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 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.
The Food of Worms.—The Nereidiformia are mostly carnivorous, and feed on small Crustacea, Mollusca, sponges, and other animals; and Polynoids are even said to eat one another. Many worms do not disdain various seaweeds, whilst the Spioniformia and Scoleciformia, which burrow in mud and sand, and are without biting organs, swallow the mud and digest what animal or vegetable débris it may contain. The Terebellids and Cryptocephala depend on minute organisms which may be driven into the mouth by the action of the cilia of the gills or tentacles.
In the case of deep-sea forms, it is an interesting fact that the intestines are not unfrequently crammed with Radiolaria and Foraminifera in a fairly fresh, uninjured condition, indicating that these Rhizopods do not merely sink to the bottom, but must actually live there.[[355]]
The economic purposes to which Polychaetes are put are few; they are used either as bait for fishes or as food for man.
One of the commonest baits used for certain fish, as all who have done any sea-fishing off the piers of our coasts know, is the common lug-worm (Arenicola marina), whilst Nephthys caeca and Nereis fucata are also used in some places; and for whiting Nereis cultrifera and N. diversicolor. Marphysa sanguinea, known to the fishermen in some parts as "varme," is less frequently used.