Chapter XVI: Introduction: Nematoda—anatomy—embryology—classification—ascaridae (6)

Whilst some species, such as the common British _N. diversicolor_, undergo no change, and others become modified as just described, others, again, are polymorphic. Claparède was the first to show that _N. dumerilii_ may occur in at least five different mature forms; these differ from one another in size, colour, mode of life, character of the eggs, etc. The immature forms may become ripe and lay eggs while still retaining the "Nereid" characteristics, or these immature forms may become "Heteronereids"[333] whilst the sexual elements are ripening. There are then three different kinds of males and of females in this one species, some being found at the bottom of the sea, as the large Heteronereid form, while the small Heteronereid swims on the surface. The relations of these various forms to one another, and the causes leading to the {278}assumption of a Heteronereid condition in some cases and not in others, are unknown.

A somewhat similar phenomenon is exhibited by members of the family Syllidae.[334] In this family sexual reproduction is frequently accompanied by the asexual modes of fission and gemmation. In some genera, such as _Eusyllis_, _Odontosyllis_, and _Exogone_, there occur changes quite similar to those characterising "Heteronereis"—that is, the posterior segments in which the genital organs exist become altered, so that the worm consists of two distinct regions, and is termed a "Heterosyllis." The most marked change is the appearance of a dorsal bundle of long capilliform chaetae in each of the genital segments (Fig. 148, I).

But in other genera the hinder genital region of the body becomes _separated_, on maturity, from the anterior non-sexual region. Various stages of this "schizogamy," or fission into a sexual and a non-sexual zooid, have been observed in different genera. In the genus _Syllis_ the first segment of the sexual zooid, after its separation from the asexual zooid, proceeds to bud forth a head. The character of the head is alike in both sexes, though different species present heads of different shapes; and as the worms were originally described as distinct genera, the names then given are retained as descriptive terms. Thus the "Chaetosyllis" form has only two tentacles; the "Ioda" form has three tentacles and a pair of palps. One and the same species (e.g. _S. hyalina_) may successively pass through these stages.

With regard to the asexual portion, there is a regeneration of the tail segments after the sexual zooid has separated; and the number of segments so regenerated is usually equal to those that have become sexual. After a time these newly formed segments will produce generative organs, and take on the characteristic natatory chaetae, and this region will in its turn separate.

But in other genera, such as _Autolytus_, the regeneration of segments may commence _before_ the separation of the sexual zooid; and the head of the sexual zooid becomes budded out _before_ separation from the asexual portion. So that the animal now consists of two worms, each with its own head, separated by {279}a region or zone of proliferation (Fig. 148, IV). Moreover, in some species not only is the hinder part of the body _converted_ into a sexual zooid, but the zone of proliferation becomes very active, and produces by gemmation a large number of segments, which become marked out, by the appearance of heads at intervals, into a number of zooids, in which genital organs will later make their appearance. A chain of as many as sixteen zooids may be formed in _Autolytus_ (Fig. 148, V)—the hindermost by _conversion_ of the hinder part of the body of the original "stock," the intervening zooids by _gemmation_.

I, Heteronereid or Heterosyllid stage. A, Non-sexual; A', sexual region of the body, with modified parapodia.

II, _Syllis_. The hinder sexual region, B, is similarly modified, and will separate from the parent zooid, A, and become an independent zooid.

III, _Autolytus_. The hinder zooid, B, develops a head by budding before separation.

IV, _Autolytus_, etc. A zone of budding (_z_) makes its appearance in front of the head of B, and by its growth will give rise to a series of new segments in the middle of the body.

V, _Myrianida_, _Autolytus_, etc. From this zone of budding a very large number of segments have been formed, which have, further, become grouped so as to form three individuals, C, D, E; B is the hindmost zooid, which is either formed from the hinder segments of the parent zooid or is produced by budding, like C, D, E.]

One original "stock," or asexual zooid, thus produces several sexual zooids, but these are only of one sex for a given stock. The males differ in several important characters from the females; so different, indeed, are the two sexes that before their history was {280}worked out by Agassiz[335] they were placed in different genera. The male zooid has thus come to be known as _Polybostrichus_ (Fig. 149, B). It has three tentacles and two bifid palps; there are two pairs of peristomial cirri; the testes are confined to the four anterior segments, which are without natatory chaetae. The female is termed _Sacconereis_, owing to the possession of a great ventral brood sac; its head possesses no separate palps; the peristomium carries only one cirrus on each side; ova occur in every segment of the body, and may even extend into the hinder segments of the asexual zooid (Fig. 149, C).

A further development of this process of gemmiparity is exhibited by _Myrianida_. Here, there is no conversion of the hinder segments, but the normal preanal zone of proliferation gives rise to a large number of new segments. After a time the most anterior of these becomes a head, and thus a new zooid {281}is marked out. The zone of proliferation immediately in front of the new head now proceeds to form new segments, and a second zooid results. This process goes on till a considerable number of new worms have been formed at the tail of the original one, the oldest of these new ones being the most posterior, the youngest next the original "stock." In each zooid there is a zone of activity which adds to its number of segments, so that as we pass backwards the zooids increase in size. As many as twenty-nine such zooids may be formed in this way entirely by gemmation; and as each zooid becomes completed, genital organs make their appearance, and when these are ripe the zooid separates from the "colony" and leads an independent life. Here, as in _Autolytus_, the sexes are dimorphic, the male and female resembling those of that genus.

