A peculiar worm—Palolo viridis—is used as food by the natives of Samoa and Fiji. The worm is similar to our Eunicid Lysidice ninetta, and lives in fissures among corals on the reefs, at a depth of about two fathoms. At certain days in October and November they leave the reefs and swim to the shores of the above islands, probably to spawn; and this occurs on two days in each of the above months—the day on which the moon is in her last quarter, and the day before. The natives, who call the worm "Mbalolo," give the name "Mbalolo lailai" (little) to October, and "Mbalolo levu" (large) to November, thereby indicating the relative abundance of the worms in these two months. The natives eat them either alive or baked, tied up in leaves; and they are esteemed so great a delicacy that presents of them are sent by the chiefs who live on shore to those living inland. A dark green-blue Phyllodocid, which is called "A'oon," occurs in abundance off Mota Island, amongst the New Hebrides, has similar habits, and is also eaten.[[356]]
Associated Worms.—A considerable number of worms live in association with other animals, either as commensals or as parasites, and it is not in every case possible to decide in what relation the two animals stand. Labrorostratus parasiticus, a Eunicid, is parasitic in the body-cavity of Odontosyllis ctenostomatus (Fig. 158); such an association between two members of the same group of animals is peculiar; but still more exceptional is the occurrence of Haematocleptes terebellides, as a parasite in Marphysa sanguinea, for both parasite and host are members of the same family, the Eunicidae. Another Eunicid, Oligognathus bonelliae, occurs in the body-cavity of the Gephyrean Bonellia.
The Polynoid Acholoe astericola and the Hesionid Ophiodromus flexuosus occur as ectoparasites (or perhaps commensals) in the ambulacral grooves of the starfish Astropecten aurantiacus. An Amphinomid is stated to live in the branchial chamber of the barnacle, Lepas anatifera. Alciopina parasitica lives, during the early stages of its life-history, within Cydippe, and it is possible that most of the Alciopids thus make use of Ctenophores as their nurseries.
A considerable number of the Polynoids are ectoparasitic: P. castanea lodges in the peri-oral region of Spatangus purpureus, and in the ambulacral grooves of Astropecten; P. (Halosydna) bairdi lives between the mantle and foot of the mollusc Fissurella cratitia; P. pentactes is found on the body of the Holothurian Cucumaria pentactes, and appears to be protectively coloured. P. (Antinoë) parasitica lives under the elytra of another Polynoid, and P. acanellae on the coral Acanella normani.[[357]]
Fig. 158.—Odontosyllis ctenostomatus, with (L) Labrorostratus parasiticus in its body-cavity. The parapodia and cirri are omitted from the greater part of the body. (After St. Joseph.) × 4.
As commensals there may be mentioned Nereis fucata, which lives in the upper coil of whelk-shells which are inhabited by a hermit crab. The same shell usually bears a particular sea-anemone, so that there are three animals living together in or upon the cast-off house of a fourth. Siphonostoma is found in the "nests" made by the mollusc Lima. A Eunice is constantly associated with the coral Lophohelia prolifera, amongst the branches of which the worm twines its tube; whilst another Polychaete inhabits a tube formed by the interweaving of the fine branches of the coral Antipathes filix,[[358]] found in the West Indian seas. A species of Polydora forms its tube in Heliopora. The Polynoids present many instances of commensalism, a few of which may be here mentioned. P. johnstoni Marenz. is only found in the tubes of Terebella nebulosa; other species occur in the tubes of other Terebellids. P. marphysae lives in tubes of the Eunicid Marphysa sanguinea. Two species live in the tubes of Chaetopterus. P. extenuata has been found in tubes of Serpula vermicularis, while P. arenicolae occurs on the body of the common lug-worm, with the colouring of which it closely harmonises.
Worms as Hosts.—The Polychaeta serve not only as food for fishes, Crustacea, and other predatory animals of larger size, but are also liable to be the hosts of parasites[[359]] such as Gregarines, and even, as we have seen, of other members of their own group. Sundry ectoparasitic Copepoda have been found attached to worms between the parapodia or to the sides of the feet, and an unnamed Copepod occurs attached, sometimes in considerable numbers, to the sides of Nereis cultrifera. The Polychaeta also act as protectors to other animals, for on the under surface of elytra of sundry Polynoids may very frequently be found specimens of Loxosoma, which may also be attached to gills of Eunicids; whilst below those of Aphrodite echidna and Hermadion pellucidum, Pedicellina belgica occurs. Under the felt of A. aculeata the Sabellid Branchiomma vigilans forms its tube, and Vorticellids may be found on chaetae, gills, or other parts of the body of sundry worms.
Distribution.—Very little can be said in a brief way of the geographical distribution of these worms, for many of the genera are cosmopolitan, although only a few species occur in all the great oceans, e.g. Polynoë imbricata, Hyalinoecia tubicola, Nerine (Scolecolepis) cirrata, and Terebellides stroemi.
As for species, it can be said generally that the different oceanic areas and even different coasts present different species, but we know practically nothing of variation amongst Polychaeta, and many so called species may be mere local varieties, for frequently the descriptions of "new species" are scarcely intelligible. At any rate we know that certain species occur at widely separated localities, for two or three species of Polynoids occur in Japan, and again at Dinard on the French coast. A considerable number of species are common to both sides of the North Atlantic ocean, having been obtained off Norway and in the Gulf of the St. Lawrence. A few of these which are common on our coasts may be enumerated:—Nereis pelagica, Nicomache lumbricalis, Glycera capitata, Thelepus cincinnatus, Scoloplos armiger, Sabella pavonia, Ophelia limacina, Aphrodite aculeata, Trophonia plumosa, Polynoë squamata, Capitella capitata, Sthenelais limicola.