The Annelida may be described as bilaterally symmetrical animals, with flattened or cylindrical bodies, composed of numerous soft rings, or without such. The locomotive appendages (generally furnished with bristles) are not articulated. Nervous system consisting of a cephalic ganglion or pair of ganglia, connected on each side of the oesophagus with a chain of ganglia running along the ventral aspect. Mouth ventral; alimentary canal with . an anus. Circulatory system with distinct vessels. The majority are small animals, but some reach the length of 6 feet and upwards, and are as thick as a finger.

In the present article the Annelida will be understood as comprehending the A. Poly- chaeta, A. Oligochaeta, A. Onycophoba, A. Discophoka, and A. Gephybea, the first two being often placed together in the subclass Chaetopoda.

I. In the first order, A. Polychaeta, the body (fig. 1) is elongated, segmented, more or less cylindrical (certain tubicolous forms having two well-marked regions), and almost universally furnished with uniramous or biramous bristlebearing feet (parapodia, Huxley). The first segment is modified, so as to form a head or snout, and has the eyes, tentacles, &c. The second or buccal segment bears the mouth and certain processes. The other segments have foot-tubercles, bearing bristles developed in special sacs (e, fig. 4),’ and various appendages, such as elytra, dorsal and ventral cirri, &c. The structure of the bristles (e.g. fig. 2) holds a prominent part in the discrimination of species and even genera, and the same may be said of the hooks of many of the Tubicola (fig. 3). A curious example of the Opheliidae has no bristles, and Tomopteris has only one at the base of each tentacle. Posteriorly, the diminished body generally terminates in two elongated cirri in the free forms, while in the sedentary such processes are often absent. In Poly-dor a (fig. 21) a curious funnel-like structure exists.

The body in the Polychaeta (e.g. fig. 4) is invested bv a finely fibrous, cuticular, or chitinous layer (<Z), to which the iridescence in many forms is due. This is perforated at somewhat regular intervals by two series Of pores, and cilia are generally present at certain points. In the sedentary annelids the cuticle is thinner, a feature probably in connection with their protective tubes. Beneath the cuticle is a cellulo-granular hypoderm, with tubular glands, bacilliparous and glandular follicles in certain cases, as well as the deposits of pigment. The muscular system is well developed, and the bands variously arranged in the body-wall. A well-marked circular layer is generally present externally, while in others it would seem to be represented only by oblique bands. The longitudinal bundles follow different plans, such as forming an almost continuous coat, or split up into various distinct ribands (α and l>, fig. 4, the former representing one of the dorsal bands, the latter one of the ventral). Schneider thought that the arrangement of the muscular system might form the basis of a classification of this and allied groups; but for sound reasons it has not been adopted. There is no tendinous raphe at each segment, though fibres from the dissepiments pass through the longitudinal layer, in some cases even to the hypoderm. The muscular fibres have a hyaline connective tissue. In Myxicola and Protula the fibres of the longitudinal coat have a dendritic appearance in transverse section. Besides the foregoing, many have a powerful oblique band on each side, passing down to be inserted near the nerve-cord, or elsewhere; and the proboscis (a, fig. 5), bristles, branchiae, alimentary canal, and other organs, have a series of special muscles. By the muscles of the body-wall the Polychaeta execute all their movements in the water and on land, the feet-papillae with their bristles playing quite a subordinate part. A few progress in addition by special organs, such as the tentacles of the Terebellae.

The cutaneous and muscular tissues enclose a space termed the perivisceral cavity, which stretches from the anterior to the posterior extremity. This chamber is lined by a special membrane, which likewise envelops the alimentary canal, vessels, and other organs. It is generally divided into various transverse spaces by muscular dissepiments or screens (which permit communication), and occasionally there are longitudinal partitions. In this cavity the highly organised perivisceral corpuscles float in a coagulable lymph, which has no relation with sea-water. The lining membrane is ciliated in Aphrodita, Glycera, Polycirrus, Tomopteris, and Terébella ; and it is curious that in these the blood-vessels proper are absent, with the exception of the first and last. Ray Lankester has found that haemoglobin occurs in the perivisceral corpuscles of Glycera and Capitella. Professor Huxley, after Μ. de Quatrefages, is inclined to think the perivisceraj. fluid the true blood of the group, but this is doubtful.