The process of gemmation, as seen in _Autolytus_, closely resembles that exhibited by certain Oligochaeta (_Naididae_), where there exists a definite alternation of generations; the production of new individuals by gemmation occurring throughout the greater part of the year, and sexual reproduction recurring only at certain intervals. In the Polychaeta such alternation exists in _Myrianida_; but it is only the terminal link of a series, which takes its starting-point in the process exhibited by the majority of Annelids, where no sexual character marks maturity. The next stage is presented by "epigamous" forms like Heteronereis and Heterosyllis; then "schizogamy" makes its appearance in certain Syllidae, resulting in the formation of two morphologically and physiologically distinct individuals which lead independent lives. The appearance of a head and of a zone of proliferation leading to the formation of a chain of sexual zooids is accompanied by a delay in the appearance of the genital organs, for in _Autolytus_ these arise _during_ the formation of the new individuals, as part of the general process of new formation; whilst in _Myrianida_ the delay is prolonged, and the generative elements do not make their appearance till _after_ the new individuals have reached some size.

More simple cases of the separation of the body into two parts, sexual and asexual, occur also in some of the Serpulidae. Thus in _Filigrana_ and _Salmacina_ the generative elements make their appearance in the hinder segments, as they do throughout the Sabelliformia; and this hinder part of the body separates {282}from the anterior region after the formation of a new head between the two regions.[336]

Another modification of the process of budding and fission is exhibited by _Syllis ramosa_, one of the most interesting forms of animal life which was obtained by the "Challenger." This worm consists of a main stem, whence arise a number of lateral branches, which may also branch so as to give rise to an arborescent colony (Fig. 150). The branches of the first and second and higher orders arise by budding from the sides of the original form or branches of lower order; and some of these branches develop generative products, and bud forth a head near the point of attachment. These sexual branches, no doubt, separate from the colony and distribute the ova. The worm lives in a Hexactinellid sponge, _Crateromorpha meyeri_, living in depths of 95 to 140 fathoms in the Eastern seas.[337]

REGENERATION OF LOST PARTS.—The process of budding and fission of the worm into two parts is merely an extension of that resulting in the formation of new segments when the worm is injured. In most of the Nereidiform Polychaetes the number of segments forming the body continues to increase throughout life by the formation of new segments between the anal segment and the one in front of it; that is to say, there is normally a process of budding taking place at this point. Now in many of the longer worms it may be noticed that the segments of the hinder end suddenly become smaller than the rest; these are segments newly formed to replace those lost by the worm. But this "regeneration," though the same in principle as ordinary growth {283}at the penultimate segment, is due to activity in a segment (any segment) further forwards; in other words, in the less modified worms every segment has the power of forming new tissues, just as each of the joints of a crab's leg has the power of forming the remaining joints when injured. It is not therefore surprising that a "zone of budding" arises in an uninjured worm at certain seasons, viz. that of reproduction; it is a property that each worm possesses, though generally it remains latent till injury provides the stimulus.

Moreover, not only can new segments arise at the hinder end, but a new head can be formed at the anterior end, as has been observed in worms belonging to many families—in the less modified Syllidae,[338] in others of the Nereidiformia, and even in Sabellids, where the greatly specialised gill filaments can be reproduced. Thus Sir J. Dalyell[339] noted in _Dasychone_ that the crown of branchiae was regenerated in about a month in springtime, while in winter the process occupied 116 days. He cut a _Dasychone_ into three pieces; the hindermost produced a head, the anterior piece developed an anus, and the middle portion formed both a head and tail!

These regenerated heads are of course at first smaller than the rest of the body, but soon grow to a normal size. Naturally this extensive power of regeneration is of extreme value to the Polychaetes, for if a fish or other enemy bites the head off a worm, a new one can form; and it is not difficult to see in this the origin of the reproduction by fission as a normal process.

{284}CHAPTER XI

NATURAL HISTORY OF POLYCHAETES—GENERAL HABITS—CHARACTER OF TUBE AND ITS FORMATION—COLOURING—PROTECTIVE AND MIMETIC DEVICES—PHOSPHORESCENCE—FOOD —USES—ASSOCIATED WORMS—WORMS AS HOSTS—DISTRIBUTION—FOSSIL REMAINS.

All the many hundreds of species of Polychaetes are marine, with a very few exceptions, which have been in recent years recorded from fresh (_i.e._ drinkable) water, viz. a species of _Nereis_ from a lake in Mingrelia, another _Nereis_ and a _Lumbriconereis_ from running water in Trinidad,[340] a Sabellid, _Manayunkia speciosa_,[341] from Philadelphia; and another Sabellid, _Coabangia_,[342] from fresh water at Tonquin, which lives in borings in shells of _Melania_; and it is by no means improbable that other fresh-water Polychaetes exist in Lake Tanganyika in Africa, where a Medusa has recently been discovered.

In brackish water of various densities many Polychaetes live; _Arenicola_ especially is regardless of the character of the medium, and _Nereis diversicolor_ appears to withstand considerable admixture of fresh water.

The majority of the Polychaetes occur "inshore," that is, between tide-marks and in shallow water down to 20 fathoms; but they occur at all depths more or less abundantly, and some have been dredged from depths of more than 3000 fathoms.

