1902 Encyclopedia > Worm

Worm




WORM. This word has no definite significance in modern zoological classification; it is constantly applied to several phyla of the animal kingdom which have for the most part no special relations to each other. By Linnaeus the Latin equivalent " Vermes " was applied to the modern
divisions Of MOLLUSCA, CCELENTERA, PROTOZOA, TlJNl-
CATA, ECHINODERMATA (qq.v.), as well as to those animals which are in many current text-books of zoology grouped together under the same name. The group Vermes as used, for example, by Claus includes several distinct phyla, viz., NEMATOIDEA (q.v.), Platyhelminthes (see PLANARIANS, TAPE-WORMS, and TREMATODA), NEMER-TINES (q.v.), Chxtognatha (see SAGITTA), Gephyrea (see ANNELIDA), ROTIFERA (q.v.), Discophora (see LEECH), Ghxtopoda.
All these 1?1G' 1-—J*'a£rams °f Various Earthworms, to illus-trate external characters. A, B, C, anterior seg-ments from the ventral surface; D, hinder end of body of Uroehieta. A. Lumbricus: 9, 10, seg-ments containing spermathecte, the orifices of which are indicated; 14, si-gment bearing ovi-ducal pores; lo, segment bearing mate pores; '¿o2, 37, first and last segments of clitellum. B, Acantliodrihis: cp, orifices of spermathecai; 9, ovidueal pores; £ , male pores; on 17th and 19tli segments are the apertures of the atria. C, Peri-chseta: the spermathecal pores are between seg-ments G and 7, 7 and 8, 8 and 9, the ovidueal pores upon the 14th and the male pores upon the 18th segment. In all the figures the nephridial pores are indicated by dots and the setas by strokes.
The Ghxtopoda are divided into Oligochxta and Polychxta, which have been shortly treated of, together with Dis-cophora and Gephyrea, in the article ANNELIDA (q.v.). The leech and its kindred (Discophora) have been more fully described in another article (LEECH, q.v.). The present article will treat of the earthworm and its immediate allies. The earthworm belongs to the order Oligochxta, which also includes a number of freshwater forms; these latter Oligochmta have been distinguished as "Limicolx" from the earthworms or " Terricolx." There are, how-ever, no structural peculiarities of any importance which ab-solutely distinguish the terrestrial from the aquatic forms. Earthworms are, it is true, characterized by the simplicity of their setae, by the absence of cilia upon the body, by the thickness of the body-wall, which im-plies an increased thickness of the mus-cular layers, and by the thickness of the intersegmental septa, particularly in the anterior region of the body.
structural modifica-tions, however, are so obviously connected with the density of the medium in which they live that they cannot be held to be of primary import-ance. On the other hand, there is no deep-seated anatomical character which distinguishes earth-worms from the freshwater Oligochxta. An article on earthworms must therefore necessarily include an account of the aquatic and mud-inhabiting Oligochxta.
The Oligochxta range in size from a few lines to several feet in length; the large earthworm from the Cape Col-ony (Microchxta rappi) measures 5 or 6 feet in length when fully extended. On the whole the terrestrial forms (earthworms) are larger than the aquatic forms. The Oligochxta are found all over the globe, but at present no details of value can be given as to their distributional

areas. As might be imagined from their soft perish-able bodies, nothing is known respecting the distribution of the Oligochxta in past time.
The most prominent characteristic of the Oligochxta, as of the Chxtopoda generally, is the segmentation of the body : the body is divided into a series of segments or metameres, which resemble each other most closely in the lowest forms. This metamerism is seen externally in the presence of transverse furrows, corresponding with the internal divisions of the body cavity, and in the dis-position of the setae. The month opens into the first segment, which is usually unprovided with setae ; in front of the mouth is a preoral lobe ; this latter is aborted in some Oligochxta (Urochxta, Thamnodrilus).
Body- Body- Wall.—In all Oligochxta three layers can be distinguished wall. in the body-wall—(1) an outer epidermis, which secretes a delicate cuticle, (2) a circular muscle layer, (3) a longitudinal muscle layer. Within the last-named is the peritoneal lining of the ccelom.
Clitellum.—During sexual maturity certain of the segments of the body in Lumbricus and other earthworms undergo a change in appearance which is caused by the development of several layers of unicellular glands beneath the epidermis. Amongthe "Limicolie " (e.g., in Limnodrilus, Ehynchelmis, Enchytrxidx) the clitellum is, on the contrary, furnished with an epidermal layer only one cell thick ; some of these cells become large and glandular. In these points Criodrilus agrees with earthworms. The secretion of these glands may form the cocoon in which the eggs are deposited, but it appears to be also used to attach individuals together during copu-lation. The clitellum is universal among the Oligochxta. Monili-gaster and Criodrilus were for some time considered as an exception to this rule, but a clitellum has recently been demonstrated in these two genera by Bourne (17) and Benham (7). The clitellum in earthworms never occupies less than two segments, and in Trigaster it extends over twenty-seven.
Setse.—These are universally found in the Oligochxta, but differ in their shape, as well as in their number and arrangement, in different families. In the majority of forms the setse are disposed in four longitudinal rows ; the seta; in eaeh of these rows may be comparatively numerous (Naidomorpha), or may be limited to two

FIG. 2.—Seta? of Oligochxta. a, peuial seta of Perichseta ceylonica ; b, extremity of penial seta of Acanthodrilus (after Horst); c, seta of Urochxta (Perier); d, seta of Lumbricus-, e, seta of Criodrilus; f, g, seta? of Bohetuilla comata; h, setse of Psammoryctes barbatus (/to.? after Vejdovsky).
