fig144Fig. 144.—Ammotrypane aulogasterRathke, enlarged. (From Cuningham.) Anterior end.a, Prostomium;b, everted buccal region;c, notopodial cirrus; X, ciliated organ everted; I, II, III, first three segments.
Fig. 144.—Ammotrypane aulogasterRathke, enlarged. (From Cuningham.) Anterior end.a, Prostomium;b, everted buccal region;c, notopodial cirrus; X, ciliated organ everted; I, II, III, first three segments.
Fig. 144.—Ammotrypane aulogasterRathke, enlarged. (From Cuningham.) Anterior end.a, Prostomium;b, everted buccal region;c, notopodial cirrus; X, ciliated organ everted; I, II, III, first three segments.
Reproductive Phenomena.—With a few exceptions mentioned below, the Polychaeta are unisexual. The sexual cells are developed in all cases from the lining epithelium of the body-cavity. The exact spot at which this occurs varies in different cases; it may be, though rarely, on the floor of the body-cavity; it is more usually on the wall of some blood-vessel, either the ventral vessel or on branches of it; or on the many blind blood-vessels ofAphrodite. The number of such genital organs is very great in most worms, but in those presenting two regions of the body they are confined to the posterior segments (Sabelliformia, Terebelliformia, Capitelliformia). The number is very limited inArenicolaand other worms presenting but few nephridia: in the former genus there being six pairs, inTrophoniaonly one pair.
The following genera are hermaphrodite:—Amphiglena,Salmacina,Protula,Spirorbis, belonging to the Sabelliformia, towhich must be added some Hesionidae. In this family ova and spermatozoa are developed around the same blood-vessel. But in the former group of worms (as also inOphryotrocha) the two kinds of cells are produced in different regions of the body. Thus inProtulathe anterior abdominal segments are male, the posterior ones female, while inSpirorbisthe reverse arrangement holds; and inSyllis corruscansthe anterior segments of the body contain eggs, whilst the posterior region contains spermatozoa, and this region separates and becomes a male worm.
The eggs and spermatozoa in the Polychaeta are discharged into the sea either by rupture of the body-wall or through the nephridia; the male and female elements unite, and the resulting fertilised eggs undergo development, either floating separately in the water, or embedded in jelly, or attached to the body or to the tube of the worm.
The result of the segmentation of the egg is a free-swimming larva known as a "Trochosphere," similar to that ofPolygordius. The larvae of different species present various more or less marked departures from this type, for instead of the two girdles of cilia there may be only the anterior girdle, or there may be several complete or incomplete girdles between the two typical ones, or there may be (Chaetopterids) only a single girdle of cilia about the middle of the body, the two typical girdles being absent.[328]The postoral region, after elongation, generally becomes marked out into three segments, and these segments develop chaetae, which are usually temporary and specially long.
The little animal is thus equipped for an independent life: the provisional chaetae help in keeping it balanced; and in some cases (Spionidae) serve to protect the little soft creature, for when it is touched it curls up, and its chaetae stick out at the sides, so that it looks like a hairy caterpillar. But the larva is quite at the mercy of the sea, for it is carried hither and thither by currents, and in this way the species is disseminated. The larvae of the Polychaetes, like those of other animals, occur at certain periods of the year in large quantities at the surface of the sea, and serve as food for various larger animals.
fig145Fig. 145.—A, Trochosphere ofNephthys. × 65.a, Anus;b, apical plate (brain);c, apical tuft of cilia;c', girdle of cilia;i, intestine;m, mouth;st, stomach.B, Larva ofSpio, with three segments, eight days old. × 100.c, Preoral girdle of cilia;c', preanal girdle;ch, long provisional chaetae;pr, prostomium with eyes. (From Claparède and Metschnikoff.)
Fig. 145.—A, Trochosphere ofNephthys. × 65.a, Anus;b, apical plate (brain);c, apical tuft of cilia;c', girdle of cilia;i, intestine;m, mouth;st, stomach.B, Larva ofSpio, with three segments, eight days old. × 100.c, Preoral girdle of cilia;c', preanal girdle;ch, long provisional chaetae;pr, prostomium with eyes. (From Claparède and Metschnikoff.)
