ANNELIDA , a class of the sub-kingdom Annulosa, the title being derived from Lamarck’s term Annélides (Annellus, a little ring), given to Cuvier’s red-blooded worms. The latter were included by Linnaeus under his Vermes, along with intestinal worms, molluscs, zoophytes, and sponges. The labours of Pallas, Baster, Otho Fabricius, and especially 0. F. Müller, enabled Gmelin, in the 13th edition of the Systema Naturoe of the great Swede, to make some improvements and many additions. Cuvier ranged the Annelida under his Articulata, giving them the chief position on account of their red blood; but they are now generally classified as a separate type below the latter. It would be out of place to enumerate all the advances that have occurred since Cuvier’s time, but among those whose names stand prominently forward in this respect are Lamarck, De Blainville, Bose, Audouin, Milne-Edwards, Dugès, Moquin-Tandon, and De Quatrefages in France; Van Bene-den, Morren, and D’Udekem in Belgium; Gruithuisen, Oken, H. Rathke, Grube, Max Schultze, Hoffmeister, Hering, Kölliker, Schmarda, Keferstein, Ehlers, and Ratzel in Germany; Claparède in Switzerland; Malmgren, Metschnikoff, and Kowalevsky in Russia; Sars in Norway; Kinberg and Lovén in Sweden; Delle Chiaje in Italy; Stimpson, Leidy, A. Agassiz, and Verrill in North America; and Montagu, Dalyell, G. Johnston, J. R. Johnson, Williams, Huxley, Baird, and Ray Lankester in our own country.
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 pass from the ventral to the intestinal wall, inosculating with others from the dorsal, and forming a rich capillary network. In the Lumbrici there is also a third longitudinal vessel underneath the nerve-cord, and it is in communication with both dorsal and ventral, especially by the cutaneous system. The blood is generally reddish, devoid of corpuscles, and coagulable. Respiration seems to be effected by the cutaneous and special plexuses, aided, probably, in some by the currents of water in the ciliated digestive canal.
The digestive system is in the form of a simple straight, more or less muscular tube, differentiated into oesophagus, crop, gizzard, and sacculated intestine in Lumbricus, and coated internally with the usual glandular elements (certain glands anteriorly being supposed to be salivary), and lined internally with cilia. In the same group a peculiar closed fold of the chamber constitutes the typhlosolis of authors. The anus is situated at the posterior end of the body. In the Limicolous group the structure is less complex—pharynx, oesophagus, and intestine only being present. The nervous system consists of a pair of cephalic ganglia, from each of which a trunk passes by the side of the oesophagus to join the ventral chain, which gives off branches in each segment to the surrounding parts. In Lumbricus another small gangliated chain lies over the commencement of the alimentary tract. The nervous system agrees in intimate structure with that of the Polychaeta. Besides touch, and in some sight, the special senses are not much developed. Several have epidermal papillae connected with cutaneous nerves, which probably aid tactile sensibility.
The Oligochaeta are hermaphrodite, the sexual elements being developed in certain anterior segments in the form of testicles, deferent canals, receptacles, copulatory papillae in some males, and ovaries and accessories in the female. In each segment, with a few exceptions (while there are two in each in Lumbricus), the segmental organs occur. Claparède thought that in the Limicola the latter are absent from the segments bearing the oviducts and seminal receptacles, but present in those having male organs; while in Lumbricus they are present in all the segments except the four last. In some, special glands exist for secreting the egg-capsules. A cincture or clitellum occurs in certain segments anteriorly (in Lumbricus from the eighteenth to the twenty-ninth or thirtieth, and in the others from the tenth to the fifteenth segment). The ova are deposited in chitinous capsules (containing one or more), and the young issue therefrom in a tolerably complete condition. Besides the ordinary development by ova, it has long been known that Nais and Chaetogaster exhibit fissi-parous reproduction. In Nais, after a certain degree of growth has been reached, budding takes place, so that several tolerably complete young forms may be found attached to the adult. The process goes on until the Nais has been reduced to twelve or fourteen rings; then a pause occurs, and the animal increases in length to forty or fifty rings, when a new cycle commences by division in the middle of the body. Chcetogaster shows similar features, the budding taking place between the third and fourth segments in the androgynous form—many zoids or buds being attached in line; but while the second in position is one of the newest, the second in age is near the middle of the series.
Reproduction of lost parts readily takes place in this group. Parasites, such as Gregarinoe, Opalinoe, Nema-toids, and larval cestodes, are met with internally, and Vorticelloe are common externally. Commensalism is seen in Chcetogaster, which lives on the pond-snails (Limnoeus and Planorbis), and Stylaria is stated also to be ectopara-sitic.
They are distributed over the land and fresh waters (in sand and mud) of the whole world: very few are marine.