The circulation of the blood is effected by a system of closed vessels with muscular walls (fig. 5), the funda mental arrangement consisting of a dorsal and a ventral trunk (v), with communicating branches in each segment—the current in the former being directed forwards, and in the latter backwards. Branches from the main trunks ramify on the wall of the alimentary canal, body-wall, dissepiments, and other parts. The dorsal gives branches to the branchiae, whence the blood passes by other twigs to the ventral. Sometimes dilatations, termed “hearts,” occur at the bases of the branchiae —as in Eunice; and in Arenicola and Polyophthaltnus similar dilatations exist in other parts. Various modifications of the system take place, such as the bifurcation of the dorsal vessel anteriorly or posteriorly, the presence of blood-sinuses or lacunae round the intestine in certain sedentary forms, and great diversity in regard to capillary distribution. The blood is usually reddish in colour, and devoid of corpuscles—the latter, however, being present in Syllida, some of the Ophe-liidae, Cirratulidae, and in Staurocephal·us. It is colourless in Aphrodita, greenish in Stylaroides and certain Sabellidae. Finally, Glycera and Polycirrus are devoid of a true circulatory apparatus. This system is compared by Professor Huxley to the water-vascular system of the Echinoderms, Trematoda, and other groups; but, as formerly mentioned, this is doubtful. Haemoglobin has been found by Ray Lankester in. the blood of many of the Polychaeta.

The Annelida composing this group generally possess a special branchial apparatus, and where this is absent (e.g. Lumbriconereis), the skin and ciliated digestive chamber seem to carry on the function. So characteristic are the branchiae, that the term Dorsibranchs was aptly bestowed on one section (see fig. 4), and Cephalobranchs on the other (see fig. 6). The branchiae have the form of simple filiform organs, or they are pinnate, bipinnate, pectinate (e, fig. 4), flabelliform, or arbuscular processes. To each the blood is carried by a branch of the dorsal vessel, which traverses the organ to its extremity, and, looping, carries the aerated fluid to the ventral vessel. Vascular loops pass between the two branches in the branchiae, and only very thin cuticular tissues covered with cilia intervene between the water and the blood, so that due oxygenation occurs. In the Ser-pulidae the branch from the dorsal vessel is continued directly into the twig going to the ventral, and from their point of union a single vessel traverses the branchia and sends processes into the pinnae, the blood, moreover, in these presenting undulatory movements. A cartilaginous framework exists in the branchial processes of the latter group and the Sabellidae (fig. 6). According to Fritz Müller the opercular plug in certain forms is developed by a gradual metamorphosis of the branchia.

The digestive system consists of a rounded alimentary canal, commencing at the second and terminating in an anus at the last or the penultimate segment. The mouth opens in the form of a simple slit or grooved dimple on the under surface of the second segment. In many the first region is a complex muscular and protrusible proboscis (fig. 5, a, pharynx and oesophagus of some, and the oesophagus of others), with papillae, homy or dense calcareous teeth (fig. 7), with a narrow oesophageal region (α') behind. Certain glandular organs in the oesophageal region of certain forms are termed salivary. The walls of the succeeding portion (α") are often marked by numerous sacculations from constrictions at the dissepiments; and in some (e.g., Aphrodita) large diverticula occur. The inner surface bears vibratile cilia, the walls are glandular, and there are also circular and longitudinal muscular fibres. The gland-cells contain numerous refracting granules, and the tissue is by some held to be homologous with the liver of the higher forms. Many modifications of this system exist: thus, for example, there is in Syllis a firm, smooth anterior region (pharynx'), often armed, then a region provided with glands (proventriculus or gizzard), a stomach with a glandular sac on each side, and lastly the intestine. The food of the group consists of both animal and vegetable substances, and sandy mud is swallowed by others for the nutrient particles it contains.

The nervous system has the form of a pair of cephalic ganglia, and a chain of ganglia placed along the ventral aspect of the body (n, fig. 4). The former lie over the oesophagus, and send branches to the eyes, tentacles, and other organs. A cord (buccal) joins them on each side to the ventral chain, and thus the oesophagus is encircled by a nervous collar. In each segment the ventral ganglia give out branches for the supply of the neighbouring organs and feet. The nerve-cells chiefly occur on the ventral surface and sides of the ganglia. In some (e.g., Terebella), while the nerve cords are united in the anterior region, they are separate posteriorly. The eyes are much developed in Alciope, certain Syllidae, and others; but, on the other hand, many are eyeless. Most of the eyes are mere masses of black pigment (sometimes capsulated) situated on the head, destitute of means of accommodation, with certain nerve-threads going to them. In Alciope, sclerotic, lens, retina, and other parts exist. In Branchi-omma the eyes occupy the branchiae, and in Fabricia the posterior end of the body. Otolithes are stated to be present in a few {e.g. Arenicola), but on these and other minute points further investigation is required. The sense of touch is extremely delicate in the group; on the surface at large, as well as in the tentacles and cirriform processes Evame, Nephthys, Ophiodromus, and others, exhibit remarkable irritability.