Lumbriculidx and many earthworms). In Acanthodrilus multi-poms and other earthworms the eight setae are no longer in pairs, but separated by nearly equivalent intervals. In XJrochseta (fig. 1, D) and Diachxta each segment has also eight seta;, but these are dis-posed more or less alternately in successive segments. In the former of these two genera, as also in Eudrilus, peculiar structures of a chitinous nature exist between the individual setae of a segment; these are similar to certain structures described in Anachxta by Vejdovsky (15) as abortive setae ; their presence in Urochseta and Eudrilus may indicate that the number of setae in these worms has been reduced from a continuous circle round each segment, such as exists in the family Perichxtidx (fig. 1, C). Among the Naidomorpha there are delicate hair-like setae which pass by numerous intermediate forms into setae with a bifurcate extremity; in many limicolous forms, as in earthworms, the seta? are simple in form, ending in a slightly curved extremity. The setae are developed in the interior of cells of ectodermic origin ; special muscles effect their movements.
Ccelom.—The Oligochieta, like other Annelids, have a ccelom Ccelom. which is formed by the excavation of paired mesoblastic somites ; the intersegmental septa represent the walls of each two adjacent somites ; the dorsal and ventral mesenteries, wdiich in the Archi-annelida suspend the gut from the body-wall, are largely absent in the Oligochxta, the cavities of each pair of somites becoming continuous. Traces of the dorsal mesentery are met with in some forms ; in almost all the ventral mesentery persists to a great extent in a sheet wdiich suspends the ventral blood-vessel from the intestine.
The ccelom communicates with the exterior by the nephridia o and ducts of the reproductive organs and by certain dorsally-placed pores. These latter are sometimes present only on the head seg-ment (Criodrilus and many aquatic genera), sometimes on the body segments also (Enehytrseida); in the majority of earthworms they appear to exist only on the body segments, and the first one does not usually appear before the third and fourth segment. In Pon-todrilus and a few other species the dorsal pores are entirely absent. The ccelom is lined with a peritoneum, the cells of which exhibit different characters in different parts of the body. The intestine is covered with a layer of large cells containing numerous granules; this cellular investment was originally described as hepatic, but it is now known to have no relation to the alimentary tract. The investigations of Kukenthal (16) show that these cells are concerned with the excretory function.
Nervous System.—This consists of (1) a pair of cerebral ganglia Nervous connected by a circumcesophageal ring with a chain of ventral system, ganglia arranged in pairs—a pair to each segment; (2) a system of smatll ganglia and nerves arising from the cerebral ganglia and innervating the anterior part of the alimentary tract; and (3) two lateral ganglionated cords, which have a special interest for those who believe that the segmented worms are the Invertebrate group from which the Clwrdata (including the Vcrtebrata) have sprung. The discovery of Eisig ("Die Capitelliden," Naples Monographs), that this lateral cord is on both sides connected with segmentally arranged sense-organs in the Capitellidee, is an additional argument for considering this lateral system as the homologue of the lateral line in fishes.
The nervous system of JEolosoma is probably degenerate ; it con-sists merely of a pair of cerebral ganglia, which are situated in the procephalic lobe in connexion with the epidermis. In Ctenodrilus the nervous system, like that of the Arehiannelida (see below), is imbedded in the epidermis throughout its whole extent. In the higher forms, in fact in all the remaining Oligochxta, the central nervous system has lost its primitive connexion with the epidermis. Moreover, in these forms the cerebral ganglia, originally developed in the procephalic lobe, have moved back, and may lie as far back as in the fourth segment.
Vascular System.—All the Oligochxta possess, in addition to the Vascular corpusculated fluid of the ccelom, a system of closed vessels which system, in the higher forms attains to a very highly developed condition. This vascular system contains in nearly all the Oligochieta, as in the Polychxta and Hirudinea, a red-coloured fluid, which has been proved to owe its coloration to haemoglobin, and in which are suspended corpuscles. Aiolosoma has a colourless pseudhaemal fluid. In the lower forms the walls of the blood-vessels are exces-sively delicate, and contain no muscles ; in the higher forms (e.g., Lumbricus) the blood-vessels are furnished with muscular tissue as well as with an epithelial lining. The cells of the latter give rise to the corpuscles of the blood, which consist of little more than the nucleus.
The simplest form of vascular system occurs in JEolosoma and Ctenodrilus. The alimentary tract is surrounded with a network of blood capillaries, which in the oesophageal region unite to form a dorsal vessel; this passes along the oesophagus, but is situated between the walls of the intestine and its covering of peritoneal cells ; beneath the cerebral ganglia the dorsal vessel gives off a branch on either side ; these unite with a ventral vessel, which passes beneath the intestine, and gives off branches to it, which are regularly arranged in pairs. In the Enchytrxidx the dorsal vessel is also restricted to the anterior segments of the body, and originates from a blood sinus in the walls of the alimentary tract. The dorsal vessel gives off anteriorly two branches which unite to form the ventral vessel; three pairs of slender vessels, a pair to each segment, originate from the dorsal vessel, and are connected with these two branches. In all the higher Oligochxta there is reticulum of blood capillaries developed in the walls of the

tary canal, but the dorsal vessel, although connected with this network, does not originate from it, but passes from end to end of the body. The dorsal vessel is also connected with the ventral vessel by paired trunks, which are segmentally disposed, i.e., a pair to each segment. In the Naidomorpha and Lumbriculidm, the vascular system consists only of these parts, together with some few branches which penetrate the layers of the body-wall and reach to the epidermis. Among earthworms the vascular system is more complex ; our knowledge of the details of the circulation in certain tropical genera (Urochseta, Pontodrilus) is due to Perrier, and but little of importance has been added to his descriptions. The circulation of Lumbricus is known principally from the investi-gations of D'Udekem, Claparede, and recently of Horst. In Lum-bricus there are three longitudinal trunks (fig. 3) which run from end to end of the body—(1) dorsal, (2) supranervian, (3)subnervian. The dorsal vessel is connected with the supranervian by seven

FIG. 3.—Diagrammatic Transverse Section through one of the Posterior Segments of Lumbricus (partly after Marshall and Hurst), n, nephridium ; /, funnel of nephridium ; n.c, nerve cord; e, epidermis; m, transverse muscles ; m', longitudinal muscles ; s, s', ventral and dorsal pairs of setaa ; t, typhlosole ; d, dorsal blood-vessel, connected by a vertical branch with typhlosole and by branches with intestinal blood-plexus ; a.n, supranervian vessel; v, infranervian vessel. On the left side are indicated the chief vessels given off from the main trunk to the body-wall and nephridium,
