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POLYCHAETA

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Originally appearing in Volume V05, Page 795 of the 1911 Encyclopedia Britannica.
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POLYCHAETA .—ThiS See also:

group may be thus defined and the See also:definition contrasted and compared with those of the other divisions of the See also:Chaetopoda. Setae always See also:present and often very large, much varied in See also:form and very numerous, See also:borne by the dorsal and ventral parapodia (when present). The prostomium and the segments generally often See also:bear processes sensory and branchial. Eyes often present and comparatively complicated in structure. Clitellum not present as a definite See also:organ, as in See also:Oligochaeta. The anus is mostly terminal, and there are no anterior and posterior suckers. See also:Nervous See also:system often imbedded in the epidermis. Vascular system generally present forming a closed system of tubes. Alimentary See also:canal rarely coiled, occasionally with glands which are See also:simple caeca and sometimes serve as See also:air reservoirs; jaws often present and an eversible pharynx. Nephridia sometimes of the type of those of the Oligochaeta; in other cases See also:short, wide tubes with a large See also:funnel serving also entirely or in See also:part as gonad ducts. Frequently reduced in number of pairs; rarely (Capitellidae) more than one pair per segment. Gonads not so restricted in position as in Oligochaets, and often more abundant; the individuals usually unisexual.

No specialized system of spermathecae, sperm reservoirs, and copulatory apparatus, as in Oligochaeta; development generally through a larval form; See also:

reproduction by budding also occurs. Marine (rarely fresh-See also:water) in See also:habit. The Polychaeta contrast with the Oligochaeta by the See also:great variety of outward form and by the frequency of specialization of different regions of the See also:body. The See also:head is always recognizable and much more conspicuous than in other Chaetopoda. As in the Oligochaeta the peristomial segment is often without setae; but this See also:character is not by any means so See also:constant as in the Oligochaeta. The prostomium bears often processes, both dorsal and ventral, which in the Sabellids are split into the circle of branchial plumes, which surround or nearly surround the mouth in those See also:tube-dwelling Annelids. Tomopteris is remark-able for the fact that the See also:hammer-shaped prostomium has paired ventral processes each with a single seta. It is held, however, that these are a pair of parapodia which have shifted forwards. The presence of parapodia distinguish this from other See also:groups of Chaetopoda. Typically, the parapodium consists of two processes of the body on each See also:side, each of which bears a bundle of setae; these two divisions of the " See also:limb " are termed respectively notopodium and neuropodium. The notopodium may be rudimentary or absent and the entire parapodium reduced to the merest See also:ridge or even completely unrepresented. Naturally, it is among the See also:free living forms that the parapodium is best See also:developed, and least developed among the tubicolous Polychaeta.

To each See also:

division of the parapodium belongs typically a See also:long tentacle, the cirrus, which may be defective upon one or other .of the See also:noto- See also:podium or neuropodium, and may be developed into an arborescent gill or into a See also:flat See also:scale-like See also:process, the elytron (in Polynoe, &c.). There are other gills developed in addition to those which represent the cirri. Setae.—The setae of the Polychaeta are disposed in two bundles in many genera, but in only one bundle in such forms as have no notopodium (e.g. Syllis). In some genera the setae are in See also:vertical rows, and in certain Capitellidae these rows so nearly meet that an arrange- ment occurs reminiscent of the continuous circle of setae in the perichaetous Oligo- chaeta. The setae vary much in form and are often longer and stronger than in the Oligo- chaetes. ointed setae and TMf" very short hooks or " uncini " (see fig. 3) are among the most remarkable forms. Simple bifid setae, such as those of Fm. 3.—a, Bristle of Pionosyllis Oligochaetes, are also present Malmgreni; b, See also:Hook of Terebella. in certain forms. Among the burrowing and tubicolous forms it is not uncommon for the body to be distinguish-able into two or more regions; a " See also:thorax," for example, is sharply marked off from an " See also:abdomen " in the Sabellids. In these forms the bundles of setae are either capilliform or uncinate, and the dorsal setae of the thorax are like the ventral setae of the abdomen. It is a remarkable and newly-ascertained fact that in regeneration (in Potamilla) the thorax is not replaced by the growth of uninjured thoracic segments; but that the anterior segments of the abdomen take on the same characters, the setae dropping out and being replaced in accordance with the See also:plan of the setae in the thorax of uninjured See also:worms.

