fig156Fig. 156.—Lepidasthenia elegansGr., × 2, to illustrate colour-markings: the dark bands in the anterior part of the body occupy two elytriferous, and the intermediate segments. In the hinder region, where the elytra are in every third segment, this one is dark.el.12, The twelfth elytron.
Fig. 156.—Lepidasthenia elegansGr., × 2, to illustrate colour-markings: the dark bands in the anterior part of the body occupy two elytriferous, and the intermediate segments. In the hinder region, where the elytra are in every third segment, this one is dark.el.12, The twelfth elytron.
Fig. 156.—Lepidasthenia elegansGr., × 2, to illustrate colour-markings: the dark bands in the anterior part of the body occupy two elytriferous, and the intermediate segments. In the hinder region, where the elytra are in every third segment, this one is dark.el.12, The twelfth elytron.
Protective and Mimetic Devices.—From the point of view of "protection" in the evolutionist's sense of the word, we cansay 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 ofPsammolyceby the attachment of sand grains to little cups on the elytra, so that the back of the animal is concealed. Certain commensals, such asPolynoë 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 inSabella pavoniathey 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out as the gills are in life, they are in many cases difficult to recognise; it is rather their movement as they are withdrawn that attracts one's attention to them, as the tubes of these worms frequently serve for the attachment of brownish seaweeds, to which the gills bear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 asTomopterisand 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 ofMyxicolawhich 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 aurantiacusis 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 wrapsitself 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.
fig157Fig. 157.—Chaetopterus variopedatusRen. × ½. On the left the entire animal, with the three regions A, B, C.c, Peristomial cirrus;d, "sucker";e, the great "wings";f, "fan";m, mouth. On the right the animal is represented in the dark, under stimulation, so as to exhibit the phosphorescent portions of the body. (From Panceri.)
Fig. 157.—Chaetopterus variopedatusRen. × ½. On the left the entire animal, with the three regions A, B, C.c, Peristomial cirrus;d, "sucker";e, the great "wings";f, "fan";m, mouth. On the right the animal is represented in the dark, under stimulation, so as to exhibit the phosphorescent portions of the body. (From Panceri.)
Fig. 157.—Chaetopterus variopedatusRen. × ½. On the left the entire animal, with the three regions A, B, C.c, Peristomial cirrus;d, "sucker";e, the great "wings";f, "fan";m, mouth. On the right the animal is represented in the dark, under stimulation, so as to exhibit the phosphorescent portions of the body. (From Panceri.)
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]ProbablyChaetopterusis 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 ofPolynoëexhibit 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 aurantiacusproduces a beautiful violet phosphorescence; usually its many tentacles alone show the light, but under strong stimulation the entire body takespart 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. InTomopteristhere 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" ofPolyophthalmus, for their structure recalls that of the light-organs of deep-sea fishes.
As many of the phosphorescent Polynoids are commensals, whileChaetopterusinhabits 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.
TheFood 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 merelysinkto the bottom, but must actuallylivethere.[355]
Theeconomic purposesto which Polychaetes are put are few; they are used either as bait for fishes or as food for man.
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), whilstNephthys caecaandNereis fucataare also used in some places; and for whitingNereis cultriferaandN. 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 EunicidLysidice 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 ofOdontosyllis ctenostomatus(Fig. 158); such an association between two members of the same group of animals is peculiar; but still more exceptional is the occurrence ofHaematocleptes terebellides, as a parasite inMarphysa 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 GephyreanBonellia.
The PolynoidAcholoe astericolaand the HesionidOphiodromus flexuosusoccur as ectoparasites (or perhaps commensals) in the ambulacral grooves of the starfishAstropecten aurantiacus. An Amphinomid is stated to live in the branchial chamber ofthe barnacle,Lepas anatifera.Alciopina parasiticalives, during the early stages of its life-history, withinCydippe, 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. castanealodges in the peri-oral region ofSpatangus purpureus, and in the ambulacral grooves ofAstropecten;P.(Halosydna)bairdilives between the mantle and foot of the molluscFissurella cratitia;P. pentactesis found on the body of the HolothurianCucumaria pentactes, and appears to be protectively coloured.P.(Antinoë)parasiticalives under the elytra of another Polynoid, andP. acanellaeon the coralAcanella normani.[357]
fig158Fig. 158.—Odontosyllis ctenostomatus, with (L)Labrorostratus parasiticusin its body-cavity. The parapodia and cirri are omitted from the greater part of the body. (After St. Joseph.) × 4.
Fig. 158.—Odontosyllis ctenostomatus, with (L)Labrorostratus parasiticusin its body-cavity. The parapodia and cirri are omitted from the greater part of the body. (After St. Joseph.) × 4.