The Polychaeta are for the most part dioecious, the sexual glands being variously developed on the inner surface of the body-wall or on the dissepiments, and developing in the forms of ruffs round the vascular axes. The products are detached into the perivisceral cavity. The ciliated and looped segmental organs of Dr Williams occur very generally, and may conduct outwards the sexual elements, but they have also been supposed to be excrementitious. In Tomopteris their ciliary current carries spermatozoa inwards, and the same may occur in other forms. The structure and deposition of the egg, and the changes in its development, agree with those in other classes. Certain Eunicidae are ovo-viviparous. Occasionally the ova are borne on the back (fPolynoe), or in a ventral pouch (Auto-lytus cornutus), until hatched, while in Spirorbis a modification of the operculum acts as an ovisac. The young escape as ciliated organisms with a long whip anteriorly; but, as development progresses, the cilia are confined to certain zones. A few immature forms, again, such as the young of Nerine, carry remarkable bristles, which are shed before the adult form is reached. Some have arranged the larval forms according to -the zones of cilia and the temporary bristles. In the growth of the young animal the development of the new segments takes place between the first and last In certain Nereidae, e.g. Nereis Du-merilii, at least two sexual forms exist,—viz., a small adult which develops either ova or spermatozoa in the usual way (except that in the male the elements occur in two testicles placed in one segment); and, secondly, another which becomes transformed into a Heteronereis before the sexual elements are developed. Metschnikoff further found another Nereis which is hermaphrodite. The phenomena of alternate generations is also observed in Autolytus, and fissiparity in Filigrana, —the latter, with Spirorbis and J77φΛ(<7Zenα,beingalsohermaphrodite. The female Autolytus quite differs from, the male, and has its body loaded with ova, which pass into a pouch in A. cornutus, Agass., and the products by-and-by get exit as fιee-swimming embryos. The male is wide in front, and has large tentacles, the sperm-sacs occupying the first five bristled segments. From the young of the foregoing is developed—so as to complete the series—the third kind, viz., the “parent-stock” of A. Agassiz, which differs from both the previous forms. No sexual elements occur in the parent-stock, but the males and females are produced from the body by transverse fission (fig. 8).

Regeneration of mutilated parts is common in the Polychaeta, even to the reproduction of a head.

The tubes formed by many exhibit an amount of precision and skill not far removed from the powers of the most ingenious insects. In common with the Nemerteans, their skin exudes a tenacious secretion which coheres under water, and this alone forms the protective tube in some; while in others it is strengthened with mud (fig.

9), sand, gravel, shells, and stones. The secretion varies from the most delicate film in the Syllidae, to the tough parchment-tubes of Ghoetopterus, or the rigid crow-quills of Hyalin<xcia (fig. 10). The eyeless Pectinaria belgica (fig. 11), again, fashions a tube like a straight horn (fig. 12) of minute pebbles or large grains of sand, carefully selected and admirably fixed to each other by a whitish secretion. In placing the grains together in the wall there is no haphazard, but angle fits angle as in a skilfully built wall. Beautiful semi-transparent tubes are constructed by an allied species in the deep sea from the siliceous spicules of sponges. The remarkable arborescent or pectinate processes which ornament the free ends of the tubes of certain Terébelloe are also noteworthy. The calcareous tubes of the fixed Serpu-lidae. or the free (e.g. Ditrypa), differ from those of the molluscs in the absence of organic connection between the animal and its protective sheath. Lastly, large coherent masses of coarse gravel and sand-tubes are formed on various beaches by Sabellaria.

The annelids are subject to many parasitic inroads, as by Gregarinoe, Opalinoe, and Nematoids, internally; various vegetable growths, zoophytes, Loxosomoe, and the crustaceans Selius, Selenium, Nereicola, Terebellicola, Sdbelliphilus, Chonephilus, and Sdbellacheres, externally. Commensalism is likewise not uncommon; thus Polyno'è scolopendrina haunts the tubes of Terebella nebulosa, and it also alternates with Harmothoë mar phy sae in the tunnels of Marphysa sanguinea; Acholoe astericola frequents the ambulacra! rows, of Astropecten, Malmgrenia castanea, the peri-oral region of Spatangus purpureus, and Nereilepas fucata associates itself with the hermit-crab in Buccinum; while Alcio-pina parasitica lives in the interior of Pleurobrachia, and one of the Amphino-midae in the respiratory cavity of Lepas.