The nature of the soil composing the shore has a good deal to do with the number of worms to be found there; thus in calcareous districts they are fewer than in places where harder rocks, {285}such as granite, form the shore line, for the chalk or limestone wears away more quickly, and exposes to destruction the worms which may have sheltered in its crevices: further, it does not give so permanent a place of attachment to seaweeds, on which many Polychaetes feed. The calcareous rocks, too, are more likely to be traversed by springs of fresh water, which is not to the taste of the worms. The sand resulting from the destruction of the rocks, whether hard or soft, is of itself unsuitable to the majority of worms, which are most abundant where mud containing decaying vegetable matter is mixed with the sand: this, which gives a firmer consistency to the soil, so that the burrows retain their form better, supplies food for the burrowers.

GENERAL HABITS.—The division of the Polychaetes into the "Errantia" or free-swimming and wandering forms, and "Sedentaria" or tubicolous and sedentary forms, is a misleading mode of classification, for as a matter of fact only a comparatively few forms are really free-swimming throughout life; the majority, even if they do not form definite tubes, burrow galleries for themselves in the soil, and these burrows are in many cases only rarely left; this is true of both groups. Amongst the "errant" Polychaetes nearly all the Eunicidae secrete a parchment-like tube, and some Polynoids form mud tubes. Among the "sedentary worms" there are forms which merely burrow; while _Myxicola_ readily leaves its gelatinous tube and swims freely; _Pectinaria_ carries its house with it as it moves about, and _Polycirrus_, a Terebellid, does not form any tube at all.

Owing to their sedentary habits, quite a representative collection of genera may be made, especially at a spring tide, at any seaside place which is provided with a sandy shore, and with rocks and seaweed. The larger species, however, require to be dredged, and the best time is at night, for then many forms which during the day are concealed in their burrows, will be issuing forth to obtain food.

It may be useful to give instances of worms occurring in various situations between tide-marks. Throughout pretty well the whole of the area left uncovered by the tide, even up to nearly high-water mark in many parts of the coast, the cylindrical "castings" of sand and mud, forming little heaps, indicate the burrows of _Arenicola_, the common "lug-worm"; these "castings" have passed through the worm's body, having been {286}swallowed during the process of burrowing as well as for the purpose of obtaining food, as in the case of the earthworms. Rather nearer the water may be seen little tufts of sand-threads, about an inch high, springing from a short piece of cylindrical, sandy tube rising up out of the sand; this is the head end of the tube of _Terebella conchilega_ (Fig. 153).

Amongst the rocks may be found loose stones of different sizes; on lifting them up, various kinds of worms may be brought to light, according to the locality, the time of year, the position with respect to the sea, and so on. _Polynoë_ is pretty sure to be present somewhere near low-tide mark; the number of species is considerable, and their colouring very varied: but as the worms have a habit of remaining still on the under surface of the uplifted stone, the observer may easily overlook them.

Other worms occur below the stones, more or less buried in the sand or mud; for instance, a small _Nereis_ may be lying in its temporary burrow immediately underneath, and will at once withdraw from the now injured part of the burrow; while deeper in the mud or sand, especially in rather highly-smelling mud, little red worms are abundant, such as _Scoloplos_, _Nerine_, _Capitella_, and others. By digging near low water one may find _Nephthys_, _Glycera_, and others burrowing or hiding in the soil.

In rock pools, or sandy stretches amongst rocks kept moist and cool by abundant _Fucus_, one may see under stones the red or yellow gill filaments of _Cirratulus_ and of Terebellids protruding from their burrows and tubes, while other worms are to be met with in clefts of the rocks, and amongst the roots of _Laminaria_.

Still farther out, below low-water mark, where one must wade, can be seen the beautiful branchial crowns of various Sabellids protruding from their tubes; but care is necessary on approaching these worms, as eyes are, in many cases, present on the branchiae and a shadow is readily perceived; then the brightly-coloured tuft disappears, and only a piece of sandy or muddy cylindrical tubing remains to tell where the _Sabella_ has withdrawn. In order to obtain the worms one must dig quickly and deeply before they have been disturbed; for the tube is of considerable length, and the inhabitant withdraws to the bottom of it. Some of these soft-skinned worms have the power of boring into hard rocks,[343] though by what means they do so is {287}uncertain.[344] _Polydora ciliata_ makes a tube of mud projecting from the mouth of U-shaped galleries in chalk, limestone, shells, and even shale; it has no hard jaws or other structures sufficient to account for the holes, but it is possible that the specially strong chaetae on the sixth segment may be of some use in this work. Other lithodomous worms are _Sabella saxicava_ and _Dodecaceria concharum_, which is a common little borer, forming galleries in oyster-shells, etc.

The TUBES formed by these Polychaetes are very varied in constitution.[345] In some cases a mucus, which hardens to form a firm protective envelope, is secreted from special parts (_e.g._ the ventral gland shields of Terebellids and Sabelliformia), or from the greater part of the general surface of the body; in other cases the secretion serves to stick together particles of mud or sand, or shelly fragments, so as to form a more or less cylindrical tube (rarely branched), which is lined internally by the hardened "mucus," having the appearance of silk.