Among the Oligochaeta the sexually mature See also:

worm is distinguished from the immature worm by the clitellum and by the development of genital setae. Among the Polychaeta the sexual worm is often more marked from the asexual form, so much so that these latter have been placed in different See also:species or even genera. The alteration in form does not only affect structures used in See also:generation; but the form of the parapodia, &c., alter. There are even dimorphic forms among the Syllids where the sexes are, as in many ' Polychaets, See also:separate. Nephridia.—The nephridia of the Polychaeta have been generally dealt with above in considering the nephridial system of the Chaetooda as a whole. They contrast with those of the Oligochaeta and fIirudinea by See also:reason of their frequently See also:close association with the gonads, the same organ sometimes serving the two functions of See also:excretion and See also:conveyance of the ova and spermatozoa out of the body. On the See also:hypothesis that such a form as Dinophilus (see See also:Haplodrili) has preserved the characters of the See also:primitive Chaetopod more nearly than any existing Polychaet or Oligochaet, it is clear that the nephridia in the Oligochaeta have preserved the See also:original features of those See also:organs more nearly than most Polychaeta. Thus Nereis among the latter worms, from the resemblance which its excretory system bears to that of the Oligochaeta, may be made the starting-point of a See also:series. In this worm the paired nephridia exist in most of the segments of the body, and their form (see fig. 2) is much like that of the nephridia in the Enchytraeidae. The funnel, which is not large, appears to open, as a See also:rule at least, into the segment in front of that which bears the See also:external orifice. Quite See also:independent of these are certain large dorsally situate funnel-like folds of the coelomic epithelium, ciliated, but of which no duct has been discovered leading to the exterior.

It is possible that we have here gonad ducts distinct from nephridia which at the See also:

time of sexual maturity do open on to the exterior. In Polynoe the nephridia are short tubes with a slightly folded funnel whose lumen is intercellular, and this intercellular lumen is characteristic of the Polychaetes as contrasted with leeches and Oligochaetes. Among the Terebelloidea there is a remarkable differentiation of the nephridia into two series. One set lies in front of the See also:diaphragm, which is the most anterior and See also:complete septum, the See also:rest having disappeared or being much less developed. The anterior nephridia, of which there are one to three pairs, contrast with the posterior series by their small funnels and large See also:size, the posterior nephridia having a large funnel followed by a short tube. In Chaetozone setnsa the anterior nephridia occupy five segments. There is usually a See also:gap between the two series, several segments beingwithout nephridia. It seems that the posterior nephridia are mainly gonad ducts, and the gonads are developed in close association with the funnels. The same arrangement is found in some other Polychaetes; for instance, in Sabellaria there is a single pair of large anterior nephridia, which open by a See also:common See also:pore, followed after an See also:interval by large-funnelled and short nephridia. This differentiation is not, however, See also:peculiar to the Polychaetes; for in several Oligochaetes the anterior nephridia are of large size, and opening as they do into the buccal cavity clearly See also:play a different See also:function to those which follow. In Thamnodrilus, as has been pointed out, there are two series of nephridia which resemble those of the Terebelloidea in the different sizes of their funnels. In Lanice conchilega the posterior series of nephridia are connected by a thick See also:longitudinal duct, which seems to be seen in its most reduced form in Owenia, where a duct on each side runs in the epidermis, being in parts a groove, and receives one short tubular nephridium only and occupies only one segment.