pairs of stout trunks in the outer part of the body ; from the dorsal vessel in front of the last of these originates on either side a lateral longitudinal vessel which passes along the sides of the oesophagus and gives off branches to it. In the intestinal region the dorsal vessel is connected by branches with the capillary network of the intestine, and also directly by lateral branches with the sub-nervian vessel; the latter gives off branches to the body-walls. The majority of earthworms possess in addition a small vessel running above the alimentary canal, but below the dorsal vessel; this is especially concerned with the blood supply of the alimen-tary tract, and in the intestinal region it runs in the interior of the typhlosole; the presence of this supra-intestinal trunk was first noted by Perrier, who also discovered that more or fewer of the large contractile "hearts" of the anterior segments are connected with this vessel as well as with the dorsal vessel. The subuervian vessel is absent in Pontodrilus and other genera. The immense development of the integumental capillary system is characteristic of earthworms, and is no doubt to be explained by the much greater thickness of the body-wall; it has been already stated that among the lower Oligochseta the integumental capillary system is present though feebly developed. In some genera of earthworms the capillaries penetrate the epidermic layer, as in some of the aquatic genera and in many leeches. It has been recently stated by Sarasin that these epidermic capillaries open by pores on to the exterior. That the vascular system of Alolosoma represents a primitive condition is shown by the investi-gations of Vejdovsky (15) into the development of RAynchelmis. In this worm the dorsal vessel is at first only visible in the anterior region of the body, where it lies upon the 03sophagus below the peritoneal covering of the latter; posteriorly it communicates with a blood sinus surrounding the intestine ; this stage exactly corresponds to the adult Aiolosoma, and the blood is also colour-less ; subsequently the dorsal vessel becomes connected with the ventral by a few lateral trunks ; this stage is retained in the Enchylrseidse. Moreover, in the Chlorsemidse a similar condition exists (Horst, Zool. Anzeig., viii. p. 12), and in the larval Tcrebclla.
Excretory Organs. —It has been recently shown by Vejdovsky Excretory (15) that the Oligochseta, like Polygordius, many Polychseta, organs. Gephyrea, awi Hirudinea, possess temporary larval excretory tubules as well as the definitive nephridia. These organs have been found in lihynchelmis, Nais, Chsetogaster, and Alolosoma. In the first mentioned type they appear as a pair of fine tubes with a ciliated lumen, without any apparent internal aperture ; they run on either side of the pharynx, and each opens by a pore placed at the side of the mouth. In the young asexually produced individual of Nais, Chsetogaster, and jEolosoyna similar organs are to be seen. The permanent excretory organs consist in the majority of Oligochseta of three parts:—(1) a funnel-shaped expansion furnished with cilia, opening into the body cavity; (2) a coiled glandular tube; and. (3) a terminal vesicle furnished with muscular layers, and opening on to the exterior of the body. The latter section may be absent, and in the relative proportions of the different parts as well as in certain details of their structure there are great differences. As a general rule, the funnel lies in a segment anterior to that which bears the external pore ; but in Plutellus Perrier states that the whole organ lies in one segment. In the majority of forms there are not more than a single pair of nephridia to each segment (except in Perichseta, &e.). In the limicolous Oligochseta nephridia are generally wanting in the anterior segments of the body, and disappear in those which contain the generative ducts when the latter are developed ; in the terricolous forms, on the other hand, the nephridia usually commence in the second or third segment. Ctenodrilus is remarkable for the fact that it only possesses a single pair of nephridia, the funnel of which is situated on the anterior side of the first dissepiment; but in no form are these organs entirely wanting. The glandular part of the organ consists, as in the Hirudinea and most Platyhelminths, of a row of cells placed end to end, which are perforated by the lumen; the lumen of the tubule is therefore, as was first discovered by Claparede, intra-cellular; in this particular the nephridia of the Oligochseta differ from those of the Polychseta, where the walls of the duct are made up of rows of cells, the lumen therefore being Mifcr-cellular.1 While in the greater number of Oligochseta the lumen of the tubule is simple and unbranched, in Chsetogaster fine branches are given off, which ramify in the substance of the cells; this is an important point of resemblance to the nephridia of the Hirudinea, where Vejdovsky and Bourne (Q. J. M. S., 1884) have described a similar branching of the duct. The glandular part of the nephridium is often, as in Lumbricus, differentiated into two parts ; the anterior section is composed of more delicate cells, the posterior of larger and more glandular cells ; the lumen is furnished with cilia. The external surface of the organ is frequently covered with rounded cells of a glandular appearance, which are to be looked upon as modified peritoneal cells; in certain cases (e.g., Pontodrilus and many "Limicolse") those cells form a solid mass in the interior of which are concealed the windings of the excretory tubule. Special dilatations of the tubule are occasionally met with, as in Rhynchelmis ; and, among leeches, Clepsine, Pontobdella, &c, show a similar dilatation, which, as in Rhynchelmis, comes imme-diately after the funnel. The terminal section of the nephridium, the "contractile" vesicle, is more marked among earthworms than in the "Limicolse"; it is lined by a delicate layer of cells and furnished with muscular fibres; in Urobenus (Benham, 7), as well as in many other species, this region of the nephridium is very largely developed, and is furnished with a long sac-like diver-ticulum. The differences in structure between the various parts of the nephridium are due to their different origin : the funnel is formed independently of the glandular tubule, though both take their origin from the mesoblast; the contractile vesicle is inva-ginated from the ectoderm.
The relation of the nephridia of the Chsetopoda to those of the Platyhelminths, on the one hand, and to those of the Hirudinea and Gephyrea are variously interpreted. It has been proved that in Polygordius, many Chsetopoda, and many Hirudinea and Gephyrea, the larvae, like those of the Oligochseta, possess excretory organs, which are constructed on the type of the nephridia in Platyhel-minths. This is at any rate the case with Polygordius, the Chajtopod larva, and the larva of Echiurus; in all these types the nephridia are paired branched tubes, wdiich open separately on to the exterior; they have, however, no internal openings, the flagellate cells of the Platyhelminth, with their single llagellum and funnel-shaped peril Nussbaum has lately (Arch. Slav, de Biol., i.) found that in the leech a number of cells fuse to form a single drain-pipe cell. This would tend to prove that the intercellular lumen preceded the intracellular.
