CHAPTER X

CLASSIFICATION OF THE POLYCHAETA—SHAPE—HEAD—PARAPODIA—CHAETAE—GILLS—INTERNAL ORGANS—JAWS—SENSE ORGANS—REPRODUCTION—LARVAL FORMS—BUDDING—FISSION—BRANCHING—REGENERATION.

The Polychaeta are marine worms whose bodies are usually elongated and cylindrical; they either lead a free life, swimming in the open sea, or crawling along the bottom; or they pass their life in burrows or definite tubes of various kinds.

Each segment is normally provided on each side with a single or a couple of bundles of chaetae, by means of which locomotion is effected. These, in the free-living forms, are carried at the ends of lateral muscular outgrowths of the body, known as "parapodia," which are practically limbs.

The "head" of the worm generally carries eyes, and frequently more or less elongated tactile organs, the "tentacles" dorsally and "palps" ventrally. The foregut is frequently provided with a masticating apparatus in its anterior region, which is capable of protrusion; but this apparatus is absent in many burrowing and tubicolous forms. The sexes are separate, so that there is no such complicated system of generative organs as occurs in the Oligochaeta. The nephridia usually act as genital ducts. In the majority of cases the egg develops into a larva, the "Trochosphere," which leads a free life and undergoes a greater or less metamorphosis into the adult condition.

The classification of Polychaeta adopted in this work is as follows:^[312]—

Comparative Anatomy of the Polychaeta.

General Shape of the Body.—The majority of the Polychaeta have an elongated and very mobile body, like that of Nereis, consisting of an indefinite and usually of a considerable number of segments; a few, however, have a shorter body, with fewer segments, definite in number, for instance Aphrodite and Polynoë, which have thirty to forty segments; and some Hesionids, with only some seventeen to twenty segments.

In Aphroditidae and certain Amphinomidae the body is more or less oval in shape. In Lipobranchius and Sternaspis it is grub-like, short, and cylindrical, with rounded ends; in the former it is difficult to distinguish head and tail, or dorsal and ventral surfaces.

The segments composing the trunk may be all alike, or may constitute two more or less sharply marked regions, the thorax and abdomen, differing in the character of the chaetae, or in their arrangement, or in some other way, as in the Sabelliformia and the Capitelliformia.

As peculiar cuticular structures, the curious shields of Sternaspis, and of certain of the Maldanidae may be mentioned.

The posterior extremity is generally more or less narrowed, and most of the Nereidiformia are provided with special elongated cirri, borne by the anal segment. In the Maldanidae and others the body terminates in a funnel, at the bottom of which is placed the anus. Only in a few cases is the anus not terminal; in Notopygos and other Amphinomidae, as well as in some species of Polynoë, it is dorsal.^[313] In Sabellaria and Pectinaria the hinder end of the body undergoes great degeneration; in the former it is achaetous, but cylindrical and bent forwards alongside the body (Fig. 131). In Pectinaria (Fig. 177), this region, which is called the "scapha," is leaf-like, and serves to close the narrower end of the tube in which the worm lives. Arenicola marina, and some Terebellids have no chaetae in the hinder, narrower part of the body.

The Head.—The prostomium is, in the majority of cases, rounded or conical, though it may be square (Nephthys) or elongated and jointed (Glycera), or even hammer-shaped (Tomopteris); or it may be fused with the peristomium, and apparently absent (Arenicola). In the great group Cryptocephala, the peristomium grows forwards so as to hide the prostomium entirely.

In a few of the Nereidiformia the prostomium is compressed, and in the Amphinomidae it is provided with a dorsal ridge or "caruncle," which is a leaf-like process overlapping three or more segments. In many Aphroditidae (as well as in Polydora) there is a peculiar "frontal" ridge passing forwards from the prostomial tentacle, and downwards into the mouth (Figs. 132, c, and 133, A, x).

