ANNELIDA - POLYCHAETA The Polychaeta include the marine bristle-worms. A very few species have adopted a freshwater habitat. They differ from the other great subdivision of the Annelida, the Oligochaeta, in the following ways. In the Oligochaeta there are no appendages, and the bristles are embedded directly in the body-wall ; there is a clitellum and hermaphroditism is the normal condition. In the Polychaeta there are almost always appendages of various kinds, the numerous bristles are borne by special organs, the parapodia; there is no clitellum and the sexes are separate.
The general form varies greatly accord ing to the mode of life of the animal, which may be active or sedentary. The body is usually divided into the cephalic lobe or prostomium, the buccal segment or peristomium, together usually called the head (fig. 4) (in forms where cephalization is advanced this term includes additional elements), the metastomium or body proper which is often marked off into thorax and abdomen, and the pygidium or terminal segment. The prostomium may assume a variety of forms, square, conical, complex or annulated. It is usually provided with eyes, which range from simple pigment spots to complex organs with a crystalline lens. The head often carries a number of appendages, the tentacles, palps, tentacular cirri, which are innervated from different sources. Morphologi cally the appendages in the Polychaeta are divisible into two groups, those which are purely epidermal and solid and those which are hollow and contain all the elements of the body-wall. In the majority of the appendages these two groups are corn bined, the hollow basal portion being designated by the termina tion—phore, and the solid terminal portion by the termination— style. Thus there are palpostyle and palpophore, cirrostyle and cirrophore.
The parapodia or feet in which the bristles are implanted are hollow outgrowths of the body-wall. They are typically composed of two branches, the dorsal or notopodium and the ventral or neuropodium. The foot may be a very complex structure as in the Nereidae, with various membranous expansions and lamellae. (Fig. 5.) Each branch of the parapodium is sup ported internally by stout chitinous rods, the acicula, and there are usually a dorsal and a ventral cirrus : on the upper surface of the foot in some groups simple or ramified branchiae are present. In the sedentary orders the ventral lobe is reduced to a transverse ridge or torus uncinigerus, in which are embedded numerous small hooks.
Bristles.—The chaetae or bristles display an immense variety of form. They are divisible into bristles proper and hooks. The bristles proper may be simple or compound. Simple bristles may be smooth and capillary, or toothed in an almost indefinite num ber of ways. They may be shaped like a paintbrush as in Pan thalis; pectinate as in the Eunicidae; winged as in the Terebel lidae; transversely excavated as in Neplithys; or paddle-shaped as in the heteronereids. The compound bristles consist of a basal article with which is articulated a terminal piece. The uncini or hooks of the Sedentaria are small chitinous plates the external edge of which is broken up into a series of minute teeth ; or there may be one main tooth with rows of smaller teeth above it. (Fig. 6, a to h.) Morphology.—The outer layer of the body-wall consists of an epidermis, which is sometimes ciliated, and a cuticle. The epidermis is composed of supporting cells and gland cells. The bases of the supporting cells may form a basal layer, and in cer tain regions the epidermis may be greatly thickened as in the gland shields of the Terebellids which secrete the mucus em ployed in the building of their tubes. Beneath the epidermis there are three muscle-layers, a circular, a longitudinal, and an oblique, of which the longitudinal is the most important. The muscles are unstriped. The body-wall is lined by a coelomic epithelium and besides the sexual elements the coelom contains amoebocytes, leucocytes, phagocytes, etc. In the Sedentaria there is a tendency for the segmental septa to break down and for the coelom es pecially in the thoracic region to become a continuous cavity extending over many segments, as in Arenicola.
The alimentary canal is usually a straight tube from mouth to anus; but in some of the Seden taria, notably in the Amphictenidae, it is much coiled. In a num ber of families the digestive tube begins with an exsertile pro boscis provided sometimes with complex chitinous jaws and sometimes with soft papillae. It can often be divided up macroscopically into a number of regions, a pharynx, gizzard, stomach, intestine, etc. Nereis for example has a stout armed proboscis, an oesophagus, a thick walled gizzard with caeca and a moniliform intestine. Syllis has a buccal bulb followed by a pharynx armed with a tooth; then comes a curious barrel-shaped proventriculus, a ventriculus with a pair of T-shaped caeca and a moniliform intestine. In the Capitellidae there occurs an acces sory intestine or siphon similar to that in the Echiuridae.
