The varied colouring in the Polychaetes, as in other animals, is due to a variety of causes. The red is in many cases due to haemoglobin of the vascular system showing through the transparent body; the green of the tentacles of the Sabellids and Chlorhaemids is similarly due to chlorocruorin. In other cases the contents of the intestine or the tint of the coelomic fluid may affect the colour of the worm. In Capitella the coloured excretory products are regained in the skin; in an Eunicid living in a yellow sponge, on which it feeds, the colouring matter is extracted and stored in the skin; in the same kind of way green caterpillars may owe their tint to feeding on green leaves. But many of the Polychaetes possess distinct pigments in the skin; thus in Arenicola the dark pigment melanin has been recognised; in Cirratulus and Nereis certain lipochromes; whilst Eulalia viridis contains a pigment allied to bonellein. These various pigments yield different absorption bands when a solution is examined with the spectroscope; others, however, give no bands, but are distinguished by different chemical reactions.[[350]] The colour of the intestine of Chaetopterus has been stated to be due to "modified chlorophyll," but it is quite a different substance.
When seen in the living and healthy condition, however, these Polychaete worms vie with the very butterflies in their brilliant and beautiful colourings, and though our own worms are not lacking in beauty, many tropical and southern forms exceed them in gayness of tint. Bright reds, orange, yellows, greens, blues, rich violets, and sombre browns are all displayed.[[351]]
The handsome Terebella nebulosa of our own coasts is coloured bright red, sprinkled with white spots. Nicomache lumbricalis is pink, with red girdles. Eunicids are frequently red or brown, and the red gills along each side, together with a brilliant iridescence, render these worms very beautiful. Nereids present a great range of coloration, from light green to sundry tints of brown and red in various combinations. Amongst the Serpulids our common S. vermicularis is a very showy little worm, with its orange body, its red gills splashed with orange, and its orange operculum streaked with red; and a Southern form, Placostegus coeruleus, occurring at the Cape of Good Hope, is provided with beautiful lavender-blue gills. Our own Sabellids present examples of beautiful markings on the gills, in different colours or in different shades of the same colour. Amongst Polynoids, P. leucohyba, from the Antilles, has blue elytra; Hemilepidia erythrotaenia, a long worm from the Cape of Good Hope, has the anterior end of its body covered with light blue elytra, whilst the uncovered part is orange, with a broad magenta-red band along the dorsal surface.
The Phyllodocids are mostly very brightly coloured. The common P. lamelligera of our coast has a bluish-green body, with olive-green parapodia; but Lopadorhynchus erythrophyllum, from Jamaica, has a blue body with red parapodia; whilst Notophyllum myriacyclum has a brown body with longitudinal dark-brown stripes and yellow parapodia. Both these worms live in coral reefs, where brilliancy of colour is one of the characteristic features of the fauna. Other worms are of various shades of green: the dark green Arenicola with red gills; the bright green Eulalia viridis; the deep green Amphinome smaragdina, from Jamaica; Gnathosyllis diplodonta, with its green and yellow body, serve as examples.
Patterns or "markings" may be exemplified by Lepidasthenia elegans (Fig. 156), and Myrianida fasciata, which has a bright red band on each segment (Fig. 149, p. [280]). From this brief list of examples it will be seen that beautiful, and even brilliant, coloration is not confined to any particular mode of life; many of the most typically tubicolous forms, like the Terebellids and Serpulids, are as brilliantly coloured as the most typically free-swimming genera, like the Phyllodocids. Carnivorous forms like Amphinomids and Syllids present as wide a range of tint as the limivorous forms like Cirratulus, Sabella, or Maldanids. Shore-lovers, and deep-sea dwellers, and surface-swimmers, all exhibit equally bright or equally sombre tints; it is therefore difficult and rash to dogmatise on the "use" of these colourings to these animals, or to point to this worm as being protectively, to the other as being warningly, coloured; for we are too ignorant as to the habits of the worms.
Fig. 156.—Lepidasthenia elegans Gr., × 2, to illustrate colour-markings: the dark bands in the anterior part of the body occupy two elytriferous, and the intermediate segments. In the hinder region, where the elytra are in every third segment, this one is dark. el.12, The twelfth elytron.
Protective and Mimetic Devices.—From the point of view of "protection" in the evolutionist's sense of the word, we can say but little. Protective resemblance there is undoubtedly amongst the Polynoids, for the scales of these forms resemble more or less closely the stones or sand amongst which they live; in the same species there is great variety in coloration. This protective habit is carried still further in the case of Psammolyce by the attachment of sand grains to little cups on the elytra, so that the back of the animal is concealed. Certain commensals, such as Polynoë arenicolae, P. pentactes, are coloured so as to resemble their associates. In a few cases it is possible that the gills of Sabelliformia are protectively coloured; for in Sabella pavonia they vary from a light yellowish tint to a deep violet-brown, and the dark markings on them are therefore more or less distinct. Spread out as the gills are in life, they are in many cases difficult to recognise; it is rather their movement as they are withdrawn that attracts one's attention to them, as the tubes of these worms frequently serve for the attachment of brownish seaweeds, to which the gills bear resemblance. But, as a matter of fact, little work has been done in this direction, and speculation on the matter without evidence is worthless. Many pelagic forms, being transparent, such as Tomopteris and Alciopids, are no doubt protected by their lack of colour; yet these forms present brightly-coloured spots,—the light-producing organs in the parapodia of the former, and the large dark eyes of the latter.
Semper[[352]] mentions a case of possible mimicry in a species of Myxicola which lives in the clefts of a coral, Cladocora. The branchial funnel, when expanded, resembles very closely the expanded coral in size, colour, etc.; but he points out that the species occurs in other situations, where its colouring is not protective. Probably the "mimicry" is in other instances merely accidental.
No doubt many Polychaetes may be "warningly coloured," but experimental evidence is incomplete. Polycirrus aurantiacus is bright red, with orange tentacles; these worms were rejected by certain fish.[[353]] The animal has given up living in tubes as all its allies do, and it is the tentacles which appear to be distasteful to its enemies, for when irritated it coils itself up and wraps itself round with its tentacles. Moreover, when the tentacles were cut off the fish did not reject the body of the worm. The tentacles are thus coloured in such a way that fish recognise them, and associate with the colour some distasteful property.