But the process of tube-making is not a simple one, for in many cases, at least, the worms exhibit definite powers of choice. Thus some species of _Sabella_ choose only the very finest particles of mud; _Terebella conchilega_ chooses fragments of shell and grains of sand; _Onuphis conchylega_ employs small stones more or less of a size; _Sabellaria_ makes use only of sand grains. Whilst some worms, like _Terebella_, _Nicomache_, and others, make a very irregular tube, _Pectinaria_ builds a most remarkably neat house, open at each end, which it carries about with it, the narrow end uppermost (Fig. 152); the grains of sand are nearly all of the same size and only one layer in thickness, embedded in abundant "mucus," and with the outer surface quite smooth.

Sir J. Dalyell[346] made some most interesting observations on the method followed by sundry tube-formers in the building of {288}their tenements, and these observations, though made nearly half a century ago, have required very little addition or correction in modern times. In speaking of _Sabella_, he writes as follows:—

"Let a tall and ample crystal jar containing a _Sabella_ be emptied of its contents and speedily replenished with sea-water; the animal, if in view, has retreated during the short interval; the orifice of the tube is closed, all is at rest. But soon after {289}replenishment it rises, to display its branchial plume still more vigorously than before, and remains stationary, as if enjoying the freshness of the renovated element, always so grateful—the harbinger of health and strength to those whose dwelling is there. The passing spectator would conclude that he now beholds only a beautiful flower, completely expanded, inclining towards the light like some of those ornaments of nature decorating our gardens. He pauses in admiration. But if a drop of liquid mud falls amidst the element from above, disturbing its purity, then, while the plume unfolds to its utmost capacity, does the animal commence a slow revolution, the body also passing around within the tube. Now are the thousands of cilia fringing the ribs [_i.e._ the secondary filaments] of the branchiae discovered to be in vigorous activity, and their office to be wondrous. A loose muddy mass is soon afterwards visibly accumulating in the bottom of the funnel; meantime the neck or first segment of the body, rising unusually high above the orifice of the tube, exhibits two trowels beating down the thin edge as they fold and clasp over the margin, like our fingers pressing a flattened cake against the palm of the hand. [This refers to the lappets of the peristomial collar.] During these operations muddy collections are seen descending between the roots of the fans [right and left gills] towards the trowels, while another organ, perhaps the mouth, is also occupied, it may be, in compounding the preparation with adhesive matter. Still does the partial or complete revolution of the plume above, and of the body within the tube, continue; the bulk of the muddy mass diminishes, activity abates; it is succeeded by repose, when the tube is found to have received evident prolongation."

The Terebellids use their numerous tentacles in searching {290}for particles of sand, etc.; each tentacle is grooved along its ventral surface, and the particle is conveyed along the furrow to the mouth. These particles are actually taken into the mouth, and mixed with some sort of secretion; on ejection again, each particle is placed by another tentacle in its position at the edge of the tube, and by means of its lower lip the Terebellid works it into place.[347]

But whereas the greater number of tubicolous worms make use of adventitious material wherewith to strengthen the wall of their tube, the Serpulidae secrete carbonate of lime from their tube-glands, and mould a tube of this substance. Amongst the Eunicidae the secreted substance is of itself strong enough to protect the animal; for in _Hyalinoecia_ and species of _Eunice_ the tube consists of a translucent, tough, parchment-like material.

Chemical analysis has been employed in a few cases to determine the substance composing the tube. In the case of _Hyalinoecia_ (sometimes erroneously called _Onuphis_) the material consists of a phosphoric salt containing magnesia and a characteristic organic substance "onuphin"[348]; in _Spirographis_, a Sabellid, the name "spirographin" is given to its special secretion, whilst in Serpulids the organic base of the calcareous tube is "conchiolin."

The majority of worms are solitary, but there are a few instances of social worms—not that there is any co-operation or distribution of labour amongst the individuals, but they merely occur together in quantities; thus the sandy tubes of _Sabellaria_ may form compact masses of several cubic feet, which, left uncovered by the receding tide, look like rocks upon the shore; as, for instance, at Paignton and Torquay. _Filigrana implexa_ and _Serpula uncinata_ similarly intertwine their calcareous tubes to form masses.

{291}Whereas most worms live at the bottom of the sea, at various depths, a few are to be found at the surface. Purely pelagic habits are confined to a few families, viz. Tomopteridae, Typhloscolecidae, and the Alciopids and others amongst the Phyllodocidae; though _Nectochaeta_, one of the Polynoidae, and _Ophryotrocha_, one of the Eunicidae, are modified for this mode of life.[349] Several genera become pelagic during the breeding season. All these forms are excellent swimmers, and many of them are transparent.

The COLOURING OF POLYCHAETES.—The majority of Polychaetes quickly lose their colour in spirits, and become uniformly dull or light brown in museums. There are a few, however, which retain their brilliancy, like _Aphrodite_ and _Chloeia_, but in both cases the coloration is due to the beautiful hair-like bristles ranged along each side of the animal; in the former the colours of the rainbow flash from specimens which have been kept in spirit for any length of time. The Polynoids, too, with their golden chaetae and pigmented scales, retain to some extent their characteristic colouring. But the colours of most Annelids are due to pigments in the skin, together with the haemoglobin of the blood, which are soluble, or otherwise changed, in alcohol; for instance, the bright greenish-blue tint of the common _Phyllodoce_ of our coasts is changed to a rich chocolate brown; but such cases are rare, most worms becoming more or less decolorised.