Fig. 158.—Odontosyllis ctenostomatus, with (L)Labrorostratus parasiticusin its body-cavity. The parapodia and cirri are omitted from the greater part of the body. (After St. Joseph.) × 4.
As commensals there may be mentionedNereis 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.Siphonostomais found in the "nests" made by the molluscLima. AEuniceis constantly associated with the coralLophohelia 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 coralAntipathes filix,[358]found in the West Indian seas. A species ofPolydoraforms its tube inHeliopora. The Polynoids present many instances of commensalism, a few of which may be here mentioned.P. johnstoniMarenz. is only found in the tubes ofTerebella nebulosa; other species occur in the tubes of other Terebellids.P. marphysaelives in tubes of the EunicidMarphysa sanguinea. Two species live in the tubes ofChaetopterus.P. extenuatahas been found in tubes ofSerpula vermicularis, whileP. arenicolaeoccurs onthe 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 ofNereis 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 ofLoxosoma, which may also be attached to gills of Eunicids; whilst below those ofAphrodite echidnaandHermadion pellucidum,Pedicellina belgicaoccurs. Under the felt ofA. aculeatathe SabellidBranchiomma vigilansforms 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 thegeographical distributionof 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, andTerebellides 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.
As forbathymetricaldistribution,[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 TerebellidLeaena abyssorumand the SerpulidPlacostegus benthalianuswere 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 byP. tricuspidata.
Amongst our own fauna, a few examples may be given of the "replacement of species."[361]The littoralSthenelais boais represented byS. limicolain deeper water;Sabellaria alveolatabyS. spinulosa;Polynoë imbricataby several deep-water species. Similarly with genera: the littoralPomatocerosis replaced bySerpulain deeper water; and the HesionidPsamathebyCastalia.
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 manyPolychaetes are very "plastic," and can adapt themselves to changed conditions of life with considerable ease; forNereis 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 generaSerpula,Spirorbis,Ditrupa, and others.[362]
Spirorbisis 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 timesSerpulawas rare, as it was too in the Trias and Lias, but in the Jurassic strata it becomes abundant. In the chalk,S. socialismay occur in masses likeS. uncinataof the present day, forming "Serpulite chalk." In the older tertiaries the genus is represented bySpirulaea.
Terebella lapilloidesoccurs in the Lias as a cylindrical, more or less curved tube of sand-grains.
fig159Fig. 159.—Eunicites avitusEhl. A fossil worm from the lithographic slate of Solenhofen: the jaws are seen in front, and the acicula along each side. (From Ehlers.) Natural size.
Fig. 159.—Eunicites avitusEhl. A fossil worm from the lithographic slate of Solenhofen: the jaws are seen in front, and the acicula along each side. (From Ehlers.) Natural size.
Fig. 159.—Eunicites avitusEhl. A fossil worm from the lithographic slate of Solenhofen: the jaws are seen in front, and the acicula along each side. (From Ehlers.) Natural size.
Amongst the Nereidiformia the remains are fewer, but theacicula 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 namesLumbriconereites,Oenonites, andArabellites, 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 namesCrossopodia,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, othersU-shaped, have received such names asArenicolites,Scolithus,Histioderma; whilst under the nameLumbricariacertain 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; thusCornulitesis 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.
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, thePhanerocephala, 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, theCryptocephala, 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 asSyllis, 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 branchPhanerocephalacontains the following five sub-Orders, though it is possible that the Capitelliformia deserves a more important position in the system:—
Sub-Order 1.—TheNereidiformiahave well-developed tentacles and palps; the peristomium almost invariably possessesspecial 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.—TheSpioniformiapossess 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 aberrantChaetopterus.[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."
Sub-Order 4.—TheCapitelliformiahave 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.—TheScoleciformiapossess 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 branchCryptocephalacontains 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."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]
Fam. 1.Syllidae.—These are small worms, the majority being less than an inch long, so that they are not easily observed.The body consists of a fair number of segments.[368]In many genera a dorsal bundle of unjointed, natatory chaetae makes its appearance at maturity. The palps, which are grooved, are in some cases so united with one another and with the prostomium as to be scarcely recognisable. (For head see p.262, and for feet see p.264.) The pharynx is armed with one or more teeth. There is a special gizzard, following the pharynx, and provided with thick, muscular walls of peculiar structure. Following the gizzard, the oesophagus receives in many genera a pair ofT-shaped diverticula, that are used for storing water, which is swallowed with food. These diverticula are absent inAutolytusand other free-swimming forms. The reproduction of the members of this family is interesting, and has already been described (p.278).
fig160Fig. 160.—Syllis armillarisMüll. × 2. (From Johnston.) The head is towards the right.
Fig. 160.—Syllis armillarisMüll. × 2. (From Johnston.) The head is towards the right.
Fig. 160.—Syllis armillarisMüll. × 2. (From Johnston.) The head is towards the right.
