Alimentary Canal.—The alimentary canal of Polychaetes is usually a straight tube running from the anterior mouth to the posterior anus. But in some forms, e.g. Sternaspis, the gut is coiled. In others, again, e.g. Cobangia, the anus is anterior and ventral. A gizzard is present in a few forms. The buccal cavity is sometimes armed with jaws. The oesophagus is provided often with caeca which in Syllids and Hesionidae have been found to contain air, and possibly therefore perform the function of the fish’s air-bladder. In other Polychaetes one or more pairs of similar outgrowths are glandular. The intestine is provided with numerous branched caeca in Aphrodite.
Reproduction.—As is the case with the Oligochaeta, the Polychaeta furnish examples of species which multiply asexually by budding. There is a further resemblance between the two orders of Chaetopoda in that this budding is not a general phenomenon, but confined to a few forms only. Budding, in fact, among the Polychaetes is limited to the family Syllidae. In the Oligochaetes it is only the families Aeolosomatidae and Naididae that show the same phenomenon. It has been mentioned that in the Nereids a sexual form occurs which differs structurally from the asexual worms, and was originally placed in a separate genus, Heteronereis; hence the name “Heteronereid” for the sexual worm. In Syllis there is also a “Heterosyllid” form in which the gonads are limited to a posterior region of the body which is further marked off from the anterior non-sexual segments by the oak-like setae. In some Syllids this posterior region separates off from the rest, producing a new head; thus a process of fission occurs which has been termed schizogamy. A similar life history distinguishes certain Sabellid worms, e.g. Filigrana. Among the Syllids this simple state of affairs is further complicated. In Autolytus there is, to begin with, a conversion of the posterior half of the body to form a sexual zooid. But before this separates off a number of other zooids are formed from a zone of budding which appears between the two first-formed individuals. Ultimately, a chain of sexual zooids is thus formed. A given stock only produces zooids of one sex. In Myrianida there is a further development of this process. The conversion of the posterior end of the simple individual into a sexual region is dispensed with; but from a preanal budding segment a series of sexual buds are produced. The well-known Syllid, discovered during the voyage of the “Challenger,” shows a modification of this form of budding. Here, however, the buds are lateral, though produced from a budding zone, and they themselves produce other buds, so that a ramifying colony is created.
| Fig. 5.—A, Autolytus (after Mensch) with numerous buds. B, Portion of a colony of Syllis ramosa (from M‘Intosh). b.z, Budding zone; p, anterior region of the parent worm; 1-5, buds. |
Quite recently, another mode of budding has been described in Trypanosyllis gemmipara, where a crowd of some fifty buds arising symmetrically are produced at the tail end of the worm. In some Syllids, such as Pionosyllis gestans, the ova are attached to the body of the parent in a regular line, and develop in situ; this process, which has been attributed to budding, is an “external gestation,” and occurs in a number of species.
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| Fig. 6.—A, Side view of the larva of Lopadorhynchus (from Kleinenberg),showing the developing trunk region. B, Side view of thetrochophore larva of Eupomatus uncinatus (from Hatschek). | |
A, Anus. E, Eye. M, Mouth. ap, Apical organ. h, “Head Kidney.” i, Intestine. | me, Mesoblast. ms, Larval muscle. o, Otocyst. pp, Parapodium. pr, Praeoral ciliated ring, or prototroch. |
| Fig. 7.—Nereis pelagica, L. (After Oersted.) |
As is very frequently the case with marine forms, as compared with their fresh-water and terrestrial allies, the Polychaeta differ from the Oligochaeta and Hirudinea in possessing a free living larval form which is hatched at an early stage in development. This larva is termed the Trochosphere larva, and typically (as it is held) is an egg-shaped larva with two bands of cilia, one preoral and one postoral, with an apical nervous plate surmounted by a tuft of longer cilia, and with a simple bent alimentary canal, with lateral mouth and posterior anus, between which and the ectoderm is a spacious cavity (blastocoel) traversed by muscular strands and often containing a larval kidney. The segmentation is of the mesoblast to begin with, and appears later behind the mouth, the part anterior to this becoming the prostomium of the adult. The chief modifications of this form are seen in the Mitraria larva of Ammochares with only the preoral band, which is much folded and which has provisional and long setae; the atrochous larva, where the covering of cilia is uniform and not split into bands; and the polytrochous larva where there are several bands surrounding the body. There are also other modifications.