In all the Nereidiformia, as well as in Sabelliformia and Chlorhaemidae, the prostomium bears sensory processes of two kinds, viz. dorsal tentacles and ventral palps. The latter are invariably two in number, and are particularly well developed in Aphroditidae, Nereidae, Syllidae, some of the Eunicidae, and in Chlorhaemidae. Even when they are apparently absent, as in Nephthys, it is possible that they are represented by certain lobes at the sides of the mouth, for in many Syllidae they are so fused with the prostomium as to be scarcely distinguishable. In the Chlorhaemids the palps^[314] are grooved, and in the Sabelliformia they become considerably branched, and extend round the prostomium so as to nearly meet dorsally and ventrally. Each palp is, in this sub-Order, represented by a greater or smaller number of long, mobile filaments, arising from a common base; they are grooved along the inner side, ciliated, and provided with secondary processes. The crown of "gills," in fact, is nothing more than the greatly subdivided and enormously elongated palps, as both Pruvot^[315] and Meyer^[316] have shown. In such forms as Haplobranchus and Amphicorine the process of subdivision (branching) has only gone a short way. In all the Sabelliformia each filament, in addition to its sensory function, aids in conveying food to the mouth by the action of the cilia, and has a blood-vessel within, thus acting as a respiratory organ. The filament may carry compound eyes (Fig. 143) either at its apex (Branchiomma) or at intervals along its course (Dasychone).

In the family Serpulidae one (rarely two) of the most dorsally placed gill filaments is enlarged terminally, and acts as a stopper or "operculum," which closes the mouth of the tube when the animal withdraws into it. Further, in Spirorbis this operculum is grooved on one side, and serves as a brood pouch in which the eggs undergo development (Fig. 184, p. 341). It will be seen, therefore, that the palps may be very important organs for the life of the worm, and they are no less interesting to the comparative anatomist, serving as they do as an excellent illustration of the various uses which Nature finds for one and the same organ.

In the other sub-Orders the prostomium carries neither palps nor tentacles.

The tentacles in the Nereidiformia present a wide variation in number; probably the typical number is three, one of which is median and two lateral—as in Polynoids, Syllidae, and some Eunicidae. Further, there is a certain amount of evidence in the nerve supply of the median tentacle to show that it was originally double. The presence of four tentacles, then, as in Nephthys, Phyllodoce, and Glycera, may be a primitive condition. By the disappearance of the paired lateral tentacles the worm possesses a single median one, as in Aphrodite and Amphinomids;^[317] whilst a duplication of these lateral ones leads to the condition of Eunice and Hyalinoecia, which have five tentacles. In the Chlorhaemidae the number is further increased to five or more on each side,^[318] and in the Terebellidae these prostomial processes become very numerous.

In the Cryptocephala there is never more than a single pair of tentacles, and these are generally reduced to a group of sensory cells, though in Sabellaria they retain a considerable size.

In a few genera, such as Aphrodite, Nephthys, Capitella, the first postoral segment is distinguished from the succeeding segments only by its position with regard to the mouth (Fig. 132) and by its smaller size. But in the remainder of the Polychaeta, with here and there an exception, the peristomium is achaetous in the adult.^[319]

Except in the Nereidiformia, peristomial or tentacular cirri are rare, being represented in the Spioniformia by the very long "tentacles." In the Nereidiformia one or more of the following segments may be added to the peristomium, and share in the "cephalisation," which is so characteristic a feature in this group. In Amphinomids the first three or four chaetigerous segments are incomplete ventrally, owing to the shifting of the mouth backwards; these segments form lateral lips, but they are not otherwise modified. In Phyllodoce, however, there are four cirri on each side of the mouth, and from the arrangement in the Alciopids we are justified in concluding that the segment which carries the four pairs of cirri is really made up of three segments (Fig. 134, E). Among the Hesionids there are four such "cephalised" achaetous segments with long cirri.

In a few cases, such as the Chlorhaemids and Sternaspis, and to a slight degree in Arenicola, the "head" and even the anterior part of the worm is capable of being withdrawn into the body.

The Parapodia and Chaetae.—The typical parts of a parapodium have been described in the preceding chapter; here it is only necessary to refer to the series of diagrams (Figs. 136, 137) representing the parapodia of the more common Polychaetes, and to add a few remarks about them.