Two of the most important classifications are those of D’Udekem and Claparède. The former arranges the suborders according to the gemmiparous, or non-gemmiparous condition of the constituents. The latter group live in earth or mud, are unable to swim or follow their prey, and have always their organs of generation. The former are elegant little worms, living in stagnant or running water, able to swim, and their generative organs are developed only at certain periods. Claparède divides them into two families, viz., the 0. Terricola (including the earthworm and its allies), and the 0. Limicola, an arrangement much resembling Grube’s Lumbridna and Naidea.
III. The A. Onychophoea are represented by a single aberrant family, Peripatus. They are hermaphrodite forms (with claws instead of bristles) approaching the Arthropods, and inhabiting land and moist places in Chili, the West Indies, and other parts. The ventral nerve-cords remain separate.
IV. The A. Discophoea (leeches) are ringed forms (fig. 17), without lateral appendages (except in Branchellion), but possessing an anterior and posterior sucker. There is no specialised respiratory system. They are generally hermaphrodite, and often ectoparasitic.
Externally there is a delicate porous cuticle; beneath lies a thick glandular cutis, which pours out an abundant secretion of mucus, as in the Nemerteans. The muscular system consists of an external circular coat of considerable strength, from which various vertical or decussating bands pass at intervals, powerful longitudinal bundles being held in the intermediate spaces, especially laterally. There are also special muscles connected with the pharynx and other parts. Each end of the body is generally formed into a flattened muscular sucker, consisting apparently of several amalgamated segments. By aid of the latter organs they move from point to point, and they can also progress rapidly by swimming like the Nemerteans. The perivisceral space is · quite absent. The circulatory system shows a main dorsal, a ventral, and two lateral trunks, all communicating by branches in each ring. In Branchiobdella the blood is corpusculated. The blood is probably aerated on the cutaneofis, and perhaps, in some cases, on the digestive surface. The digestive system consists of a mouth opening into the anterior sucker, a muscular pharynx, oesophagus, a large stomach with various caecal diverticula, and an anus which opens in front of the posterior sucker, though sometimes into it. In some (e.g. the common leech), the mouth is furnished with three horny serrated teeth. The nervous system is composed of two cephalic ganglia, which supply branches to the eyes when present, and the usual gangliated ventral cord—giving off branches to the surrounding parts. The gullet passes through the connecting trunks as in the former groups. The last ganglion in the ventral cord of Clepsine is larger than the others; and, as in the Polychaeta. the ganglia do not always correspond in number with the rings. Other nerves occur in several, e.g., the leech, which has a trunk running along the dorsal surface of the digestive canal. Eye-specks are present in some, and touch is generally much developed. The cupuliform organs, in the cutaneous tissues of the head and anterior region in certain leeches are supposed to be connected with the latter sense.
The Discophora are hermaphrodite (with the exception of the Malacobdellidoe and Ilistriobdellidae), with the male organs ranged along the ventral surface in the form of a series of testes, which are connected with a common vas deferens leading into a vesicula seminalis on each side. The latter conveys the fluid into the intromittent organ, and its secretion agglutinates the spermatozoa into a seminal rope or spermatophore. The female organs consist of two ovaries leading to a common duct, opening into the vagina, which receives the spermatophore. The segmental organs are present, but not in direct communication with the reproductive structures, nor do they carry the products to the exterior, since they are often caecal. The foregoing diverges from the arrangement in the other Annelida, since the generative products are not extruded into a perivisceral cavity, but pass outwards by a particular apparatus. The ova are deposited in capsules formed by special secretions, those of the leech being called cocoons. No metamorphosis occurs in the development of the young, which by-and-by, in some cases, attach themselves in crowds to the abdominal surface of the parent.
The Discophora do not possess much power of regenerating lost parts. They are distributed everywhere, chiefly in fresh water and moist places; some are marine. A few attain a large size, e.g., a species from Valdivia is described as being two and a half feet long. Many are ectoparasitic, living on fishes, crabs, and even in jellyfishes (Bolinidae). In swampy ground in tropical countries certain leeches are often troublesome to travellers. Fossil leeches have been found in the lithographic stones of Germany They have been grouped into five families, viz., Malacobdellidae, Histriobdellidae, Acanthobdellidae, Branchiobdellidae, and Rhynchobdellidae—the latter including the medicinal leech of Europe, the green leech (fig. 18), the Bdella nilotica of Savigny (fig. 19), the horse-leech, skate-leech, pond-leech, which is devoid of an anterior sucker, and other well-known forms.
V. The A. Gephyrea seem to approach the Echinoderms through the Holothuroidea. The body is more or less cylindrical, and, though corrugated, is not definitely segmented. There is generally a protrusible proboscis, having the mouth at the end or at its base; and the anus is terminal or dorsal.