The varied colouring in the Polychaetes, as in other animals, is due to a variety of causes. The red is in many cases due to haemoglobin of the vascular system showing through the transparent body; the green of the tentacles of the Sabellids and Chlorhaemids is similarly due to chlorocruorin. In other cases the contents of the intestine or the tint of the coelomic fluid may affect the colour of the worm. In _Capitella_ the coloured excretory products are regained in the skin; in an Eunicid living in a yellow sponge, on which it feeds, the colouring matter is extracted and stored in the skin; in the same kind of way green caterpillars may owe their tint to feeding on green leaves. But many of the Polychaetes possess distinct _pigments_ in the skin; thus in _Arenicola_ the dark pigment {292}melanin has been recognised; in _Cirratulus_ and _Nereis_ certain lipochromes; whilst _Eulalia viridis_ contains a pigment allied to bonellein. These various pigments yield different absorption bands when a solution is examined with the spectroscope; others, however, give no bands, but are distinguished by different chemical reactions.[350] The colour of the intestine of _Chaetopterus_ has been stated to be due to "modified chlorophyll," but it is quite a different substance.

When seen in the living and healthy condition, however, these Polychaete worms vie with the very butterflies in their brilliant and beautiful colourings, and though our own worms are not lacking in beauty, many tropical and southern forms exceed them in gayness of tint. Bright reds, orange, yellows, greens, blues, rich violets, and sombre browns are all displayed.[351]

The handsome _Terebella nebulosa_ of our own coasts is coloured bright red, sprinkled with white spots. _Nicomache lumbricalis_ is pink, with red girdles. Eunicids are frequently red or brown, and the red gills along each side, together with a brilliant iridescence, render these worms very beautiful. Nereids present a great range of coloration, from light green to sundry tints of brown and red in various combinations. Amongst the Serpulids our common _S. vermicularis_ is a very showy little worm, with its orange body, its red gills splashed with orange, and its orange operculum streaked with red; and a Southern form, _Placostegus coeruleus_, occurring at the Cape of Good Hope, is provided with beautiful lavender-blue gills. Our own Sabellids present examples of beautiful markings on the gills, in different colours or in different shades of the same colour. Amongst Polynoids, _P. leucohyba_, from the Antilles, has blue elytra; _Hemilepidia erythrotaenia_, a long worm from the Cape of Good Hope, has the anterior end of its body covered with light blue elytra, whilst the uncovered part is orange, with a broad magenta-red band along the dorsal surface.

The Phyllodocids are mostly very brightly coloured. The common _P. lamelligera_ of our coast has a bluish-green body, with olive-green parapodia; but _Lopadorhynchus erythrophyllum_, {293}from Jamaica, has a blue body with red parapodia; whilst _Notophyllum myriacyclum_ has a brown body with longitudinal dark-brown stripes and yellow parapodia. Both these worms live in coral reefs, where brilliancy of colour is one of the characteristic features of the fauna. Other worms are of various shades of green: the dark green _Arenicola_ with red gills; the bright green _Eulalia viridis_; the deep green _Amphinome smaragdina_, from Jamaica; _Gnathosyllis diplodonta_, with its green and yellow body, serve as examples.

Patterns or "markings" may be exemplified by _Lepidasthenia elegans_ (Fig. 156), and _Myrianida fasciata_, which has a bright red band on each segment (Fig. 149, p. 280). From this brief list of examples it will be seen that beautiful, and even brilliant, coloration is not confined to any particular mode of life; many of the most typically tubicolous forms, like the Terebellids and Serpulids, are as brilliantly coloured as the most typically free-swimming genera, like the Phyllodocids. Carnivorous forms like Amphinomids and Syllids present as wide a range of tint as the limivorous forms like _Cirratulus_, _Sabella_, or Maldanids. Shore-lovers, and deep-sea dwellers, and surface-swimmers, all exhibit equally bright or equally sombre tints; it is therefore difficult and rash to dogmatise on the "use" of these colourings to these animals, or to point to this worm as being protectively, to the other as being warningly, coloured; for we are too ignorant as to the habits of the worms.

PROTECTIVE AND MIMETIC DEVICES.—From the point of view of "protection" in the evolutionist's sense of the word, we can {294}say but little. Protective resemblance there is undoubtedly amongst the Polynoids, for the scales of these forms resemble more or less closely the stones or sand amongst which they live; in the same species there is great variety in coloration. This protective habit is carried still further in the case of _Psammolyce_ by the attachment of sand grains to little cups on the elytra, so that the back of the animal is concealed. Certain commensals, such as _Polynoë arenicolae_, _P. pentactes_, are coloured so as to resemble their associates. In a few cases it is possible that the gills of Sabelliformia are protectively coloured; for in _Sabella pavonia_ they vary from a light yellowish tint to a deep violet-brown, and the dark markings on them are therefore more or less distinct. Spread out as the gills are in life, they are in many cases difficult to recognise; it is rather their movement as they are withdrawn that attracts one's attention to them, as the tubes of these worms frequently serve for the attachment of brownish seaweeds, to which the gills bear resemblance. But, as a matter of fact, little work has been done in this direction, and speculation on the matter without evidence is worthless. Many pelagic forms, being transparent, such as _Tomopteris_ and Alciopids, are no doubt protected by their lack of colour; yet these forms present brightly-coloured spots,—the light-producing organs in the parapodia of the former, and the large dark eyes of the latter.