In Aeanthodrilus multiporus Beddard found that the number of nephridial pores in the anterior region of the body to each segment was more than 100. In Typhseus an almost identical arrangement exists, and in Perichseta (Beddard, 4). In the last-named genus, as well as in an Australian form, Megascolides (Spencer, 12), the nephridial system forms a continuous network of tubules, uninter-rupted by the septa. In Aeanthodrilus the network of each segment is independent. In this genus, as in Perichmta, there are numerous ciliated funnels in each segment.

foration, being abseil t. Tlie larval excretory organs of the Hirudinca, like these of the Oligochseta referred to above, are to be looked upon as in a more rudimentary condition; they are unbranched, and are sometimes without an external orifice. Moreover, essentially similar organs are found in the larval mollusc. In many of the above-cited examples it is certain that the larval excretory organs have no connexion with the permanent excretory organs ; they atrophy before the latter appear. Hatchek, however, has stated that there is a connexion between the larval and permanent excretory organs in Polygordius; doubts have been thrown upon this observation by some who believe that the facts already cited showed (1) that the excretory system of Annelida and Hirudinca is a new formation, while (2) the excretory system of their Platyhelminth ancestors is represented by the transitory excretory system of the Annelida, which has therefore naturally no connexion with the permanent excretory system. This view has the merit of explaining the presence of apparently similar structures (i.e., the larval nephridia) in such diverse types as Mollusca, Hirudinca, and Gephyrea, and is perhaps further supported by the high development of the larval excretory organ in the active larvae of Polygordius, Echiurus, and the Ohsetopoda, and its rudimentary character in the embryo Oligochseta. It follows from this that the permanent nephridia of the Chsetopoda are new structures, unless the views of Bergh (18) be accepted, who would derive these organs from the generative ducts of the Platyhelminths.
Against this hypothesis may be urged (1) the unlikelihood of a new formation of excretory organs in Annelids, and the probability of these organs being really homologous with those of their Platy-helminth ancestors, and (2) the fact that the larval excretory organs of Polygordius, Polychseta, and Oligocliseta and Hirudinca are con-nected with the permanent system or at least are not in any way replaced by the permanent excretory system; in the Oligochseta the larval excretory organs appear comparatively late, and the segment occupied by them never gives rise to a pair of permanent nephridia.
The following facts lead to another hypothesis, which is in many respects more acceptable.
A connexion between the nephridia of consecutive segments has been recently stated by Wilson to occur in the embryo Lumbricus. Meyer and Cunningham have observed the same in Tercbella. Vejdvosky (15) has recorded in Anaehseta bohcmica a connexion between the nephridia of the 21st and 22d segments, and moreover the first pair of nephridia has two internal funnels. In the leech Pontobdella the nephridia (Bourne, Q. J. Min. Sci., 1884) form a network, the internal funnels and external apertures alone being arranged metamerically. The presence of numerous external pores to each segment in certain earthworms, and the continuity of nephridia of adjacent segments are facts to be referred to the same category. Eisig's discovery of the presence of many nephridia in each of the seg-ments of the Gapitellldse, which are connected together, is also, like the other facts referred to, in harmony with the supposition that the excretory system of the Annelida has been directly derived from that of the Platyhelminths somewhat as follows. The excretory system is at first, as in the Platyhelminth, a continuous system, opening by numerous apertures into the body cavity by a single orifice or a pair of orifices on to the exterior. Secondary external apertures are then formed, which are irregular in their disposition (these actually occur in certain Platyhelminths), and more or less numerous; this condition is largely retained in Acanthodrilus and Perichseta; the secondary external apertures as well as the internal apertures then become reduced in number and metamerically arranged ; this condition occurs in Pontobdella and Terebella, and to a very limited extent in Anachsela. The connexion between the nephridial system of succession segments then disappears, and the characteristic Annelid excretory system is arrived at. Alimen- Alimentary Tract.—The alimentary canal of all the Oligochseta tary is a straight tube running from mouth to anus, but even in the tract. lowest forms is specialized into different regions. A pharynx, oesophagus, and intestine can be recognized universally ; the pharynx is formed by the stomodieal invagination of the epiblast, while the terminal section of the intestine is formed by the proc-todaeal invagination; the rest of the alimentary canal is hypo-blasts. In A^olosoma the pharynx is restricted to the first segment . of the body, a condition which is seen in the embryonic stages of other Oligochasta, but also in the adult Polygordius (see below). This is followed by the narrow oesophagus, which leads, in the fourth segment, into the intestine; the intestine is at first wide, hut afterwards becomes narrower; the whole alimentary canal is ciliated. In the higher types the pharynx occupies several seg-ments, and is preceded by a buccal cavity, the epithelium of which is not ciliated ; in many Oligoehmla (e.g., Enchytrseus) the pharynx is protrusible. It is frequently furnished with glands, which are of two kinds, and probably not morphologically comparable. In many Enchytrseidse and Naidomorpha certain of the anterior segments contain glands attached to the anterior faces of the intersegmental septa; these have been termed septal glands. In Anachsela there are only two pairs of these glands, but four in Pachydrilus ; the glands of each side of the body are connected by a continuous longitudinal
duct which opens into the pharynx. Similar glands appear to occur in most Lumbricidse in the form of unicellular glands attached to the pharynx.
Another series of glandular structures are connected with the pharynx, which have been termed by Vejdovsky salivary glands ; these are found in the Enchytrseidse, and consist of simple or branched tubes, which open into the hinder end of the pharynx at each side by a single duct ; since these glands agree in their minute structure with nephridia, which are not found in the segments which contain the glands, it is probable that they represent slightly metamorphosed nephridia (Vejdovsky). Among earthworms Uro-chseta, Diachseta, and Acanthodrilus multiporus possess a pair oï glands at the anterior end of the body (glandes à mucosité, Perrier), which are larger and more complicated than the nephridia, though their structure is the same ; in Acanthodrilus these glands open into the buccal cavity ; in Urochseta they are branched and open on to the surface of the body on the one hand, and into the ccelom by several funnels (Beddard). It is possible that they are the homo-logues of the salivary glands in the Enchytrseidse.