This system is very variable. It is nor mally closed, and in the widest terms consists of a dorsal and a ventral vessel with a number of transverse connectives. In some groups it is rudimentary or even absent as in the Glyceridae and the Capitellidae. In certain families of the Sedentaria there is a special contractile body known as the heart which may enclose a cardiac body. The blood is usually bright red : it may be colour less as in the Syllids, or green as in the Serpulidae. The latter colour is due to the presence of chlorocruorin in solution in the plasma. In a few species in addition to a closed vascular system there are red blood corpuscles in the coelomic fluid. A similar con dition is found in certain echiuroids.
In many Polychaeta there are no special or gans of respiration, this function being performed by the vascu larized integument of the entire body. In those groups in which there is no differentiated vascular system the hollow outgrowths of the body-wall filled with coelomic fluid only are known as lymphatic branchiae. The true branchiae enclose an expansion of the vascular system in the form of a loop. Branchiae may be simple filaments as in Nephthys, ramified as in Terebella or pectinate as in Eunice. In the Sabellidaa and Serpulidae they form in front of the head a large plume composed of numerous filaments provided with ciliated barbules. Generally in the Sed entaria they are confined to the anterior end of the body and in the Errantia they are metamerically arranged on the dorsal sur face of the feet and may occur on a large number of the body segments. (Fig. 7, a to c.) Excretion.—Excretion is principally carried out by the seg mental organs or nephridia. These consist typically of a ciliated tube often with an intercellular lumen opening into the coelom by a ends blindly in branches provided with vibratile flame-cells or solenocytes. In the Phyllodocidae at the time of sexual maturity this blindly ending nephridium fuses with the large ciliated funnel and thus forms a passage for the escape of the genital products. In the Capitellidae the genital and excretory passages remain separated, and in many of the Sedentaria the nephridia are re duced in number and confined to the thoracic region. In the Terebellidae also the anterior pairs are purely excretory and the posterior act as gonoducts, a specialization of function having taken place.
The nervous system consists of a dorsal nerve mass or brain which may be formed by the fusion of sev eral pairs of ganglia, and the perioesopha geal commissure or nerve loop joining the brain to the main system, the ventral nerve-chain. This chain consists of two longitudinal cords with segmental gangli onic swellings and segmental lateral nerves. The two ventral cords may be widely sep arated as in Serpula or completely fused into one as in Arenicola, which has no ganglionic swellings in its nerve cord. There are many intermediate stages be tween these two extreme conditions. (Fig. 9, a and b.) Sense Organs.—Under this heading may be classed the eyes, the nuchal organs, the lateral organs and the tactile organs.
Eyes are present in most Polychaeta dor sally on the prostomium; they occur also in other parts of the body. In certain Ophe liidae they are ranged segmentally on the side of the body. In Amphiglene the pygidium is furnished with eyes. In many Serpulidae the branchiae are provided with eye spots. The eyes themselves show every stage of development be tween the simple pigment spot and the complex organs of the pelagic Alciopidae, which have a crystalline lens, a retina and a vitreous body.
The otocysts, which are really organs of equilibrium rather vibratile funnel and to the exterior by a small pore, the nephridio pore. The segmental organs are as a rule complex organs formed by the junction of an excretory tube with a genital funnel. (Fig. 8, a and b.) In Nephthys in addition to the segmental organ there is a large ciliated organ in close connection with it. E. S. Goodrich considers this to be a genital funnel modified to form a phagocytic organ, for it acts as a leucocyte-collector. The leuco cytes gather together the waste products and there may be a physiological connection between it and the nephridium. In Nephthys as in the Glyceridae and Phyllodocidae the nephridium than of hearing, are confined to the Sedentaria. There are three types of this organ, a sac communicating with the exterior by a ciliated canal and containing a foreign body such as a grain of sand, which functions as an otolith (Arenicola marina, Branch iomma) ; a closed ciliated cavity containing an otolith which has been secreted (Jasmineira) ; a closed cavity without cilia which contains a number of secreted otoliths (Arenicola ecaudata).