Semper[352] mentions a case of possible mimicry in a species of _Myxicola_ which lives in the clefts of a coral, _Cladocora_. The branchial funnel, when expanded, resembles very closely the expanded coral in size, colour, etc.; but he points out that the species occurs in other situations, where its colouring is not protective. Probably the "mimicry" is in other instances merely accidental.

No doubt many Polychaetes may be "warningly coloured," but experimental evidence is incomplete. _Polycirrus aurantiacus_ is bright red, with orange tentacles; these worms were rejected by certain fish.[353] The animal has given up living in tubes as all its allies do, and it is the tentacles which appear to be distasteful to its enemies, for when irritated it coils itself up and wraps {295}itself round with its tentacles. Moreover, when the tentacles were cut off the fish did not reject the body of the worm. The tentacles are thus coloured in such a way that fish recognise them, and associate with the colour some distasteful property.

PHOSPHORESCENCE.—Many worms of very different habits have the power of emitting a light from some parts of the body, and they are then said to be "phosphorescent."[354] Probably _Chaetopterus_ is most eminently photogenic; the base of the great "wings," the "fans," and other parts emit, on stimulation, an azure blue to greenish light, so bright that one may read one's watch by it. Several species of _Polynoë_ exhibit a similar phenomenon, each elytron, with the exception of the area of attachment, being brilliantly illuminated. In these species the phosphorescent elytra are frequently thrown off by the animal, so that possibly they deceive enemies. _Polycirrus aurantiacus_ produces a beautiful violet phosphorescence; usually its many tentacles alone show the light, but under strong stimulation the entire body takes {296}part in the display, and no doubt the phosphorescence has, like the colour, a "warning" purpose.

The production of the light in these various forms is apparently due to two different processes. In some cases, _e.g._ _Chaetopterus_, Syllids, Terebellids, it appears to be due to the oxidation of certain cell contents which are discharged more or less freely on irritation of the nerves; whilst in Polynoids the phenomenon is due to some purely nervous process, for the elytra have no glands, but are provided with ganglia and a nervous network.

In other worms, however, there are definite light-producing organs. In _Tomopteris_ there is on each parapodium, above and below, a brightly-coloured spherical organ, which for a long time was regarded as an eye, but from its structure appears to be a "photogen" (Fig. 167, p. 315). The same is very likely the true explanation of the segmental "eyes" of _Polyophthalmus_, for their structure recalls that of the light-organs of deep-sea fishes.

As many of the phosphorescent Polynoids are commensals, while _Chaetopterus_ inhabits tubes, and close allies of other phosphorescent worms have no power of emitting light, it is impossible to apply the same explanation of its purpose to all cases alike; in some it may be "accidental," though in others it may be of definite use in warning enemies or in attracting prey.

The FOOD OF WORMS.—The Nereidiformia are mostly carnivorous, and feed on small Crustacea, Mollusca, sponges, and other animals; and Polynoids are even said to eat one another. Many worms do not disdain various seaweeds, whilst the Spioniformia and Scoleciformia, which burrow in mud and sand, and are without biting organs, swallow the mud and digest what animal or vegetable débris it may contain. The Terebellids and Cryptocephala depend on minute organisms which may be driven into the mouth by the action of the cilia of the gills or tentacles.

In the case of deep-sea forms, it is an interesting fact that the intestines are not unfrequently crammed with Radiolaria and Foraminifera in a fairly fresh, uninjured condition, indicating that these Rhizopods do not merely _sink_ to the bottom, but must actually _live_ there.[355]

The ECONOMIC PURPOSES to which Polychaetes are put are few; they are used either as bait for fishes or as food for man.

{297}One of the commonest baits used for certain fish, as all who have done any sea-fishing off the piers of our coasts know, is the common lug-worm (_Arenicola marina_), whilst _Nephthys caeca_ and _Nereis fucata_ are also used in some places; and for whiting _Nereis cultrifera_ and _N. diversicolor_. _Marphysa sanguinea_, known to the fishermen in some parts as "varme," is less frequently used.

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 {298}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]

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 {299}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_.

{300}As for _bathymetrical_ distribution,[360] many genera occur at all depths, though Polychaetes appear to be most abundant, as far as we know at present, in "shallow water"—that is, down to twenty fathoms or so; but this may be due to the greater facility of collection on shore and in these slight depths, for the "Challenger" obtained considerable numbers of new species at greater depths.

The "deep-sea" forms are chiefly tubicolous, and since these tubes are fixed and partially embedded in the bottom, probably comparatively few are brought up. Some genera occur at very great depths; thus the Terebellid _Leaena abyssorum_ and the Serpulid _Placostegus benthalianus_ were brought up from 3125 fathoms—the greatest depth from which Polychaetes were obtained by H.M.S. "Challenger"; and it is interesting to note that species of each of these two genera occur in shallow water, the Serpulid being represented in our own coast fauna by _P. tricuspidata_.

Amongst our own fauna, a few examples may be given of the "replacement of species."[361] The littoral _Sthenelais boa_ is represented by _S. limicola_ in deeper water; _Sabellaria alveolata_ by _S. spinulosa_; _Polynoë imbricata_ by several deep-water species. Similarly with genera: the littoral _Pomatoceros_ is replaced by _Serpula_ in deeper water; and the Hesionid _Psamathe_ by _Castalia_.