The cuticle is chitinous, has numerous processes of similar composition, longitudinal and transverse rugae, and many pores. Beneath is a hypoderm containing certain glandular organs or sacs, and, in some, bodies like trichocysts. Bristles occur in Echiurus and Bonelha. The muscular system consists of external circular and internal longitudinal fibres, and special groups of retractor and other muscles of the proboscis. In some the longitudinal layer is arranged in separate bands, stretching from one end of the body to the other. The circulatory system shows a dorsal and ventral vessel, both in the Sipunculidae communicating with a circular vessel (ciliated internally) surrounding the oesophagus, and sending prolongations into the ciliated tentacles. The latter contains a corpus-culated fluid. Certain ciliated infundibuliform organs also' occur on the intestinal mesentery of Sipunculus, and are thought to be connected with the so-called water-vascular system. In Echiurus there is a more distinct circulation, consisting, according to De Quatrefages, of three longitudinal trunks—a dorsal, ventral, and intestinal. The perivisceral cavity is large, with rudimentary dissepiments in some, and contains a corpusculated fluid, which in the living animal shows very lively currents—most marked posteriorly, and generally in a longitudinal direction. In Boneilia the respiratory structures open into the latter chamber. Two kinds of excretory organs occur—in some opening into the rectum, and in others into the alimentary cavity anteriorly. The protrusible proboscis is often armed with chitinous processes. The mouth opens at the base of the proboscis in the Echiuridae, but at its tip in the Sipunculidae, the latter also having short ciliated tentacles surrounding the aperture. It is followed by a pharynx and much-convoluted alimentary canal lined with cilia. The anus is either terminal, or situated dorsally at a point near the anterior third of the body. ‘ The walls of the alimentary canal are glandular, and there are also muscular fibres. The nervous system consists of a ventral cord giving off various branches, but showing no distinct ganglionic enlargements, nor indication of a fusion of two cords. There is an oesophageal collar, but the cephalic ganglia do not seem to be always distinct. There are no organs of the special senses except those of touch, which is fairly developed, and in a few eye-specks, especially in young forms.
The A. Gephyrea are dioecious, and have structures homologous with the segmental organs of the other groups, in the form of a series of tubes or caeca. In the Sipunculi, according to Keferstein and Ehlers, there are two testicles, and the ova are developed in ovaries attached to the wall of the body, but they vary in situation in other families. The products fall into the perivisceral cavity. Iu some the young undergo certain metamorphoses (Actino-trocha-iona), but in others the larval condition differs from the adult chiefly in the possession of ciliated zones.
The Gephyrea are widely distributed on the surface of the globe, generally iu muddy regions, and some are common in empty univalves. They are all marine. They have been grouped in three families: (1.) Echiuridae, containing forms with bristles, such as the common spoon-worm (Echiurus vulgaris) and Boneilia; (2.) Sipunculidae, with a dorsal anus, e.g., Phascolosoma Bernhardi of the univalve shells; (3.) Priapulidae, with a terminal anus, e.g., Priapulus caudatus. Sternaspis has lately been removed to the Poly-chaeta, and Phoronis has been included in the group as a tubicolar Gephyrean.
The Chaetognatha and the higher Turbellaria approach the Annelida proper very closely, though from different points of view, and may be regarded as intermediate between them and the Nematodes, Tematodes, and Cestodes. The Nemerteans (the highest group of the Turbellaria) especially come near the Annelida, notwithstanding the condition of the nervous system. They have cilia externally, and a cutis which secretes similar hyaline tubes to those of many Annelida. The muscular system is greatly developed, and in definite layers, and many swim as freely as the most active leeches. The digestive system intimately agrees, having cilia on the inner surface of the canal, a muscular oesophageal region, and a sacculated intestine, while the glandular and other elements in the wall are very similar; it differs in not passing through a buccal nervous collar, although it lies beneath the nervous system in this region. The circulatory apparatus is fairly developed, and in some corpusculated. The special plexus, in certain forms, in the oesophageal part of the digestive tract points out not only the true function of the vessels (which have been considered a water-vascular system without reference to the cephalic sacs and their ciliated vessels in the Enopla), but shows a close analogy with the anterior plexuses of many Annelida proper and Balanoglossus, and even foreshadows the branchial system of certain vertebrates, as seen, for instance, in the young Petromyzon. The appending of the branchial system to the anterior end of the digestive is characteristic. Though it is quite inaccurate to say that the Nemerteans have a corpusculated fluid in the general cavity of their bodies, yet a highly organised corpusculated fluid exists in their muscular proboscidian chamber, and evidently performs important functions in their economy Moreover, the remarkable proboscis and its sheath pass through a ring of nervous tissue consisting of the superior and inferior commissures and their connections with the ganglia. The cephalic ganglia are large, and lie over the digestive system, but the nerve-cords are separated throughout. The Nemerteans are chiefly dioecious, and the products of their sexual organs, which are developed in the form of a series of sacs on each side between the muscular wall of the body and the digestive canal, find exit by lateral pores, the contents of the male organs being often vented in clouds, as in Hermella The young sometimes undergo a metamorphosis, e.g., the 2⅛√i√⅛m-development of certain Anopla. To this the 7 τ omαnα-condition of Balanoglossus and the Actinotrocha- state of certain Gephyrea show similar features; and the three forms lead by separate channels to the Annelids proper. There is little analogy with the Tunicates, but the similarity of the development of certain Echinoderms to the three forms just mentioned is eminently suggestive,
Finally the Annelida as a whole show certain general features which may be grouped under three heads— (1.) The uses of the class to man; (2.) The property of phosphoresence; and (3.) The power of boring into hard substances.