The oesophagus is ciliated in the lower forms, but among earth-worms cilia appear to be limited to that section of the cesophagus which lies behind the gizzard. The intestine, however, appears to be ciliated in all the Oligochseta. There is thus a gradual diminu-tion in the ciliation of the alimentary tract in passing from the lower to the higher forms. In Alolosoma the whole canal from month to anus is ciliated. When the buccal cavity first appears it is lined with a cuticle, and has no cilia. The pharynx loses its cilia in the Enchytrseidse, while the rest of the alimentary tract is cili-ated. Among earthworms the cilia are partially wanting in the (esophagus, while the intestine is lined with a ciliated epithelium.
In some Oligochseta the cesophagus is furnished with a muscular dilatation, which is usually termed gizzard. This organ first makes its appearance among the Naidomorpha, but is absent from other genera of " Limicolse." It is, on the contrary, present in nearly all earthworms ; the gizzard is lined with a tall columnar epithelium, which secretes a specially thickened cuticle, and the muscular layers are enormously increased. In Lumbricus the gizzard lies at the posterior end of the oesophagus, but in all other earthworms it is succeeded as well as preceded by a section of cesophagus, as in Nais. In Lumbricus, as well as in many other genera of earth-worms, the gizzard occupies two segments, and the septum dividing these segments has disappeared, or is at most represented by a few bauds of muscle, which bind down the gizzard to the body-wall. On the other hand, in some earthworms as well as in the Naidomorplia the gizzard only occupies one segment. In Digaster and Trigaster there are, as the names of these genera imply, two or three separate gizzards, while in Moniligastcr there are four or five. In Trigaster and Moniligastcr each gizzard only occupies a single segment; it is possible that the single gizzard of Lumbricus, &c., which occupies two segments, is due to the fusion of two separate gizzards lying in as many consecutive segments.
The oesophagus in most earthworms is furnished with from one to six pnirs of glandular diverticula, which are known as "calci-ferous glands" or "glands of Morren." Those glands produce a calcareous secretion.
The intestine is wider than the cesophagus, but has much the same structure ; in Molosoma the walls of the intestine consist of little more than a single layer of ciliated cells, but in the higher forms this is surrounded by a layer of circular and longitudinal muscular fibres. A remarkable peculiarity distinguishes the intestine of the majority of earthworms ; this is a longitudinal fold on the dorsal side which projects into the lumen of the intestine, and which is known as the "typhlosole" (fig. 3, t). A typhlosole does not exist in Pontodrilus nor in any known limi-colous form. In Perichseta the intestine is furnished with one or more pairs of short caîca ; in Megascolcx and other genera there are series of compact glands opening into the intestine.
Reproductive Organs.—The Oligochseta are, so far as is known, Repro-invariably hermaphrodite ; in this particular they differ from the cUictive Polychseta, where as a rule the testes and ovaries are found in organs distinct individuals. Among the Polychseta, however, I'rolula and other Serpulidss are hermaphrodite. The reproductive organs consist of testes and ovaries, with their ducts, and spermatheca;, which are filled with spermatozoa during copulation. The repro-ductive glands are developed as a proliferation of the peritoneal epithelium, but are restricted to one or two segments ; these organs moreover have a definite form, and are commonly surrounded by a layer of cells of different nature from the sexual cells which make up the substance of the gland.
(1) Male Reproductive Organs—(a) Testes.—In Lumbricus these organs consist of two minute pairs of solid cellular masses attached close to the median ventral line upon the posterior face of the septa which divide segments 9-10 and 10-11 (fig. 4). These organs were first discovered by Hering. In the majority o

drilus, Eudrilus, Perichceta, he.) there is ¡111 identical number of testes occupying the same segments. It is probable but not yet proved that in Urochseta and Moniligaster, where there is only a single seminal reservoir and vas deferens on each side, there is but one pair of testes; at any rate, in Typhseus, where the seminal reservoirs and vasa deferentia are single, there is but one testis on each side placed in the 10th segment.
Among the "Limicolse" there is rarely (Phreoryctes) more than a single pair of testes, which may be in the 5th (Naiclomorpha), 9th (Lumbriculidx), 10th (Tubifex), or 11th (Enchytrseus, &c.) segment.
(b) Seminal Reservoirs.—The spermatozoa are not, however, de-
veloped within the testes, but in special receptacles, the seminal
reservoirs (seminal vesicles, vesiculse seminales). These structures
frequently enclose the testes, with which they have been con-
founded by many _writers. By the earlier writers the seminal
reservoirs were regarded as ovaries; this opinion was due to the
numerous parasitic Gregarines which these organs contain; the
encysted parasites were mistaken for ova. In Lumbricus lerrestris,
&c. (Bergh), there is a single median reservoir, which encloses
the testes, the funnels of the vasa deferentia, and the nerve cord
in each of segments 9 and 10 ; with these are connected three
pairs of lateral outgrowths situated respectively in segments 8,
10, and 11. In Allolobophorafcetida there are four pairs of isolated
seminal reservoirs in segments 8, 9, 10, and 11; there is no median
unpaired portion; and the testes as well as the vasa deferentia
lie freely in the body cavity ; the first two pairs lie on the
posterior septa of their segments, and open by an aperture into
the cavity of the segment behind; the last two pairs lie upon the
anterior septa of their segments, and open into the cavity of the
segment in front. In all cases the seminal reservoirs are formed
as outgrowths of the septa. The cavity of the seminal reservoirs
is broken up by anastomosing trabecular, in the interstices of which
the spermatozoa undergo their development. There is very great
variety among earthworms in the number and arrangement of the
seminal reservoirs, but there is no form known in which they are
absent. In Urochseta, T%j2)hxus, Titanus, and Diachxta there
appear to be only a single pair, which are of great length. In
Diachxta (Benham) they occupy 26 segments.