In most Annelids the chaetae are in two bundles on each side, but there are certain families in which the dorsal bundle, and even the notopodium itself, is absent, as in the Eunicidae, Syllidae, and Phyllodocidae; or the dorsal bundle may be absent only in certain regions of the body, as in the hind-body of Terebellids. In some Amphinomidae and Aphroditidae the notopodium is scarcely distinct as a separate lobe, being a slight tubercle on the upper surface of the neuropodium; but the notopodial chaetae are present, and indeed particularly well developed in many cases.

But whilst, in the Nereidiformia, the parapodia, whether consisting of two lobes or only one, are always well developed, and project to a more or less pronounced degree from the sides of the body, it is otherwise in the rest of the group, where the chaetigerous lobes are usually reduced to mere tubercles or ridges, no doubt in relation to their burrowing or tubicolous habits. In Sternaspis the chaetae issue directly from the body-wall.

Amongst the Nereidiformia we find examples in which the parapodia, instead of being more or less conical "legs," are flattened fore and aft so as to serve as efficient "fins," as in the active swimmers, Nereis virens and Nephthys caeca, and in the pelagic Phyllodocids, Alciopids, Typhloscolecids, and Tomopteris.

Of the typical dorsal and ventral cirri, the ventral is only absent in some Amphinomids amongst the Nereidiformia; the dorsal is absent in Nephthys and degenerate in Glycera, whilst in a very large number of families of the other sub-Orders neither cirrus is present. These cirri, though originally filamentous and sensory, may, by virtue of special blood supply, become "gills," and this occurs in several families of different sub-Orders. Thus in Eunice this gill is comb-like; in Amphinome and in Arenicola (on certain segments) it is arborescent, as it is also in one to three segments in Terebellids; whilst in Ariciidae, Spioniformia, Cirratulidae, Opheliidae, and Sabellaria it remains more or less finger-shaped or filamentous. In the family Serpulidae the thoracic cirri, both dorsal and ventral, become flattened and extended antero-posteriorly, and unite with one another to form the "thoracic membrane."^[320] In Phyllodocidae the cirri are foliaceous and natatory, and they contain a great quantity of glands of a peculiar character. The Aphroditidae are distinguished from other Annelids by the possession of "elytra" or dorsal scales, which appear to be the dorso-ventrally flattened cirri, retaining their sensory nature, but adding to this function several others.^[321]

The chaetae or bristles are mainly used in locomotion, but it is not unreasonable to believe that some of the stronger, serrated kinds may be used as weapons of offence and defence; certainly the Polynoids, bristling as they do with stiff chaetae along each side, must be rather unpleasant to their smaller enemies.

The various bristles may be placed in three chief groups, viz. (1) simple; (2) jointed; (3) uncini (see Fig. 138).

(1) The simple chaetae may be smooth and hair-shaped, i.e. "capillary," such as are present in nearly all families: or they may be forked (Amphinomidae), comb-shaped (Eunice), notched or serrated, or provided with a series of frills at right angles to their length, as in Aphroditidae; or fringed along one or both sides with a membranous expansion, as in Terebellids and Sabellids. The simple chaetae may also be short and spine-like, as in the ventral bundles of Arenicola; or they may be slightly curved at the end and notched, forming what are generally termed "crotchets," such as are common amongst Oligochaeta. These "crotchets" may be simple, or have numerous denticulations at the end (Maldanidae), or be provided with a membranous hood (Spioniformia, Capitelliformia). In Hermione peculiar sheathed, spear-like bristles occur (Fig. 138, N).

(2) Jointed chaetae have already been described (p. 246); they are confined to the sub-Order Nereidiformia, and occur only in certain families.

(3) The uncini are very short chaetae, which are simply embedded in the skin, and do not extend beyond the body-wall into the body-cavity. An uncinus is a sharply curved hook, which may have more or less numerous secondary teeth on it. They are characteristic of the Sabelliformia and the Terebelliformia.