The nuchal organs are ciliated pockets above the hinder portion of the brain. They are paired and sometimes eversible as in the Glyceridae. The lateral organs are richly innervated segmental ciliated pads found along the side of the body in a number of families. Ciliated organs may occur as in the Spionidae in very different parts of the body, and Soderstrom thinks that certain dorsal sense organs in the latter family may function as olfactory organs and help to bring the males and females together at the time of sexual maturity.
The tactile organs are the palps, tentacles, cirri and various papillae.
The Polychaeta are normally dioecious. Hermaphroditism is confined to a few species mostly belonging to the Serpulidae. The cosmopolitan Nereid, Platynereis dumerilii is notable in being polymorphic and having a hermaphrodite form. Except for the copulatory chaetae in the Capitellidae and for the differentiated gonad in Arenicola ecaudata and Hesione sicula, the Polychaeta do not possess specialized genital organs. The germ cells are proliferated from the coelomic epithelium lining the parapodial blood-vessels and drop off into the coelom in which their development is completed. There is a variety of ways by which the ripe germ-cells are released, of which the commonest is by way of the segmental organs. In a few instances (Owenia f usi f ormis, Micronereis variegate) the eggs and sperm are set free through special anal pores, and in Perinereis cultri f era (male) through the anus itself. Often the body-wall is ruptured, and sometimes as in Clistomastus lineatus and the Palolos the modi fied part of the body bearing the sexual elements breaks off in its entirety from the rest. Sexual dimorphism is the exception in the Polychaeta. Those groups which pass through an epitokous phase at the time of sexual maturity, a phase which involves a kind of metamorphosis, and those groups in which reproduction takes place by budding show external differences between the sexes. Otherwise except for the copulatory chaetae in the Capi tellids and the genital papillae in the Terebellids the sexes are distinguished by their germ cells alone. (Fig. io, a to c.) Epitoky or sexual metamorphosis is most marked in the Nereids, but it occurs also in several other families. At the time of sexual maturity a great change both external and internal takes place. The appearance of the head is much altered, the eyes grow to a great size and the palps shrink. The body appears to be made up of two distinct parts, the anterior end, sometimes filled with genital cells, in which the feet remain unmodified, and the posterior end in which the feet are furnished with enormous foliaceous lobes and with special paddle-shaped swimming chaetae.
At the same time great internal changes take place. The muscles and digestive tube undergo histolysis, which is accompanied by a certain amount of histogenesis. According to A. Dehorne the coelomic leucocytes at this period take on the role of active phagocytes. The name Heteronereis is applied to Nereids under going the sexual phase. At this time the sexes are often distin guished by differences in the pygidium.
Among the Syllids reproduction takes place by budding and by fission or division of the body. To these processes the names blastogamy and schizogamy have been given. In blastogamy a parent individual, which is asexual, buds off at its posterior end a chain of sexual individuals or stolons which break away from the parent stock to lead an independent existence. Budding takes place just in front of the pygidium. The anterior segment forms a head and the remaining segments are intercalated between this and the pygidium. In Autolytus the male stolons are known as Polybostrichus and the female as Sacconereis. In schizogamy one of the segments of the parent individual forms a head with ap pendages and the stolon is formed at the expense of the posterior region of the parent's body. Often reproduction takes place by a combination of blastogamy arid schizogamy, the schizogamous stolons themselves budding a number of segments before break ing away from the parent individual (Procerastea) .
In the Polychaeta cell-division is normally spiral, and when the embryo leaves the egg-membrane and begins to lead a free larval existence it is called a Trochophore. The Trochophore is described as atrochous (Terebella), monotrochous (Chaetopterus), polytrochous (Opliryotrocha) according to the number and position of the ciliated bands which encircle it. (Fig. I I, a and b.) The trochophore stage is followed by a phase, often free-swimming, in which the young differs greatly from the adult. In the course of time the various appendages are devel oped, the larval bristles are re placed by the adult bristles and the fully grown worm is formed.
For a histor ical account of the Polychaeta the reader is referred to the in troduction to J. H. Ashworth's Catalogue of the Chaetopoda in the British Museum (Part 1, London, 1912), and to de Quatrefage's Histoire des Anneles (Paris, 1865) ; the works mentioned in our bibliographies on PP. 994 and 998 below, should also be consulted.