In many of the " Limicolx, as was first proved by Lankester in Tubifex, seminal reservoirs are found ; and where their develop-ment has been traced they appear to originate as outgrowths of the septa. In some of the simpler forms, c. g., Chmtogaster, seminal reservoirs are not developed, but the testicular products float freely in the perivisceral cavity, where they undergo their de-velopment.
(c) Vasa Deferen-
tia.—It is a rule
without any excep-
tions among the
Oligochxta that the
spermatozoa are car-
ried out of the body
by special ducts,
which perform only
this function. In
this respect the
Oligochxta are in
marked contrast to
thePolycJmta,Vfheve FIG. 4
theripe spermatozoa
are either set free
by a rupture of the
body-wall, or are
conveyed to the ex-
terior by means of
the nephridia, which
however, may be slightly modified in relation to this function. These ducts are termed vasa deferentia; they in-variably open freely into the body cavity, and only sometimes (e.g., Lumbricus) acquire a secondary relation with the testes by w-ay of the seminal reservoirs ; they are developed independently of the testes. In the Ilirudinea and Platyhelminths, on the other hand, the efferent ducts are continuous with the testes, of which they appear to be mere outgrowths ; Nussbaum has, however, recently (Zool. Anzeiger, viii. p. 181) stated that in Glepsine the vasa deferentia are developed independently of the testes.
In their simplest form (in many Enchytrssidx) the vasa deferentia consist of a single pair of convoluted tubes, which open by a wide funnel-shaped aperture into one segment, while the external aper-ture is situated in the following segment. Very generally in those
Oligochxta the funnel-shaped expansion is continued into a wide cylindrical tube, which narrows abruptly on passing through the intersegmental septum into a slender tube ; the walls of both sections of the vas deferens are formed of cylindrical ciliated cells ; the external aperture is often surrounded by several rows of largo glandular cells. Among a large number of the lower Oligochxta there is a single pair of vasa deferentia, which occupy in the same way two segments ; the internal funnel-shaped aperture opens into one segment, while the greater part of the tube and the external orifice are situated in the following segment. A further complica-tion is, however, introduced in the form of a glandular terminal organ, which opens on to the exterior by one extremity, and com-municates with the vas deferens at the other. In Stylaria lacustris this organ is pear-shaped, narrowing towards the external aperture; it is lined by a layer of cylindrical glandular cells, outside of which is a layer of muscular fibres ; the whole organ is covered by largo peritoneal cells of a glandular appearance. This organ is termed the atrium; the vas deferens, which is short, only curved (not convoluted), opens by one end into the broad extremity of the atrium, and terminates in a ciliated funnel, which has this peculiarity that it does not lie in the segment in front, but in the same segment (in the 6th) as the atrium; it is, however, closely applied to the intersegmental septum of segments 5-6.
This condition of the vasa deferentia is exactly repeated in the earthworm Moniligaster barwelli ; the seminal reservoirs (Beddard) lie partly in the 8th anil partly in the 9th segment; the vas deferens, which is much coiled, lies in the same segment; it is con-tinuous at one end with the seminal reservoir, and at the other with a glandular body opening between segments 9-10, which has a structure apparently identical with that of the atrium of Stylaria. Chxtogastcr diaphanus (Vejdovsky, 15) has the same general dis-position of the vasa deferentia, but the atrium is divided into a glandular "vesieula seminalis," into which opens the vas deferens, and a distal non-glandular portion, which can be everted during copulation.
The reproductive ducts of the Tubificidx are still further com-plicated in their structure. In Tubifex rivulorurn the structure of the atrium at an early stage is like that of Stylaria ; a simple globular or pear-shaped atrium is formed as an invagination of the integument; later this is differentiated into two parts, as in Chmtogastcr ; an additional structure is, however, developed in

FIG. 5.—Male Genital Ducts of Various Oligochseta. In most cases the whole length of the vasa deferentia is shown; the transverse lines indicate the boundaries of the segments through which they pass. The thick black lines indicate the atria, the dotted lines the muscular part of the ati ia. In F, G, H the penial seta? are shown lying in a sac opening in common with the atria. A, Moniligaster barwelli (outside the atrium in A and C is a layer of glandular tissue); C, Phreatothrix pragensis (after Vejdovsky); V, Eudrilus sitvicola; E, Pontodrilus marionis; F, Perichxta armala; G, Typhxus gammii; II, Acanthodrilus novx-zelandix ; I, Ocnerodrilus (after Eisen).
the form of a group of unicellular glands, collectively termed the prostate, which open into the glandular distal region of the atrium ; in the adult Tubifex the proximal section of the atrium is developed into a protrusible penis; this is surrounded by a second invagina-tion of the integument, which forms a penis sheath and part of the penis proper. The glandular part of the atrium (vesieula seminalis) is ciliated, but cilia are wanting in the penis. In Telmatodrilus

(Eisen) the prostates are very numerous, and arranged in pairs ; in Psammoryctes (Vejdovsky) and in HemUubifex (Eisen) the upper part of the vesicula semiualis, into which open the vas deferens and the prostate, forms a globular chamber distinct from the re-mainder of the atrium.
Another type of efferent apparatus is found in the Lumbriculidse. In Stylodrilus there is a single pair of atria, which have much the same structure as in Stylaria, but the proximal end of the organ is less glandular and can be everted as a penis. "With each atrium, however, are connected two vasa deferentia; one of these opens by a funnel-like expansion into the segment in front, the other passes back into the segment behind that which contains the atrium, and after again perforating the septum opens by a funnel-shaped expansion into the atrial segment. Among earthworms Moniligaster, as already stated, is furnished with an efferent apparatus exactly comparable to that of the lowrer " Limicolse." In other genera the same divisions can be recognized in the efferent apparatus, which may also be single or double. In all earthworms, however, with the exception of Moniligaster, the internal funnels are situated several segments in front of the external pore, instead of being placed in the next segment. This is the case also with Ocnerodrilus (Eisen), a form usually referred to the "Limicolm." Urochseta and Typhosus are the only genera known, besides Moniligaster, in which there is a single vas deferens on each side. In all others there are a pair of vasa deferentia on each side, which open by separate funnels into two consecutive segments (10th and 11th); the vasa deferentia open on to the exterior by a common pore; they may become united into a single tube in the segment behind that which contains the posterior funnel (Lumbricus, Perichaita), or one or two segments farther back (Microchseta), or, finally, as in Eudrilus, they may unite in the terminal apparatus.
