Fig. 173.—Chaetopterus. (From Panceri.) Natural size of a young specimen. A is the anterior region of the body; B, the middle region; C, the hinder region. c, Peristomial cirri; d, "sucker"; e, the great "wings"; f, the first of the three "fans"; m, mouth.
The second region, B, is very curiously modified; it is formed of five segments. The most anterior is produced on either side into a great wing-like process, which in life is directed forwards above the region A. Each is grooved on its inner side, the ciliated grooves being continuous with a median groove running forwards along the back of A; this apparatus serves to bring food to the great funnel-like mouth. The next segment (twelfth) carries a dorsal and ventral "sucker," representing the parapodia. Each of the segments 13, 14, 15 carries a membranous fold encircling the body. By the constant movement of these "fans," which have nearly the same diameter as the tube, a current of water is constantly washed over the animal. The fans represent the notopodia; the neuropodia are bilobed rounded knobs. The region C consists in the adult of about thirty segments, all alike, and less modified than the preceding. The animal is the most truly tubicolous of the Polychaetes, and is much modified on this account. No locomotor chaetae are present, though the great wings and notopodial processes of region C contains chitinous bristles, which, however, do not project;[[384]] the anterior region with its stiff chaetae, and the neuropodial uncinal plates of the rest of the body serve in its movements up and down the tube, while the "suckers" fix the worm temporarily to the wall of its house.
Chaetopterus is highly phosphorescent (see p. [295]). It is further interesting on account of the green colouring-matter, which is extracted by alcohol. Two commensal Polynoids occur in the tube, viz. Polynoë glabra and P. cirrosa. The larva is "mesotrochal" (with a ciliated ring round its middle), that region of the body lying in front of the cilia giving rise to the region A, whilst the rest of the body gives rise to regions B and C.
Fam. 4. Magelonidae.—This family includes only the very peculiar worm, Magelona papillicornis Fr. Müll., which lives buried in sand, between tide-marks, in various parts of our coast and that of the United States. Its chief features are the large, flat, spoon-shaped prostomium; the long peristomial cirri, slightly expanded terminally, carrying papillae along one side; the enormous, eversible buccal region, which is an important respiratory organ. The blood is of a madder-pink colour, and the blood-vessels in the thorax are greatly dilated. The body of the worm is divisible into two well-marked regions, owing to differences in the chaetae.[[385]]
Fam. 5. Ammocharidae.—This family contains only one species, Owenia filiformis D. Ch. Some of the anterior segments are longer than the hinder ones, though the arrangement of chaetae is alike throughout. The mouth is wide, like that of Chaetopterus, and is surrounded, except ventrally, by a membrane, so deeply notched as to give rise to flattened filaments containing blood-vessels. These "gills" appear to belong to the peristomium. The small worm in its sandy tube is plentiful on our coasts in about 20 fathoms. Off Greenland and the Mediterranean.
Sub-Order 3. Terebelliformia.
Fam. 1. Cirratulidae.—These worms have a cylindrical body, more or less attenuated at each end; the segments are distinct, and similar throughout, with capillary chaetae on each side in two bundles, carried by small papillae. The prostomium is conical, the peristomium usually without cirri. On more or fewer segments the dorsal cirri are long and filamentous, and function as gills. There is a single pair of anterior nephridia: the septa and genital ducts are repeated throughout the hinder part of the body. The worms usually live in burrows.
Cirratulus.—The prostomium is long, sometimes annulated. In addition to the segmental filamentous "gills" there is a transverse row of long "tentacular filaments" across one of the anterior segments, and it has been suggested that they are really prostomial tentacles which have shifted backwards. They and the gills twist about in a very active fashion during life, and look like small independent worms, especially when they are detached. C. cirratus Müll, is a brown or dirty yellowish worm about 4 to 6 inches in length, usually to be found under stones, partially embedded in the mud or sand. The prostomium carries a pair of linear groups of "eye-spots"; the first chaetigerous segment carries a transverse row of tentacular filaments, and the red gill-filaments commence on the same segment. Common. C. tentaculatus Mont. is a larger worm, dark red in colour, and is distinguished from the preceding by the absence of eyes and by the fact that the tentacular filaments are on the seventh chaetigerous segment, while the gills commence more anteriorly. Atlantic and the Mediterranean.