The Polychaeta are all marine, and distributed over the whole surface of the globe,—often at very great depths,—as early shown by General Sabine, and recently by·the Norwegian dredgings, and those of the celebrated “Porcupine” and “Challenger” expeditions. Many species are common to the entire North Sea, and extend to the south as far as Gibraltar, while some northern forms enter the Mediterranean in considerable numbers. The British species are common to the North Sea, and many range to the shores of North America. The size of some of the forms greatly increases in the Arctic Sea. The tropical waters abound with lustrous and splendidly tinted speci mens, especially amongst the Amphinomidae. The borers in shell, limestone, coral, and Melobesia, seem cosmopolitan, and the swimming Tomopteris is likewise widely distributed. The forms frequenting the bottom of the sea live in mud or sand, or lurk under stones, in chinks of rocks, shells, and seaweeds. Many specially affect mud or muddy sand, e.g., the Lumbriconereidae, Nephthydidrae, Glyceridae, and the Terebellidae; while some prefer hard or stony ground, e.g. i the Polynoidae and Sigalionidae. Between tide-marks they abound in fissures of rocks, under stones in pools, under tangle-roots, and in sand or sandy mud. Many live in captivity several years, but the duration of their existence generally is unknown.

Tubicolar Polychaeta occur in various strata, from the Lower Silurian upwards, and Serpulites even appear in the quartz of Durness. Many of the tracks and burrows referred to the group (Polychaeta) are involved in considerable doubt; but the Eunicites, Lumbriconereites, and Merin-gosoma, from the Solenhofen rocks, are satisfactory evidences of fossil forms. The arrangement of the apertures of Arenicola didyma, from the Longmynd, in pairs is peculiar.

In regard to beauty of form and colour, complex structure and wonderful habits, the Polychaeta are not surpassed by any invertebrate class. The splendid bristles of the Aphroditidae, constantly glistening with all the hues of a permanent rainbow, the brilliant tints of the Phyllo-docidae, Hesionidae, and Nereidae, and the gorgeous branchial plumes of the Terebellidae, Sabellidae, and Serpulidae, can only be compared with the most beautiful types of butterflies and birds.

The class Annelida has been divided into the sections Branchiata and Abranchiata, the former comprehending the Polychaeta, the latter the other groups; but this does not always hold good, since many Polychaeta have no branchiae. The latter, again, have been separated into the Dorsibranchs and Cephalobranchs, the former corresponding to the Maricola, Errantia, or Nereidae of others, the latter to the Tubicola or Sedentaria. Grube’s divisions, Rapacia and Limivora, are based on the nature of their food. In the present state of the department it will suffice to indicate the following families (chiefly after Malmgren) under which the Annelida Polychaeta have been ranged, and to refer for further information to the Annélides of De Quatrefages, the Familien of Grube, the Choetopoda Qi Ehlers, the Annélides Chétopodes of Claparède, the British Museum Catalogue by Johnston and Baird, and the various works of Kinberg and Malmgren: — Euphrosy- nidae, Amphinomidae (fig. 13), Aphroditidae, Polynoidae, Acoe-tidae, Sigalionidae, Nephthy-didae, Phyllodocidae, IIesio-nidae (fig. 14), Alciopidae, To-mopteridae (provisionally), Syl-lidae, Nereidae (fig. 1), Stauro- CEPHALIDAe, LUMBRICONEREIDAe, Eunicidae (fig. 4), Onuphididae (fig. 10), Goniadidae,Glyceridae, Ariciidae, Opheliidae, Sc t ALi-BREGMIDAe,TELETHUSIDAe(fig. 20), Sphaerodoridae, Chloraemidae, Sternaspidae, Chaetopteridae, Spionidae (fig. 21), Cirratu-lidae, Capitellidae, Maldanidae (fig. 9), Ammocharidae, Hermel-lldae, Amphictenidae (figs. 11 and 12), Ampharetidae, Terebellidae, Sabellidae (fig. 16), Eriographidae, and Serpulidae (fig- 15). · . . .

II. The A. Oligochaeta are annelids without tentacles, cirri, or specialised branchial processes. Bristles, variously grouped, from two to eight and upwards in each transverse series. They are hermaphrodite, and the young undergo no metamorphosis.

The body is enveloped in a delicate cuticle resembling that of the Polychaeta, and pierced by many pores. Underneath is a cellulo-granular hypoderm, with numerous glands. The setae are simple, bifid, or hair-like. The muscular layers are an external circular, and an internal longitudinal, marked by the bristle sacs. The foregoing tissues enclose a perivisceral space, with the usual septa and the characteristic corpuscles in a coagulable lymph, which performs important functions in the economy.

The circulatory system consists essentially of a dorsal trunk situated over the digestive chamber, carrying the blood from behind forwards, and a ventral or sub-intestinal conveying the fluid in the opposite direction. The former is broken up into a plexus of vessels in the anterior segments, and generally gives off a large branch (perivisceral) on each side in the others. The periviscerals in certain segments behind the anterior plexus are enlarged, and so evidently contractile as to have received the name of “hearts,” and propel the blood into the ventral vessel. In the other segments the periviscerals pass to the latter, either with or without division into branches—in the form of cutaneous plexuses, which are greatly developed in this group. Branches...