The structure of the vasa deferentia and the funnels corresponds to that of the lower Oligochseta, except that the funnels are usually much plicated. In Lumbricus, Urochseta, &c., the vasa deferentia open directly on to the exterior; in other types, however, an atrium can be recognized. The most primitive form of these organs (in some respects) is seen in the genus Eudrilus (Beddard, 3); the two vasa deferentia open into the interior of a glandular organ, which is probably the homologue of the atrium; this organ is divided into two parts by a longitudinal septum, but is covered by continuous layers of muscles ; it communicates with two muscular tubes, also covered by a continuous layer of muscle, which unite into a single muscular penis, which is probably eversible. The penis, like the atrium, is lined by a single layer of non-ciliate cells; the cells of the glandular portion are non-ciliate, aud are arranged in two layers ; the penis projects into the interior of a cavity open to the exterior ; this corresponds to the penis sheath of the Naidomorpha. With the penis sheath ("bursa copulatrix," Perrier) is also connected a small rigid diverticulum with muscular walls.
It is not certain whether the partial longitudinal division of the atrium and vesicula represents the partial fusion of the primitively distinct atria or the commencing separation into two parts of a single atrium; the latter alternative, on the whole, is the more probable.
In Perichieta, Acanlhodrilus, &c, certain glandular bodies are connected with the termination of the vas deferens, which have been termed "prostates." These glands are of two kinds: (1) in Acan-thodrilus, Pontodrilus, &c, they consist of a single somewhat con-voluted tube of uniform diameter ; (2) in Periehseta, &c, they have a racemose lobulated appearance. It is probable that these structures are homologous. The prostates of Periehseta (fig. 5, F) are made up of numerous ductules,.which are connected with groups of cells that have the character of unicellular glands ; each cell is connected by a long stalk with the termination of the ductule ; in Acanlhodrilus, &o., the prostate is lined by a double layer of cells which surround a central lumen ; the innermost layer of cells are narrow and columnar, the outer layer are large pear shaped glandular cells. The difference between the two organs is this: in Perichieta the glandular cells have become segregated into groups, while the lumen of the gland is branched. In at least one species of Perichaita both these differences from Acanlhodrilus are less marked.
The so-called prostates of Acanlhodrilus and Pontodrilus agree in their minute structure with the vesicula semiualis of Eudrilus, and they open on to the exterior, like the corresponding structures in Periehseta, by a thick-walled muscular tube ; the vas deferens, however, is connected in Pontodrilus with the muscular part of the atrium, and not with the vesicula ; the condition of the efferent apparatus is therefore a more modified one. In Typhseus this modification is carried still further: the vas deferens enters the body-wall independently of the vesicula and atrium, and only joins the latter below the epidermis just before its opening on to the exterior. If Vejdovsky be right in interpreting the so-called receptaculum seminis of Ocnerodrilus (Eisen) as the atrium, the relations of the several parts of the efferent system in this worm are much the same as in Typhxus. In Acanthodrilus (fig. 5, H) the two pairs of atria open on to the 17th and 19th segments respectively; the vasa deferentia open on to the 18th independently of them. (d) Genital Setse. —It is commonly the case among the Oligochseta
R M
that the seta? upon the clitellum, in the neighbourhood of the spermathecse and the male sexual apertures, undergo a certain amount of modification. Thus in Lumbricus the setse in these regions of the body are longer and more slender than the setae else-where (Hering); in Urochseta aud Thamnodrilus the setae upon the clitellum are ridged. Analogous modifications are found in Nais and other "Limicolse." The function of these setse is prob-ably, as Lankester has suggested, to assist in attaching the worms together during copulation. Before the individual is mature these setse are absent, and the ordinary setse occupy their place ; when the clitellum is developed the ordinary setae drop out, and are replaced by the genital setse (Vejdovsky).
In Acanthodrilus, Typhseus, and some species of Periehseta the apertures of the atria are furnished with a thin-walled muscular diverticulum in which are found a bundle of extraordinarily long setse ; these can be protruded through the sexual orifice and may possibly serve to assist in the transference of the sperm to the spennathecse of another individual. These setse may be termed "penial" setae (Lankester) (fig. 2, a, b).
(2) Female Reproductive Apparatus—(a) Ovaries.—The ovaries of Lumbricus were first discovered by D'Udekem ; they consist of a pair of minute pear-shaped bodies attached to the anterior septum of the 13th segment (fig. 4, 0); their position exactly corresponds to that of the testes, and like those organs they are covered by a delicate layer of flattened peritoneal cells. In Perichaita and Acanthodrilus, where the testes are prolonged into numerous digi-tate processes, the ovaries have an identical form ; finally, the contents of the young ovaries and testes consist of entirely similar germinal cells (Bergh); these facts all tend to prove the serial homology of the ovaries aud testes. With the exception of Eucli-jndrilus and Phreoryctes, the Oligocholia possess but a single pair of ovaries, which appear to be invariably placed behind the testes.
(b) Receptaculum Ovorum.—Corresponding to the seminal reser-voirs are a pair of outgrowths from the posterior side of the septum which separates segments 13-14 ; the ripe or nearly ripe ova are stored in these receptacles. Their presence in Lumbricus was first discovered by Hering. They have the same structure as the seminal reservoirs; their cavity is similarly divided by anastomosing trabecular; aud they have been proved by Bergh (19) to originate in the same way. In Moniligaster (Horst) these bodies are of large size, and therefore resemble more closely the seminal reservoirs. They are generally present in earthworms. Among the Naido-morpha organs are met with which appear to resemble the recep-tacula. In Stylaria lacustris a pair of delicate sacs are developed in the same segment as that which contains the ovaries ; in these the ova undergo their development ; the extremity of the sac encircles the ovary, so that the ova can readily find their way into it. This organ is, however, compared by Vejdovsky (15) to the delicate peritoneal covering of the ovary in Lumbricus, and more particularly to a tube-like projection of this peritoneal covering at the free extremity of the ovary where the ripe ova are found.