The chaetae appear as solid, usually fibrillated structures, of a yellow or golden tint, transparent and refringent. Chemically they consist of chitin, and each chaeta is the product of a single cell. The chaetae of Euphrosyne are hollow and calcareous, being peculiar in both characters.

Certain modifications of the chaetae presented by various worms deserve mention. In Polydora (Fig. 133, A) and in Chaetopterus (Fig. 173, p. 324) those of one segment are especially strong, but their significance is uncertain. In Capitella those of the notopodium of the eighth and ninth segments are specially modified; they are analogous to the copulatory chaetae of Oligochaeta. In Aphrodite, in addition to the ordinary locomotor chaetae, there are brilliant, iridescent bristles and peculiar felting threads arising from the indistinct notopodium; these latter, however, are not true "chaetae," but are separate chitinous filaments similar to the constituent fibres of an ordinary chaeta.^[322]

While the chaetae in the Nereidiformia and others are grouped in bundles, those of many other families are in vertical, transverse rows, as in Maldanidae and in Arenicola. The uncini are always embedded in such rows, usually slightly raised from the general level of the body surface, each being termed a "torus uncinigerus." These tori are usually limited to the sides of the body, but in Myxicola and in Notomastus they encroach upon the dorsal surface, and in Chaetozone, also upon the ventral, so as nearly to encircle the body, recalling the "perichaetous" condition of some earth-worms.

Gills.—We have already seen that several different organs, e.g. the palps in Sabelliformia, the prostomial tentacles of Chlorhaemidae, and the notopodial cirri of sundry other Polychaetes, may take on a respiratory function. There are, however, certain "gills" developed either on the parapodium itself or elsewhere on the body which it is difficult to homologise. Such are the retractile gills on the parapodia of the Glyceridae (Fig. 136, C); those of Dasybranchus, near the abdominal neuropodia; those of Mastobranchus, near the notopodia. Nephthys has a sickle-shaped gill on the under surface of the notopodium. The long gill filaments at the posterior end of Sternaspis, again, are only doubtfully interpreted as the dorsal cirri of some of the posterior segments.

Since primitively the whole skin of the worm is respiratory, any part of the skin may become more or less specialised for this function, and chiefly, of course, on the more actively moving parapodia. The blood-vessels constituting the essential part of the "gill" may make use of any already existing outgrowth (such as a cirrus or a tentacle), or may push the body-wall out on their own account.

Internal Anatomy.

Probably those organs which have the greatest effect in modifying the shape of the body are the septa, for we find in the long, free-swimming worms that these are regularly present throughout the body, and external "segmentation" of the body is well marked. In burrowing and tubicolous forms the septa are frequently incompletely developed, or more or fewer may be absent; and the body becomes less distinctly segmented externally, tends to vary greatly in diameter during movement, or becomes plumper. With the disappearance of the septa there is also a diminution in the number of nephridia, as in Arenicola, with only six pairs. Further, there is frequently a dimorphism of these organs; instead of all of them serving equally as excretory organs and as genital ducts, some of the most anterior in the Sabelliformia and Terebelliformia become greatly enlarged, and take on practically the whole of the former function; whilst more or fewer of the posterior nephridia dwindle in size, and become genital ducts. The absence of septa allows a free communication between the successive segments, and thus a freer flow of coelomic fluid for the distension of the anterior end of the worm during burrowing.

The alimentary system presents certain modifications of a systematic value. In the Nereidiformia the muscular pharynx, which is always protrusible and is preceded by an eversible buccal region, frequently encloses thickened cuticular plates which serve as crushing and grasping organs. The form, number, and arrangement of these "jaws" vary in the different families. They form valuable fossil records of extinct worms.

In the Scoleciformia and Capitelliformia the buccal region exists, but there are no jaws. In the Sabelliformia and Terebelliformia eversion does not take place and jaws are absent.