Fig. 145.—A, Trochosphere ofNephthys. × 65.a, Anus;b, apical plate (brain);c, apical tuft of cilia;c', girdle of cilia;i, intestine;m, mouth;st, stomach.B, Larva ofSpio, with three segments, eight days old. × 100.c, Preoral girdle of cilia;c', preanal girdle;ch, long provisional chaetae;pr, prostomium with eyes. (From Claparède and Metschnikoff.)
These larvae are at first very different from the adult animal, and the necessary changes to be passed through are more or less great according to the species. It is not our intention to describe these changes in detail.[329]The larva increases in size, the permanent chaetae make their appearance in regular order, and the body exhibits segmentation, the new segments always appearing just in front of the anal segment. The internal organs gradually develop, and the prostomial and parapodial appendages grow out in their turn. In the Sabelliformia the multifilamentous "gills" arise by the continued branching of an at first simple process (the palp) arising from the latero-ventral surface of each side of the preoral lobe.[330]These gradually encroach dorsally and ventrally till the prostomium is more or less encircled; meanwhile the peristomium grows forwards so as to conceal the prostomium, which no longer increases at the same rate as does the rest of the body.
Although most worms appear to discharge their ova directly into the sea and take no further care of them, some make provision for their offspring either by laying the eggs in a jelly, which will serve as food for the young larvae—Aricia,Ophelia,Protula,Phyllodoce—or by attaching them to their body. In certain Polynoids the eggs are attached by means of a secretion to the back, under the elytra, where they undergo development up to a certain stage. InExogoneand some other Syllids they are attached to the ventral cirri, or inGrubea limbata, all over the back. In the femaleAutolytus(Sacconereis) a ventrally-placed brood sac is formed by the hardening of asecretion; the eggs develop into embryos inside the brood sac, and then become free, with head appendages and three pairs of parapodia. Enormous numbers of such embryos may occur; for instance, some 300 were counted in a brood sac ofAutolytus ebiensis. In the case of tubicolous worms, the eggs are frequently attached to the tube, either inside or outside. InSpirorbisandSalmacinathe operculum serves as a brood pouch.
Only a very few species are known to be viviparous, viz.Syllis viviparaKr.,Cirratulus chrysodermaClap.,Marphysa sanguineaMont., andNereis diversicolorMüll.
In most genera there is no external difference between a mature worm filled with generative products and an immature one, except, it may be, in the colour; for the yolk of the eggs is frequently tinted yellow, or pink, or bluish, while the spermatozoa in mass are white; so that the normal colouring of the worm may be modified when filled with these elements. But in a few instances striking anatomical peculiarities are exhibited by the mature worm.[331]In many species ofNereis, for instance, those segments containing the generative products undergo more or less extensive changes, while the anterior ones remain unaltered. The body of the ripeNereisis then distinguishable into an anterior non-sexual region and a posterior sexual region; and so great are these changes in certain species that the mature worms were for a long time believed to belong to a different genus, and received the nameHeteronereis. But we now know their true relations, thanks to the work of Claparède and others. The males in the Heteronereid phase have fewer unaltered anterior segments than the females, so that there is a sexual dimorphism.
fig146Fig. 146.—Male "Heteronereis" ofN. pelagicaL. × 1.A, Non-sexual region;B, sexual, modified region. (From Ehlers.)
Fig. 146.—Male "Heteronereis" ofN. pelagicaL. × 1.A, Non-sexual region;B, sexual, modified region. (From Ehlers.)
Fig. 146.—Male "Heteronereis" ofN. pelagicaL. × 1.A, Non-sexual region;B, sexual, modified region. (From Ehlers.)