The limitation of species to certain regions, or to certain depths of an ocean, may appear at first sight peculiar, in view of the unrestricted communication between all its parts; but there are as efficient "barriers" there as on land, for generally a particular worm can live only in a certain temperature and at a certain pressure, and is dependent for its food on particular organisms, which in their turn depend on the depth and its accompaniments. It is, in fact, so much the more peculiar that certain species are more or less cosmopolitan, or occur at widely distant points. It is less peculiar, of course, to find different species of the same genus at different depths or in different areas, for any slight variation in a species advantageous to new conditions would readily be fixed, and give rise to a new species.

The distribution of the Polychaeta depends probably on the pelagic larvae, which are carried by currents from one part of an ocean to another. There can be little doubt that many {301}Polychaetes are very "plastic," and can adapt themselves to changed conditions of life with considerable ease; for _Nereis diversicolor_, _Arenicola marina_, and others live equally well in water of very different densities, and with a different food supply. The great variety in the "habitats," and presumably therefore in their food supply, etc., exhibited by many Polychaetes, as well as the great variation observable in some species of Polynoina, and the close affinity of the species and genera of this sub-family, lead us to the same conclusion.

EXTINCT POLYCHAETES.—The most numerous fossil records of the Polychaetes are calcareous tubes of various shapes and sizes; they are irregularly or spirally curved, and are very usually attached at one end, or by one surface, to stones or to fossils. These tubes belong to the Serpulidae, and are referred to the genera _Serpula_, _Spirorbis_, _Ditrupa_, and others.[362]

_Spirorbis_ is the oldest unequivocal representative of the Polychaetes, as its tubes are found more or less abundantly in the Silurian and other Palaeozoic strata. In Palaeozoic times _Serpula_ was rare, as it was too in the Trias and Lias, but in the Jurassic strata it becomes abundant. In the chalk, _S. socialis_ may occur in masses like _S. uncinata_ of the present day, forming "Serpulite chalk." In the older tertiaries the genus is represented by _Spirulaea_.

_Terebella lapilloides_ occurs in the Lias as a cylindrical, more or less curved tube of sand-grains.

Amongst the Nereidiformia the remains are fewer, but the {302}acicula and the hard jaws are preserved in certain rocks, and can be referred to existing families. _Eunicites avitus_[363] is represented by a double series of acicula, indicating the parapodia of the two sides; and by remains of both upper and lower jaws (Fig. 159). Four different species of the worm have been described from the lithographic slate of Bavaria, of Jurassic age; and several upper jaws of other Eunicids have been discovered in the Palaeozoic beds of Canada and Scotland, and have received the names _Lumbriconereites_, _Oenonites_, and _Arabellites_, in reference to their nearest allies amongst living genera.

There are, however, numerous remains, in the forms of tracks or casts, in the earlier rocks, which have been referred to the Polychaeta. The names _Crossopodia_, _Myrianites_, _Nereites_, _Phyllodocites_, have been given to some of these traces, though they are open to numerous other interpretations. Some of the "tracks" are similar to those made by living Crustacea in walking over wet sand; others appear to be the casts of some animals. Tubular burrows in rocks or fossils, some straight, others U-shaped, have received such names as _Arenicolites_, _Scolithus_, _Histioderma_; whilst under the name _Lumbricaria_ certain cylindrical, coiled structures, resembling worm "castings," are met with in this same lithographic stone of Solenhofen. Many of the tubes referred to Polychaetes by the earlier palaeontologists have been transferred to other groups; thus _Cornulites_ is now believed to be a Pteropod shell.

This very meagre geological record is quite insufficient to form any basis for a phylogeny of the group. And this poor supply of remains is not surprising, when we consider the soft nature of the tissues, the absence, in the majority of families, of skeleton and of other parts which could have been fossilised; yet we might have expected a greater abundance of fossilised jaws than is represented at present. But it must be borne in mind that the conditions of life of these soft-bodied animals are not conducive to their leaving abundant fossilised remains.

{303}CHAPTER XII

CHARACTERS OF THE SUB-ORDERS OF POLYCHAETES—CHARACTERS OF THE FAMILIES—DESCRIPTION OF BRITISH GENERA AND SPECIES—THE MYZOSTOMARIA.

SYSTEMATIC.—The Order Polychaeta may be divided into two branches, in one of which, the PHANEROCEPHALA, the prostomium retains its ancestral condition as a lobe overhanging the mouth, and frequently carries, in addition to paired eyes, certain sensory processes of a simple structure, the tentacles and palps; the body-segments are more or less alike, and (except in some Spioniformia, some of the Terebelliformia, and the Capitelliformia) do not present two sharply marked regions, owing to the differential arrangement or character of the chaetae. In the second branch, the CRYPTOCEPHALA, the peristomium grows forwards during development, so as to compress or even hide the prostomium, which thus becomes a very insignificant organ. The tentacles are reduced, but the palps become greatly developed and take on sundry new functions. The body in this group, by the character and arrangement of the chaetae, is distinguishable into a thorax and abdomen, presenting certain internal differences.

These two branches may be supposed to have arisen from a common ancestor having a general resemblance to a nereidiform worm, such as _Syllis_, possessing palps and tentacles on the prostomium, definite parapodia and cirri on the body, and internally, a well-marked and regular repetition of organs.