Syllis.—The tentacles and cirri are moniliform; the palps large; there is a single dorsal tooth, which is provided with a poison gland, the duct of which opens near its apex; it is used rather for stabbing its prey than for grasping and tearing.S. krohniiEhlers, is abundant under stones, and forms tubes of sand; it is nearly an inch long, and consists of some eighty-five to ninety-five segments marked with yellow bands. It may readily be identified by longer dorsal cirri, terminally dilated, alternating with shorter ones.S. cornutaRathke, has a translucent green body, about half an inch long; no alternationof cirri. Mediterranean, Atlantic, on the Norwegian coast, off Spitzbergen, and on the Madeira coast.S. armillarisMüll. is very common at low water; it is pale yellowish-brown, with a couple of dusky marks on each segment; and measures 2 inches. The dorsal cirri are quite short, consisting of only eight to ten joints. InPionosyllisthe tentacles and cirri are not moniliform; a single dorsal tooth.P. malmgreniM‘I. under stones.Sphaerosyllis.—The dorsal cirri are swollen at the base, and are not moniliform; the long palps are fused along nearly their whole extent.S. hystrixClap. is only about one-eighth of an inch in length.ExogoneOerst.GrubeaQfg.
Autolytus.—The small palps are entirely fused with the prostomium; the pharynx, which is bent upon itself, is armed with a circle of denticles. Dorsal cirri somewhat foliaceous. There are no ventral cirri. The male and female differ from one another and from the asexual "stock" (see p.279).A. pictusEhl. is abundant under stones. It measures about two-thirds of an inch in length, is darkly coloured with a median lighter band; the anterior dorsal cirri are long.A. proliferMüll. is common.
Myrianida fasciataMilne Edwards, with its foliaceous cirri, occurs off our coasts (see Fig. 149, p.280). Atlantic, Mediterranean.
Fam. 2.Hesionidae.—The body is relatively short, with only a few segments (sixteen to fifty, according to the genus); in the larger forms it is cylindrical. The parapodium is usually uniramous; the dorsal cirri are long and multiarticulate; the chaetae are jointed. The prostomium carries, in addition to four eyes, two or three tentacles, and generally a pair of jointed palps. The peristomium and two or more of the following segments are achaetous, and carry long "peristomial" cirri. The pharynx is very long but unarmed.
PsamatheJohnston, has many segments; head with two tentacles and a pair of three-jointed palps.P. fuscaJnstn. occurs amongst coralline Algae, to which it bears some resemblance, which is heightened by the moniliform cirri. It is a small worm, less than an inch in length. Mediterranean.Castalia punctataMüll. is dirty green or brownish, with a narrow purplish band on each side. It occurs in deeper water than the preceding. InOphiodromusthe head has three tentacles; the palps are two-jointed; there are six pairs of peristomial cirri; the parapodiaare biramous.O. vittatusSars is dredged in numbers off the Scotch coast, and is found also at low tides. It measures 2 inches in length. A closely allied species lives in the ambulacral grooves of the starfishAstropecten.
Fam. 3.Aphroditidae.[369]—The most characteristic feature of this family, and one by which its members are absolutely distinguished from all other Chaetopods, is the possession of scales or "elytra" on the back. These flattened dorsal cirri are of a somewhat horny texture, and are carried, generally, on alternate segments of the body; filamentous cirri occurring on the other segments. In the sub-familiesHermioninaandPolynoinathe elytriferous segments are 2, 4, 5, 7, 9, etc., up to 23; then every third segment. The worms are usually short, with some thirty-five to forty-five segments, thoughSthenelaisand a few others have many more. (For head see p.262, and for parapodium see pp.265,268.) The pharynx is very thick walled, and furnished with two pairs of jaws, which are, however, not hardened in the sub-familyHermionina. The intestine is provided with a number of paired longer or shorter caeca (Fig. 142). A considerable number of this family are commensal or parasitic (see p.297). The family is well represented on our own coasts, so that only a few of the more readily distinguishable species can be here described.
fig161Fig. 161.—Polynoë squamataL. Nat. size.c, Notopodial cirrus;e, elytron;f, parapodium;p, palp;t, tentacle. (From Johnston.)
Fig. 161.—Polynoë squamataL. Nat. size.c, Notopodial cirrus;e, elytron;f, parapodium;p, palp;t, tentacle. (From Johnston.)
Fig. 161.—Polynoë squamataL. Nat. size.c, Notopodial cirrus;e, elytron;f, parapodium;p, palp;t, tentacle. (From Johnston.)