The Annelida are not devoid of value in an economical sense. All round the British and many other coasts the lobworm {Arenicola marina, fig. 20) is used as bait; and here and there Nephthys caeca and Nereilepas fucata. In the Channel and Channel Islands two of the most plentiful of the Nereidae {Nereis cultrifera and N. diversicolor) are extensively employed in fishing. They are constantly sought for between tide-marks with a pointed instrument resembling a spear, and kept in vessels amongst sand and seaweeds. One of the most esteemed baits in ordinary and in conger fishing in the same regions is the large Marphysa sanguinea. The anterior segments of the living annelid only are preserved, since the posterior region is apt to decay and cause the death of the whole. The natives of the Fiji group much relish a form allied to our Lysidice ninetta, and they predict its annual appearance in their seas by observing the phases of the moon. It is called Palolo by the Samoans and Tonguese, and Mbalolo by the Fijians. Occurring in vast numbers, formal presents of the esteemed food are sent by the fortunate chiefs considerable distances to those whose dominions are not visited by the annelid. If the latter has similar habits to the British Lysidice, it probably leaves its retreats in the coral-reefs and rocks for the purposes of reproduction. The extensive use of the Lumbrici in fresh-water fishing, and that of the leech in medicine, need only be alluded to— the latter forming a considerable item in British importations. Echiurus is employed as bait by the Belgian fishermen, and Pallas records that the natives of the same coast formerly considered the muscular proboscis of the sea-mouse good food. .Lastly, a Sipunculus is eaten by the Chinese. An examination, again, of the stomachs of our most valuable fishes shows the important part played by the Annelida in their food-supply; and the large number of species of fish w hich can be speedily captured on a rich coast with bait of Nereis cultrifera is ample corroboration. The stomachs of cod and haddock, for instance, are often quite filled with sea-mice with Polynoidae, Tere-bellidae, Alitta virens, 0wenia, Trophonia, Phascolosoma; and in fresh water those of trout with Lumbrici.
The property of phosphorescence Occurs in the families Polynoidae, Syllidae, Chaetopteridae, and in Polycirrus and Lumbricus. In the first-mentioned, light greenish and somewhat steady scintillations are given off at the attachment of each scale, and the separate organ gleams with pulsations of light at the ruptured surface. It was probably the latter appearance which caused Μ. de Quatre-fages to state that it was emitted in muscular contraction. The synchronous emissions of light by the Italian fire-flies is interesting in this respect. In the Syllidae the light comes from the under surface of each foot; in Choetopterus the most vivid luminosity is on the dorsum of the tenth segment; and Polycirrus is so phosphorescent, that the slightest tremor - in the water causes vivid pale bluish fire to gleam along every tentacle. The exact physiology of ' the luminosity is still open to investigation, though P. Panceri of Naples connects it with certain fatty granules. The luminous emissions have no connection with light or darkness, with the capture of prey, or the alluring of their enemies, nor with the illumination of the depths of the sea.
Boring and burrowing in sand, mud, and earth are very general in the Annelida. Glycera and Nephthys, for instance, disappear with great rapidity amongst sand by boring with their proboscides, the former passing its elongated organ through a considerable space in a single thrust. Eteone and Ammotrypane carry their bodies swiftly through moist sand and gravel. Others penetrate muddy clay and the debris in fissures of rocks. The labours of the earthworm in passing through the soil only require indication; and the Gephyrea and some leeches show the same habits in clay and mud. Certain Polychaeta and Gephyrea have also the power, in common with cirripedes, molluscs, Bryozoa, and sponges, of perforating rocks, stones, shells, and other solid media. The most conspicuous forms are Poly dor a (fig. 21), Dodecaceria, Sabella saxicava, and Phascolosoma Johnstoni. Though such annelids are very abundant in corals, limestone, chalk, and shells, yet their occurrence-in large numbers in aluminous shale and sandstone shows that their perforations are not necessarily due to an acid. The influence of the boring annelids in disintegrating the foregoing substances is considerable. (w. c. Μ.)