The changes whichNereisundergoes in its transformation affect chiefly (a) the shape of the parapodia, and (b) the form of the chaetae of these parapodia. Other organs may also be affected, though less noticeably; thus the eyes become enlarged, the intestine may become so compressed by the generativeproducts as to be functionless, and the tail develops special sensory papillae.[332]
In the parapodia an increase in size and a sharper delineation of the various parts take place; then flattened foliaceous outgrowths (Fig. 147,x,y) arise from certain lobes of the feet, in which, too, the blood supply becomes greatly increased. The old chaetae are pushed out by the development of new ones of quite a different shape; these are jointed like the old ones, but the appendix is, in many species at least, flattened and oar-shaped (Fig. 123, C, p.246); and the chaetae are arranged in a fan-like manner. Both these modifications are in evident relation to the free-swimming habit which the Heteronereid now adopts. The new foot serves as a swimming organ, the old one was a walking appendage.
fig147Fig. 147.—Parapodium of male "Heteronereis" ofN. pelagicaL. × 10. (From Ehlers.)a, Notopodial cirrus;b, notopodium;c, neuropodium with new chaetae;c', foliaceous outgrowth;d, neuropodial cirrus;x,y, foliaceous outgrowths.
Fig. 147.—Parapodium of male "Heteronereis" ofN. pelagicaL. × 10. (From Ehlers.)a, Notopodial cirrus;b, notopodium;c, neuropodium with new chaetae;c', foliaceous outgrowth;d, neuropodial cirrus;x,y, foliaceous outgrowths.
Fig. 147.—Parapodium of male "Heteronereis" ofN. pelagicaL. × 10. (From Ehlers.)a, Notopodial cirrus;b, notopodium;c, neuropodium with new chaetae;c', foliaceous outgrowth;d, neuropodial cirrus;x,y, foliaceous outgrowths.
Whilst some species, such as the common BritishN. diversicolor, undergo no change, and others become modified as just described, others, again, are polymorphic. Claparède was the first to show thatN. dumeriliimay 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 theassumption 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 asEusyllis,Odontosyllis, andExogone, 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 becomesseparated, 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 genusSyllisthe 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 asAutolytus, the regeneration of segments may commencebeforethe separation of the sexual zooid; and the head of the sexual zooid becomes budded outbeforeseparation from the asexual portion. So that the animal now consists of two worms, each with its own head, separated bya region or zone of proliferation (Fig. 148, IV). Moreover, in some species not only is the hinder part of the bodyconvertedinto 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 inAutolytus(Fig. 148, V)—the hindermost byconversionof the hinder part of the body of the original "stock," the intervening zooids bygemmation.
fig148Fig. 148.—Diagrams illustrating the various stages in the asexual formation of a chain of zooids. (Modified from Malaquin.)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 ofB, 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;Bis the hindmost zooid, which is either formed from the hinder segments of the parent zooid or is produced by budding, likeC,D,E.
Fig. 148.—Diagrams illustrating the various stages in the asexual formation of a chain of zooids. (Modified from Malaquin.)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 ofB, 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;Bis the hindmost zooid, which is either formed from the hinder segments of the parent zooid or is produced by budding, likeC,D,E.
Fig. 148.—Diagrams illustrating the various stages in the asexual formation of a chain of zooids. (Modified from Malaquin.)
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 ofB, 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;Bis the hindmost zooid, which is either formed from the hinder segments of the parent zooid or is produced by budding, likeC,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 wasworked out by Agassiz[335]they were placed in different genera. The male zooid has thus come to be known asPolybostrichus(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 termedSacconereis, 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).
fig149Fig. 149.—Myrianida fasciata.(From Malaquin.) The bright red markings of the living animal are here represented black.A, An asexual individual which has produced by budding from the zone (z) a chain of twenty-nine zooids, the oldest being labelled 1, the youngest 29.B, A ripe male zooid (Polybostrichus), with three tentacles and a pair of forked palps (p). There are five unaltered anterior segments.C, A ripe female zooid (Sacconereis) with the palps fused with the prostomium;s, the ventral brood pouch projecting on each side;t, tentacles.