This connexion of successive nephridia (in Lanice) has its counterpart in Allolobophora, Lybiodrilus, and apparently in the Lumbriculids Teleuscolex and Styloscolex, among the Oligochaeta. Among the Capitellidae, which in several respects resemble the Oligochaeta, wide and short gonad ducts coexist in the same segments with nephridia, the latter being narrower and longer. It is noteworthy that in this See also:

family only among the Polychaeta, the nephridia are not restricted to a single pair in each segment; so that the older view that the gonad ducts are metamorphosed nephridia is not at variance with the anatomical facts which have been just stated. Alimentary Canal.—The alimentary canal sf Polychaetes is usually a straight tube See also: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 See also: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 See also:fish's air-See also:bladder. In other Polychaetes one or more pairs of similar outgrowths are glandular.

The See also:

intestine is provided with numerous branched caeca in See also:Aphrodite. Reproduction.—As is the See also:case with the Oligochaeta, the Polychaeta furnish examples of species which multiply asexually by budding. There is a further resem- blance between the two orders of Chae- topoda in that this budding is not a See also:general pheno- menon,but See also:con- fined 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 Aeoloso- matidae and Nai- didae that show the same phenomenon. It has been men. tioned 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 See also: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 schizo- gamy. A similar See also:life See also:history distinguishes certain Sabellid worms, e.g. Filigrana. Among the Syllids this simple See also:state of affairs is further complicated.

In Autolytus there is, to begin with, a See also:

conversion of the posterior See also:half of the body to form a sexual zooid. But before this separates off a number of other 4' ys which appears between the two first-formed znlracta, Kr. (After individuals. Ultimately, a See also:chain of sexual Malmgren.) zooids is thus formed. A given stock only produces zooids of one See also: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 b a See also:zone, and they themselves produce other buds, so that a ramifying See also:colony is created. Quite recently, another mode of budding has been described in Trypanosyllis gemmipara, where a See also: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 A of the See also:parent in a See also:regular See also:line, and develop in situ; this process, which has been attributed to budding, is an " external gestation," and occurs in a number of species. As is very frequently the case with marine forms, as compared with their fresh-water and terrestrial See also:allies, the Polychaeta differ from the Oligochaeta and Hirudinea in possessing a free living B ms, Larval muscle. o, Otocyst. pp, Parapodium.

pr, Praeoral ciliated See also:

ring, or prototroch. larval form which is hatched at an See also:early See also:stage in development. This larva is termed the Trochosphere larva, and typically (as it is held) is an See also:egg-shaped larva with two bands of See also:cilia, one preoral and one postoral, with an apical nervous See also:plate surmounted by a tuft of longer cilia, and with a simple See also:bent alimentary canal, with lateral mouth and posterior anus, between which and the ectoderm is a spacious cavity (blastocoel) traversed by See also:muscular strands and often containing a larval See also: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 See also:chief modifications of this form are seen in the Mitraria larva of Ammochares with only the preoral See also:band, which is much folded and which has provisional and long setae; the atrochous larva, where the covering of cilia is See also:uniform and not split into bands; and the polytrochous larva where there are several bands surrounding the body. There are also other modifications. See also:Classification.—The older arrangement of the Polychaeta into Errantia or free living and Tubicola or tube-dwelling forms will hardly See also:fit the much increased knowledge of the group. W. B. Benham's division into Phanerocephala in which the prostomium is See also:plain, and Crytocephala in which the prostomium is hidden by the peristomium adopted by See also: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 See also: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.

(I) 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 " See also:

appearance, and with some See also:affinities to that order. The peristomium has no setae, and the setae generally are See also:hair-like or uncinate, of ten 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 (After See also:Montagu.) Arenicola marina, L. segment. In other forms genital ducts and nephridia coexist in the same segment. The nephridia are sometimes numerous in each segment.

There is no See also:

blood system, and the coelomic corpuscles contain A, Anus. E, See also:Eye. M, Mouth. ap. Apical organ. h, Head kidney." i, Intestine. 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 Chaetoplerus, 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.

End of Article: POLYCHAETA

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