The branch PHANEROCEPHALA contains the following five sub-Orders, though it is possible that the Capitelliformia deserves a more important position in the system:—

SUB-ORDER 1.—The _Nereidiformia_ have well-developed tentacles and palps; the peristomium almost invariably possesses {304}special cirri; the parapodia are well-marked locomotor organs, supported by acicula, and carry dorsal and ventral cirri. The chaetae are usually jointed, though unjointed ones may coexist with these; uncini are never present. An eversible buccal region leads into a muscular pharynx, which in the majority is armed with chitinous jaws; the septa and nephridia are regularly repeated throughout the body. The worms lead a predaceous life, and are mostly carnivorous; a few form tubes.

SUB-ORDER 2.—The _Spioniformia_ possess neither tentacles nor palps; the peristomium usually carries a pair of long tentacular cirri, and extends forwards at the sides of the prostomium. The parapodia project only to a slight degree; the dorsal cirri may attain a considerable size, and act as gills throughout the greater part of the body. The chaetae are unjointed; uncini are only present in the aberrant _Chaetopterus_.[364] The body may present two regions more or less distinctly marked externally, but without corresponding internal differences. The buccal region may be eversible, but there are no jaws. Septa and nephridia are regularly developed. The worms are burrowers, or tubicolous.

SUB-ORDER 3. _Terebelliformia._—The prostomium is a more or less prominent lobe (upper lip) with or without tentacles but without palps. The peristomium may carry cirri or "tentacular filaments."[365] The parapodia are feebly developed; there are no ventral cirri; the dorsal cirri may exist and function as gills on more or fewer of the anterior segments. The chaetae are unjointed, and uncini are usually present. The buccal region is not eversible; there are no jaws. The septa are usually incomplete, with the exception of one strongly-developed "diaphragm" anteriorly; the nephridia are dimorphic, those of the anterior (prediaphragmatic) segments are of large size and are excretory; the posterior series are mere funnels, and act as genital ducts. These worms are burrowers or tube-formers, and in the majority the tube-forming glands are grouped on the ventral surface of the anterior segments to form "gland-shields."

{305}SUB-ORDER 4.—The _Capitelliformia_ have no prostomial processes, but possess a pair of large retractile "ciliated organs." The parapodia do not project; the chaetae are unjointed, and are hair-like in the anterior segments and hooded "crotchets" posteriorly; this external division of the body does not correspond with definite internal differences. There are no cirri, though special "gills," often retractile, are frequently present. The buccal region is eversible; there is no armed pharynx. An "accessory gut" or "siphon" exists. The nephridia are small, and sometimes more than one pair in a segment; special genital funnels exist in more or fewer of the anterior segments of the hind body. There is no system of blood-vessels; the coelomic corpuscles are red. The worms are burrowers.

SUB-ORDER 5.—The _Scoleciformia_ possess a prostomium, which rarely (Chlorhaemidae) carries any sensory processes; the peristomium is without cirri (except, perhaps, in the Chlorhaemidae). The parapodia are ill developed, and may be absent; only rarely are dorsal cirri present, acting as gills; ventral cirri are absent. The chaetae are unjointed; true uncini are not present. The buccal region is eversible, but there is no armed pharynx. The septa are not regularly developed, as more or fewer are absent, and the nephridia are considerably reduced in number, it may be to a single pair (Sternaspidae and some Chlorhaemidae), but they are all alike.[366] The worms are mostly burrowers.

The branch CRYPTOCEPHALA contains two sub-Orders:—

SUB-ORDER 1. _Sabelliformia._—The prostomium is entirely hidden by the forward extension of the peristomium; the tentacles are very small, being frequently represented merely by small knobs of sense-cells; the palps, on the other hand, are greatly developed, branched, and contain blood-vessels, acting as respiratory as well as sensory organs. The peristomium never carries cirri or chaetae, and it is usually raised up into a projecting collar, used in fashioning the lip of the animal's tube. The parapodia are but feebly developed; cirri are absent, except in the Serpulidae, where the dorsal and ventral cirri become united to form the "thoracic membrane" (Meyer). The chaetae are of two kinds—unjointed, hair-like, fringed bristles and "uncini." {306}By their arrangement the body is divided into a thorax of nine segments and an abdomen; in the former the capillary chaetae are dorsal, and in the latter ventral. The buccal region is not eversible; there is no pharynx. The septa are regularly developed in the abdomen, but are absent in the thorax; the nephridia are dimorphic; there are two large ones in the thorax opening by a median dorsal pore just above the brain; those of the abdomen are small funnels, and act as genital ducts. The worms are tubicolous; "gland-shields" are present on the thoracic segments.

SUB-ORDER 2. _Hermelliformia._—The peristomium (Fig. 135) is enormously developed, and forms a bilobed hood capable of closing over the mouth; the truncated free end of each lobe carries three semicircles of peculiar chaetae, which act as an efficient protection when the worm is withdrawn into its tube. The prostomium is very small, but retains a pair of well-developed tentacles; the palps, which are subdivided as in the Sabelliformia, have become fused with the ventral edges of the peristomium, and appear as a series of ridges on each side, carrying numerous filaments. The thorax consists of five segments, the notopodia of three of which are well developed and bear strong chaetae; dorsal cirri are present along the greater part of the body, and act as gills. The arrangement of the chaetae and of the internal organs is as in the Sabelliformia. The worms form tubes of sand.

BRANCH A. PHANEROCEPHALA.

SUB-ORDER 1. NEREIDIFORMIA.[367]