Fig. 149.—Myrianida fasciata.(From Malaquin.) The bright red markings of the living animal are here represented black.A, An asexual individual which has produced by budding from the zone (z) a chain of twenty-nine zooids, the oldest being labelled 1, the youngest 29.B, A ripe male zooid (Polybostrichus), with three tentacles and a pair of forked palps (p). There are five unaltered anterior segments.C, A ripe female zooid (Sacconereis) with the palps fused with the prostomium;s, the ventral brood pouch projecting on each side;t, tentacles.
Fig. 149.—Myrianida fasciata.(From Malaquin.) The bright red markings of the living animal are here represented black.A, An asexual individual which has produced by budding from the zone (z) a chain of twenty-nine zooids, the oldest being labelled 1, the youngest 29.B, A ripe male zooid (Polybostrichus), with three tentacles and a pair of forked palps (p). There are five unaltered anterior segments.C, A ripe female zooid (Sacconereis) with the palps fused with the prostomium;s, the ventral brood pouch projecting on each side;t, tentacles.
A further development of this process of gemmiparity is exhibited byMyrianida. 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 zooidis 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 inAutolytus, the sexes are dimorphic, the male and female resembling those of that genus.
The process of gemmation, as seen inAutolytus, 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 inMyrianida; 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 inAutolytusthese ariseduringthe formation of the new individuals, as part of the general process of new formation; whilst inMyrianidathe delay is prolonged, and the generative elements do not make their appearance tillafterthe 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 inFiligranaandSalmacinathe generative elements make their appearance in the hinder segments, as they do throughout the Sabelliformia; and this hinder part of the body separatesfrom 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 bySyllis 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]
fig150Fig. 150.—Portion ofSyllis ramosa. (Reduced from M‘Intosh.)
Fig. 150.—Portion ofSyllis ramosa. (Reduced from M‘Intosh.)
Fig. 150.—Portion ofSyllis ramosa. (Reduced from M‘Intosh.)
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 growthat 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 inDasychonethat the crown of branchiae was regenerated in about a month in springtime, while in winter the process occupied 116 days. He cut aDasychoneinto 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.
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 ofNereisfrom a lake in Mingrelia, anotherNereisand aLumbriconereisfrom 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 ofMelania; 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;Arenicolaespecially is regardless of the character of the medium, andNereis diversicolorappears 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,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; whileMyxicolareadily leaves its gelatinous tube and swims freely;Pectinariacarries its house with it as it moves about, andPolycirrus, 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 ofArenicola, the common "lug-worm"; these "castings" have passed through the worm's body, having beenswallowed 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 ofTerebella 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.Polynoëis 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 smallNereismay 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 asScoloplos,Nerine,Capitella, and others. By digging near low water one may findNephthys,Glycera, and others burrowing or hiding in the soil.
In rock pools, or sandy stretches amongst rocks kept moist and cool by abundantFucus, one may see under stones the red or yellow gill filaments ofCirratulusand 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 ofLaminaria.
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 theSabellahas 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 isuncertain.[344]Polydora ciliatamakes a tube of mud projecting from the mouth ofU-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 areSabella saxicavaandDodecaceria concharum, which is a common little borer, forming galleries in oyster-shells, etc.
Thetubesformed 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.
fig151Fig. 151.—Clymene ebiensisin its tube (t) (fromRègne Animal).a, Anterior,p, posterior end, which is, however, injured.
Fig. 151.—Clymene ebiensisin its tube (t) (fromRègne Animal).a, Anterior,p, posterior end, which is, however, injured.
Fig. 151.—Clymene ebiensisin its tube (t) (fromRègne Animal).a, Anterior,p, posterior end, which is, however, injured.
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 ofSabellachoose only the very finest particles of mud;Terebella conchilegachooses fragments of shell and grains of sand;Onuphis conchylegaemploys small stones more or less of a size;Sabellariamakes use only of sand grains. Whilst some worms, likeTerebella,Nicomache, and others, make a very irregular tube,Pectinariabuilds 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 oftheir tenements, and these observations, though made nearly half a century ago, have required very little addition or correction in modern times. In speaking ofSabella, he writes as follows:—
fig152Fig. 152.—The tube ofPectinaria auricoma. × 3. (From M‘Intosh.) This is its natural position as carried about by the animal.