Amongst the Nereidiformia the jaws are absent in the Phyllodocidae and Hesionidae; when present they are usually set in the direct course of the food. There may be one small tooth used for stabbing, as in some Syllids (Fig. 141, A); or a circle of such denticles (Autolytus, Fig. 140, D). To these are added powerful grasping jaws in Nereis (E); or the latter may alone be present, as in Glycera (F). In Polynoë the four jaws are carried by hard pieces, to which the muscles are attached (C and G). In Nephthys there is a dorsal and a ventral jaw.

In the Eunicidae, however, the numerous denticles are carried in a special pouch below the food tract, with which it communicates anteriorly.^[323] They are arranged in an upper and lower series. The lower series (L) consists of a pair of flat plates (k) on each side partially embedded in and acted upon by muscles, with a harder enamelled piece—the actual lower "tooth" (j)—at its anterior end. The upper series (U) consists of several pieces, varying in shape and size in the various genera of this family; but developmentally they result from modifications of two rows of small, similar pieces.^[324]

The intestine is generally straight and cylindrical, and is usually constricted by the septa, if these are present. In the Polynoids the intervening sacculations become so long as to receive the name of "caeca," which, in Aphrodite, become enormously elongated (Fig. 142); there are eighteen pairs of them (c), each being a slender tube bent upon itself, giving off short branches and dilated distally, where it lies in the base of the parapodium.

The intestine is looped in a few genera only, as in Trophonia, or coiled, as in Sternaspis, Petta (Amphictenid, Fig. 141, C), and Ammotrypane. In the course of the tube there may be a thick-walled muscular gizzard, with hard chitinous lining, as in certain Terebellids, where it appears to replace, in function, the pharynx of the Nereidiformia; in the Syllidae the gizzard is present in addition to the pharynx (Fig. 141, A).

Glandular appendages of the oesophagus are present in many worms. Amongst the Nereidiformia, the Syllidae and Hesionidae possess oesophageal diverticula (Fig. 141, A, d), which are used, not for secreting a digestive fluid, but as reservoirs for water and air swallowed by the worms; and are provided with muscular walls, by which their contents can be driven out. They appear, in fact, to be used like the swim-bladder of fishes.^[325] Many Chaetopods take in water by the anus—no doubt for respiratory purposes—and pass it forwards along the intestine. In the Capitelliformia a special groove conducts the water for some distance, then the groove becomes closed to form a canal, which, after a course forwards as a free tube below the intestine, again enters the latter, constituting a "siphonal apparatus," similar to that of the Echiuroids and the sea urchins.

Sense Organs.—In addition to the prostomial eyes, which are present in nearly all the Nereidiformia and Spioniformia, eyes may exist elsewhere on the body: thus Myxicola infundibulum and Fabricia possess a pair on the anal segment; in M. aesthetica Clap. there is a pair to every segment; in Branchiomma there is a compound eye near the tip of each gill filament (i.e. palp); whilst in Dasychone a series occurs along each gill filament. All these examples belong to the Cryptocephala, in which, owing to certain peculiar modes of life, these sense organs are required in correspondingly peculiar positions. It is usually stated that Polyophthalmus possesses, in addition to the usual prostomial eyes, twelve pairs on as many successive segments; but the minute structure of these organs points rather to their function as light-producing organs.

The Capitelliformia and Opheliidae possess a pair of peculiar "ciliated pits" or "nuchal organs" at the upper side of the head, between the prostomium and peristomium, and capable of eversion (Fig. 144). They are most characteristically developed in the Capitelliformia, where each organ abuts upon a special lobe of the brain. The function of these "ciliated organs," which bear a great resemblance to those of the Nemertines, is a matter of speculation. Similar organs, in the form of simple pits or grooves, occur in many of the Nereidiformia, Terebelliformia, and others.^[326]

Otocysts are rare. Arenicola possesses a pair at the base of the prostomium, each of which in some species retains an opening to the exterior.^[327] They probably serve as "organs of direction" rather than of "hearing." Aricia and Polyophthalmus likewise have such organs on the prostomium; whilst Fabricia, Myxicola, Terebella, and a few others possess them in the peristomium, or in some other segment of the body.