Fauvel following Audouin and Milne Edwards divides the Polychaeta into the Errantia and the Sedentaria. He defines the Errantia as Polychaeta in which the segments of the body are all similar except the segments adjoining the mouth and the terminal segment or pygidium ; and the Sedentaria as Polychaeta in which the body is divided into several regions, the head is small or profoundly modified, the feet are almost always simple, the ventral branches often in the form of a torus armed with hooks or uncini; the branchiae are normally confined to a definite region and the species are habitually tubicolous. W. B. Benham's arrangement of the group into seven sub-orders (Vol. II. of the Cambridge Natural History, 1896) is still commonly adopted.
There has been much speculation about the origin and affinities of the Annelids. The most recent is that of R. Woltereck based on his study of the development of the Archiannelid Polygordius. Before the separation of the mouth and the anus, the Annelid Trochophore bears some resemblance to a primitive Ctenophore, and Woltereck suggests that a primi tive Ctenophore-like ancestor took to a burrowing mode of life in its adult stage ; that segmentation was the result of the adop tion of wriggling in place of a ciliary mode of locomotion. On this hypothesis the presence of segments in the free-swimming larva has to be attributed to a telescoping of the racial history. Objections to Woltereck's hypothesis are stated at length by Soderstrom in his Das Problem der Polygordius-Endolarve (Uppsala, 1924)• Ecology.—Except for about a dozen freshwater species, mostly Nereidae, and half that number of euryhaline species, the Poly chaeta are marine. They occur most abundantly between tide marks and in shallow water near the shore, but they have also been dredged from very great depths. Their mode of life is very variable. Some live in tubes in the mud or sand, others in holes in the rock, or fixed to stones and shells, and in every kind of crevice. A number of species are pelagic. Others again attach themselves to Echinoderms or sponges.
The majority of the Polychaeta are brilliantly coloured in life, and a number, chiefly Syllids, are phosphorescent. Chaetopterus is also exceedingly phosphorescent, and its normal green colour is derived from the Algae on which it feeds.
Autotomy is of common occurrence in the group, especially among the Polynoids and the Eunicids. In fact it is usual on examination of a member of the latter group to find that it is regenerating or has regenerated its posterior end, and the Polynoids shed and regenerate their elytra or scales with great facility.
Some Polychaeta are active and predatory carnivores with large prehensile jaws, others feed on Algae.
Many of the Sedentaria feed on small organisms in suspension in the water, which become entangled in their tentacles and gill plumes. Some like the Arenicolidae ingest large quantities of mud and sand and derive nourishment from the organic matter mixed with them.
Parasitism is uncommon in the group. The only internal parasites known belong to the Eunicidae, and, with the exception of Oligo$nathus bonelliae which is parasitic in the body-cavity of Bonellia and the doubtful Ophiuricola cynips in a deep-sea Ophiuroid, they attack other Polychaeta only. Ex ternal parasites are even rarer. Icthyotomus sanguinarius is a parasite on the eel, Myrus vulgaris, and Parasitosyllis on other Polychaeta and on Nemertines.
Commensalism is of common occurrence, and of all the families the Polynoidae are the most prone to this habit. Polynoe rutilans occurs with an Alcyonarian Xenia, Scalisetosus communis is found with an Ophiuroid, and Scalise tosus echini with Echinus esculentus. Lepidasthenia pulchra lives with a Holothurian, Sticliopus, Hololepidella commensalis with a Clypeaster and Acholoe astericola in the ambulacral grooves of starfishes.
A large number of Polynoids also live in the tubes of other Polychaeta, chiefly Terebellids and Serpulids. Webster found Harmothoe parasitica living under the scales of Lepidametria commensalis which is itself a commensal with Amphitrite ornata.
