _e.g._ _Chaetopterus_, Syllids, Terebellids, it appears to be due to the oxidation of certain cell contents which are discharged more or less freely on irritation of the nerves; whilst in Polynoids the phenomenon is due to some purely nervous process, for the elytra have no glands, but are provided with ganglia and a nervous network. In other worms, however, there are definite light-producing organs. In _Tomopteris_ there is on each parapodium, above and below, a brightly-coloured spherical organ, which for a long time was regarded as an eye, but from its structure appears to be a "photogen" (Fig. 167, p. 315). The same is very likely the true explanation of the segmental "eyes" of _Polyophthalmus_, for their structure recalls that of the light-organs of deep-sea fishes. As many of the phosphorescent Polynoids are commensals, while _Chaetopterus_ inhabits tubes, and close allies of other phosphorescent worms have no power of emitting light, it is impossible to apply the same explanation of its purpose to all cases alike; in some it may be "accidental," though in others it may be of definite use in warning enemies or in attracting prey.