Fig. 152.—The tube ofPectinaria auricoma. × 3. (From M‘Intosh.) This is its natural position as carried about by the animal.
Fig. 152.—The tube ofPectinaria auricoma. × 3. (From M‘Intosh.) This is its natural position as carried about by the animal.
fig153Fig. 153.—The upper end of the tube ofTerebella conchilega. Slightly enlarged. (From M‘Intosh.)
Fig. 153.—The upper end of the tube ofTerebella conchilega. Slightly enlarged. (From M‘Intosh.)
Fig. 153.—The upper end of the tube ofTerebella conchilega. Slightly enlarged. (From M‘Intosh.)
"Let a tall and ample crystal jar containing aSabellabe 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 afterreplenishment 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."
fig154Fig. 154.—Terebella conchilegaPall. Upper end of the tube (s) showing the anterior end of the worm.h, Its head;t, tentacles collecting sand grains (y) in their grooves;x, sand grains in mouth of worm;f, filamentous fringe of tube. (After Watson.)
Fig. 154.—Terebella conchilegaPall. Upper end of the tube (s) showing the anterior end of the worm.h, Its head;t, tentacles collecting sand grains (y) in their grooves;x, sand grains in mouth of worm;f, filamentous fringe of tube. (After Watson.)
Fig. 154.—Terebella conchilegaPall. Upper end of the tube (s) showing the anterior end of the worm.h, Its head;t, tentacles collecting sand grains (y) in their grooves;x, sand grains in mouth of worm;f, filamentous fringe of tube. (After Watson.)
The Terebellids use their numerous tentacles in searchingfor 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 inHyalinoeciaand species ofEunicethe 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 ofHyalinoecia(sometimes erroneously calledOnuphis) the material consists of a phosphoric salt containing magnesia and a characteristic organic substance "onuphin"[348]; inSpirographis, a Sabellid, the name "spirographin" is given to its special secretion, whilst in Serpulids the organic base of the calcareous tube is "conchiolin."
fig155Fig. 155.—Eunice tibianaPourt. × ½. The branching tube (t) with the worm (w) protruding its head through one of several openings. (From Ehlers.)
Fig. 155.—Eunice tibianaPourt. × ½. The branching tube (t) with the worm (w) protruding its head through one of several openings. (From Ehlers.)
Fig. 155.—Eunice tibianaPourt. × ½. The branching tube (t) with the worm (w) protruding its head through one of several openings. (From Ehlers.)
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 ofSabellariamay 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 implexaandSerpula uncinatasimilarly intertwine their calcareous tubes to form masses.
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; thoughNectochaeta, one of the Polynoidae, andOphryotrocha, 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.
TheColouring 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, likeAphroditeandChloeia, 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 commonPhyllodoceof 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. InCapitellathe 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 distinctpigmentsin the skin; thus inArenicolathe dark pigmentmelanin has been recognised; inCirratulusandNereiscertain lipochromes; whilstEulalia viridiscontains 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 ofChaetopterushas 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 handsomeTerebella nebulosaof our own coasts is coloured bright red, sprinkled with white spots.Nicomache lumbricalisis 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 commonS. vermicularisis 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 commonP. lamelligeraof our coast has a bluish-green body, with olive-green parapodia; butLopadorhynchus erythrophyllum,from Jamaica, has a blue body with red parapodia; whilstNotophyllum myriacyclumhas 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 greenArenicolawith red gills; the bright greenEulalia viridis; the deep greenAmphinome smaragdina, from Jamaica;Gnathosyllis diplodonta, with its green and yellow body, serve as examples.
Patterns or "markings" may be exemplified byLepidasthenia elegans(Fig. 156), andMyrianida 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 likeCirratulus,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.