Many Polychaeta shed their eggs and sperm in the surrounding water and this constitutes their total repro ductive activity ; but an increasing number of observations show that incubation and parental care are not uncommon in the group. Several species of Polynoids incubate their eggs on their backs beneath their scales. In the Syllid Exogone gemmifera the de veloping eggs are attached in longitudinal rows to the ventral surface of the mother, where they remain until they are capable of fending for themselves. Some Syllids carry their young on their backs : the Spionids lay their eggs in the tubes which they inhabit and in some species the young remain in the tubes of their parents until an advanced stage of growth. During this time a certain number of eggs and larvae cease to develop and serve as food for the others. According to Soderstrom in Poly dora nutrix a portion of the maternal tissue undergoes histolysis and serves as a sort of nutritive mass in which the young develop. Capitella capitata and the hermaphrodite form of Nereis dumerilii build tubes at the time of sexual maturity, and do not leave them before their young are old enough to take care of themselves. In Autolytus and its allies the females at the moment of egg-laying secrete a viscous fluid which hardens on contact with the water and forms a sort of brood-pouch in which their fertilized eggs develop. C. Gravier observed a similar phenomenon in a Phyl lodocid Eteone gain. (Fig. 12, a to c.) Potamilla antarctica is said to keep its developing young inside its branchial plume. The spirorbids are unique in incubating their young in their operculum, which is the dilated end of a single branchia, normally used as a sort of stopper to their tube. Finally a small number of Polychaeta are viviparous, that is to say incu bation takes place actually within the body of the parent. The Syllids, Syllis vivipara and Ehlersia nepiotica are viviparous, as are also a few Serpulidae. M. Caullery and F. Mesnil have sug gested that the so-called viviparous Syllids are immature forms reproducing themselves parthenogenetically, and that when ma ture they are normally dioecious and behave as the rest of the family.
The tubes of the Polychaeta are varied in form. Some secrete from their mucous glands, which in the Terebellids form the thick ventral gland shields, tough parch menty envelopes, and others reinforce these with small particles from the surrounding medium. Lanice conchilega builds its tube with small shells and grains of sand, Sabella with small particles of mud. The Serpulidae secrete carbonate of lime with which their hard brittle tubes are built.
The Polychaeta have little economic importance. Arenicola, the common lug-worm, several species of Nereis, Marpliysa sanguinea and a Nephthys are used as bait by fisher men. The natives of Samoa and the adjacent islands eat the famous Palolo. At certain times of the year the posterior modi fied segment of the body of Eunice viridis, which has undergone a sexual metamorphosis, breaks off from the anterior end and laden with ripe germ-cells swarms in enormous numbers at the surface. These are collected and eaten by the natives.
Our knowledge of the geological history of the group is largely based on the study of calcareous tubes found attached to stones and to other fossils. Spirorbis is found in the Silurian and other Palaeozoic strata. Serpula is very abundant in the Jurassic where its occurrence has given rise to Serpulite chalk. The Nereidiformia, of which several species have been described from the Jurassic slate of Bavaria and from Palaeozoic beds in Canada survive mostly in the form of jaws and acicula. Walcott however in 1911 described a number of very well preserved Mid dle Cambrian Polychaeta from the Burgess shale in British Co lumbia. Many tracks and burrows in the rock of doubtful interpretation have been attributed to the Polychaeta.
Fauvel's contention that no well-defined zoo logical regions exist for the Polychaeta is now generally accepted by students of that group. Out of the five most important families of the Errantia more than one half of the total number of species known from the coasts of France are common to the Channel, to the Atlantic and to the Mediterranean. Furthermore out of 122 species collected by the Prince of Monaco at the Azores, 4o or nearly one-third are also part of the Arctic fauna and 29 are equally found in the Antarctic. Out of a total of 300 species collected by him between the Azores and Spitzbergen about 6o are common to both hemispheres. Again out of a col lection of 72 species from Madagascar 46 or nearly two-thirds were common to the northern part of the Indian ocean and to the Philippines. Finally a certain number of species, of which the most well known are Serpula vermicularis and Nereis dumerilii, are cosmopolitan. An examination of the littoral Polychaet fauna of the tropics discloses a number of species which never desert the warm zones, but are found equally in the same latitudes all over the world. With regard to bathymetric distribution the greater majority of the Polychaeta are found be tween the tide-marks or within the i oo fathom zone. Beyond the continental shelf their number decreases rapidly.
Thus out of the Prince of Monaco's collections only 25% were found between the i,000 and 2,000 metre line and only 5% be tween the 4,00o and 5,000 metre line. There are very few strictly abyssal forms, most of the examples found at great depths be longing to common littoral species.