Classification.—The older arrangement of the Polychaeta into Errantia or free living and Tubicola or tube-dwelling forms will hardly fit the much increased knowledge of the group. W.B. Benham’s division into Phanerocephala in which the prostomium is plain, and Crytocephala in which the prostomium is hidden by the peristomium adopted by Sedgwick, can only be justified by the character used; for the Terebellids, though phanerocephalous, have many of the features of the Sabellids. It is perhaps safer to subdivide the Order into 6 Suborders (in the number of these following Benham, except in combining the Sabelliformia and Hermelliformia). Of these 6, the two first to be considered are very plainly separable and represent the extremes of Polychaete organization, (1) Nereidiformia.—“Errant” Polychaetes with well-marked prostomium possessing tentacles and palps with evident and locomotor parapodia, supported (with few exceptions) by strong spines, the aciculi; muscular pharynx usually armed with jaws; septa and nephridia regularly metameric and similar throughout body; free living and predaceous. (2) Cryptocephala.—Tube-dwelling with body divided into thorax and abdomen marked by the setae, which are reversed in position in the neuropodium and notopodium respectively in the two regions. Parapodia hardly projecting; palps of prosomium forming branched gills; no pharynx or eversible buccal region; no septa in thorax, septa in abdomen regularly disposed. Nephridia in two series; large, anterior nephridia followed by small, short tubes in abdomen. The remaining groups are harder to define, with the exception of the (3) Capitelliformia, which are mud-living worms of an “oligochaetous” appearance, and with some affinities to that order. The peristomium has no setae, and the setae generally are hair-like or uncinate, often forming almost complete rings. The genital ducts are limited to one segment (the 8th in Capitella capitata), and there are genital setae on this and the next segment. In other forms genital ducts and nephridia coexist in the same segment. The nephridia are sometimes numerous in each segment. There is no blood system, and the coelomic corpuscles contain haemoglobin. (4) Terebelliformia. These worms are in some respects like the Sabellids (Cryptocephala). The parapodia, as in the Capitellidae, are hardly developed. The buccal region is unarmed and not eversible. The prostomium has many long filaments which recall the gills of the Sabellids, &c. The nephridia are specialized into two series, as in the last-mentioned worms. (5) Spioniformia (including Chaetopterus, Spio, &c.) and (6) Scoleciformia (Arenicola, Chloraema, Sternaspis) are the remaining groups. In both, the nephridia are all alike; there are no jaws; the prostomium rarely has processes. The body is often divisible into regions.
| Fig. 8.—Sabella vesiculosa, Mont. (After Montagu.) | Fig. 9. Arenicola marina, L. |
Literature.—W.B. Benham, “Polychaeta” in Cambridge Natural History; E. Claparède, Annélides chétopodes du golfe de Naples (1868 and 1870); E. Ehlers, Die Börstenwürmer (1868); H. Eisig, Die Capitelliden (Naples Monographs), and development of do. in Mitth. d. zool. Stat. Neapel (1898); W.C. M’Intosh, ”Challenger” Reports (1885); E.R. Lankester, Introductory Chapter in A Treatise on Zoology; E.S. Goodrich, Quart. Journ. Mic. Sci. (1897-1900); E. Meyer, Mitth. d. zool. Stat. Neapel (1887, 1888), as well as numerous other memoirs by the above and by J.T. Cunningham, de St Joseph, A. Malaquin, A. Agassiz, A.T. Watson, Malmgren, Bobretsky and A.F. Marion, E.A. Andrews, L.C. Cosmovici, R. Horst, W. Michaelsen, G. Gilson, F. Buchanan, H. Levinsen, Joyeux-Laffuie, F.W. Gamble, &c.