Sub-Fam. 1.Polynoina.—Body flattened, with nearly parallel sides, usually short, more rarely worm-like; three tentacles; peristomium with long dorsal and ventral cirri; the ventral cirri of the next segment are also elongated. Jaws are present. Elytra, usually twelve to eighteen pairs, the surface of which is more or less papillose, and may be "fringed" along the outer border, with long processes. The colouring of the elytra is characteristic in most cases, though liable to considerable variation in some species. The chaetae are generally strong, and of bright golden colour: they are all unjointed. ThePolynoinaare generally but feeble swimmers,and are mostly found under stones at low tide. Some species have a very wide geographical range.
Polynoë.[370]—The body is short; none or only a few segments at the end of the body are uncovered by the elytra, except in the long body ofP. johnstoni.
A. With twelve pairs of elytra.—InP. squamataLinn. the elytra entirely cover the body and conceal the head, each elytron overlapping the next posterior one, and those of the two sides overlapping. General colour sandy-brown, speckled, lighter or darker. The fringed elytra are very firmly fixed to the body. The notopodial chaetae scarcely project from below the elytra. The worm is common between tide-marks and in the coralline region, is about one to one and a half inches in length, and about one-third of an inch in width. Atlantic.P. clavaMontagu, may attain a larger size, though it is generally smaller. The elytra are dark, usually grey, mottled with white or light grey, unfringed, and do not overlap to so great an extent as inP. squamata, so that the middle of the back and the hinder part of the body is more or less exposed. This is never the case in the preceding species, but even here it is subject to variation in extent, depending on the amount of food contained by the worm or on the ripeness of the genital products. It occurs in the Mediterranean.[371]
fig162Fig. 162.—Elytra,A, ofPolynoë squamataL.;B, ofP. clavaMont. × 10.a, Area of attachment;e, external margin;f, fringe (the letter is at the posterior side of the elytron);i, internal margin. (From Bourne.)
Fig. 162.—Elytra,A, ofPolynoë squamataL.;B, ofP. clavaMont. × 10.a, Area of attachment;e, external margin;f, fringe (the letter is at the posterior side of the elytron);i, internal margin. (From Bourne.)
Fig. 162.—Elytra,A, ofPolynoë squamataL.;B, ofP. clavaMont. × 10.a, Area of attachment;e, external margin;f, fringe (the letter is at the posterior side of the elytron);i, internal margin. (From Bourne.)
B. With fifteen pairs of elytra.—P. imbricataL. is probably the commonest species of the genus, occurring nearly everywhere under stones at low tide. It is about an inch in length; the elytra are deciduous, and are very variously coloured and marked; sometimes uniformly grey or even black, sometimesmottled with brown: in other specimens each elytron has its outer half pale or white, while its inner half is darker, usually some tint of brown or olive green, so that the worm appears to have a dark band along the middle of its back. Other patterns occur. The body is entirely covered by the elytra. The chaetae project considerably, and are nearly as long as half the width of the body; those of the notopodium are brown and are directed upwards, being nearly as long as the golden neuropodial chaetae. This species has a very wide range, occurring on both sides of the Atlantic, even on the shores of Nova Zembla, and reappearing again at Japan.P. semisculptusLeach is rather larger than the foregoing. The elytra are very readily detached: they are light in colour, without a fringe, but with large papillae near the margin. The notopodial chaetae are thicker than those of the neuropodium. Several other species are also common, butP. johnstoniv. Marenz.[372]differs from the rest in having an elongated body of some seventy segments, so that the posterior half is uncovered by the elytra, which are small, greenish-grey, speckled, and have no fringe. It is common and widely distributed, but appears to be only found in the tubes ofTerebella nebulosa.
fig163Fig. 163.—Elytron ofPolynoë imbricata.a, Area of attachment to body;e, outer border;i, inner border.
Fig. 163.—Elytron ofPolynoë imbricata.a, Area of attachment to body;e, outer border;i, inner border.
Fig. 163.—Elytron ofPolynoë imbricata.a, Area of attachment to body;e, outer border;i, inner border.
C. With eighteen pairs of elytra.—P. gelatinosaSars, may attain a length of 2 inches. The elytra are very faintly coloured, transparent and soft, attached by rather long peduncles. In spirit they become swollen and folded, giving the worm a very untidy appearance. The prostomium is partly overlapped by a peculiar collar-like fold of the peristomium.
D. With numerous pairs of elytra.—Lepidastheniahas a very long body, consisting of more than eighty segments. The elytra are quite small, and occur throughout the body on the usual segments. There are no notopodial chaetae.L. elegansGr. is a very elegantly marked worm, which, however, has not been recorded from the British area; it occurs in the Mediterranean (see Fig. 156, p.293).
Sub-Fam. 2.Hermionina.—The body is short, oval anddepressed; the particularly strong notopodial chaetae are directed upwards and backwards so as to protect the elytra. The neuropodial chaetae are also strong. The prostomium carries a single tentacle and two long palps; the prostomial ridge may be well developed. The peristomium is chaetigerous, with long cirri. The jaws are represented merely by thickened prominences.