(c) Oviduct.—The oviducts in Lumbricus are two minute trumpet-shaped bodies composed of a single layer of ciliated cells ; the funnel-shaped expansion opens into the 13th segment; the tube perforates the mesentery, and opens on to the exterior in the 14th segment. The mouth of the oviduct is in close relation with the aperture of the receptaculum ovorum into the same segment. In most earthworms a pair of oviducts of similar structure have been recognized; in Periehseta the two oviducts open by a common pore situated in the median ventral line of the 14th segment. The aper-tures of the oviducts are, with the exception of Moniligaster (Horst) and Allurus (Beddard, 5), invariably placed in front of the male gene-rative pores. Among the " liuiicolous " forms oviducts have been described in Rhynehelmis, Phreoryctes, and Phreatothrix ; in these genera, however, the oviducts consist of little more than the funnel which is sessile on the ventral body-wall of the 11th segment, and opens on to the exterior in the groove between this and the following segment; in Phreoryctes, Beddard (6), there are two pairs of oviducts entirely contained in one segment. It is important to note that in Rhynehelmis and Phreatothrix the female pore is behind the male pore, while in earthworms (except Moniligaster, Allurus) it is in front. Among the lower Oligochseta oviducts are absent; their place is taken in the Enchylrseidse and others by a pair of slit-like orifices placed on the clitellar segment behind the apertures of the vasa deferentia. That these pores represent the external orifices of the oviducts in the higher forms is shown by the following considerations : (1) their position behind the male pores; (2) the probability urged by Vejdovsky that the clitellar segment really represents three fused segments, in which case the oviducal pores are really one segment behind the male efferent pores; (3) the remarkable analogy with the Oyclostomata among fishes, where the abdominal pores act as oviduct (Weber, Zeitsch.f. wiss. Zool., 1887).
The ovaries and oviducts of Eudrilus (Beddard, Horst), differ in many particulars from those of other Oligochseta. The ovaries (fig. 6) are two solid bodies situated on the anterior septum of segment

14. They are surrounded by layers of muscles, and the interior is divided by trabecular into numerous compartments ; directly con-tinuous with each ovary is a contorted tube which passes through the septum into the 13th segment, and then again passes through the septum, and opens on to the exterior in the 14th segment, in common with a largo spermathecal pouch and a small glandular body. The oviduct opens by a wide mouth into the interior of the ovary, and is lined throughout with a ciliated epithelium; it is covered by layers of muscular fibres, which are continuous with those of the ovary. The continuity of the ovary and its duct is unknown in any other Oligocholia, but is analo-gous to the condition of the ovaries and their ducts in the leeches. It is possible that the muscular wall of the ovary is to be regarded as an hypertrophy of the peritoneal cover-ing of the ovary in Lumbricus, &c., and that this has involved the ovi-duct ; or the muscular coat of the ovary may be regarded as the receptaculum ovorum, which, as in Stylaria, is developed in the same segment as the ovary; this supposition is strengthened by the fact that the oviduct traverses the septum between the 13th and 14th segments.
Spermathecal.—Of these organs there are from one to eight pairs ; they consist of spherical or pear-shaped vesicles, often furnished with accessory diverticula. They receive the semen during copu-lation and by their epithelium is fabricated the generally chitinous spermatophore in which the spermatozoa are enveloped, and in which they are conveyed to the neighbourhood of the clitellum of another individual.
The genital ducts have been compared with nephridia by many writers : there is not only a considerable anatomical resemblance, but a developmental similarity ; the vasa deferentia (and the ovi-ducts ?), like the nephridia, consist of a funnel, of a more or less elongated tubule, and of a distal vesicular part. The miracellular duct of the nephridium and the intercellular duct of the vas deferens may be explained by the different functions which the organs perform. The spermatheese, on the other hand, are comparable to the vesicle of the nephridium, which is formed in both cases by an ectodermic ingrowth. The fact that the funnel and the tubule in both cases are developed independently, but both from the mesoblast, is a striking point of similarity between the vasa deferentia and oviducts and the nephridia. The belief of Claparede that in the "Limicolse" the genital ducts were the homo-logues of nephridia, but not in the " Terricolee," was based upon the erroneous assumption that nephridia are absent in the genital segments of the former. This has been shown by Vejdovsky to be an error: nephridia are present at first in the genital segments, but degenerate and disappear when the genital ducts are formed. Moreover, enough has been already said to prove the extreme unlikelihood of a non-homology between the generative ducts in earthworms and those in the " limicolous" forms. Lankester put forward the theory that there were primitively two pairs of nephridia in each segment, each series being connected with one of the pairs of setse ; the genital ducts were supposed to be the remains of one series which had aborted, except in the genital segments. This theory was at first espoused by Perrier, who found that the nephridia were sometimes connected with one series of setse and sometimes with the other, and sometimes (Plutellus) alternated from segment to segment, opening in one segment by the ventral and in another by the dorsal setce; these facts appeared to show that one or other of the two series of nephridia was partially or entirely retained in different genera. In his later publications Perrier was not inclined to attach much weight to this hypothesis, inasmuch as it did not satisfactorily refer the genital ducts to one or other of the presumed double series of nephridia ; the apertures of the genital ducts and nephridia were found occasionally to coincide at the same pair of setse.
The discovery that each segment of a worm may contain numerous nephridial pores disposes of any a priori difficulties as to the homology between nephridia and genital ducts, though the question is far from being settled.
Classification and Affinities.—It has already been stated that the Oligocholia form a group which cannot be subdivided into Limieolm and Tcrrieolse as was proposed by Claparede. The genera of Oligochseta have been arranged in families by Vejdovsky (14), to whom the reader is referred for a classification, which is satisfactory as regards the "limicolous" forms, and some of the families {e.g., Lumbricidse and Perwhaitidse) of earthworms. Since that time, however, a large number of new genera and species of earthworms have been described, which cannot at present be satisfactorily arranged. Perrier divided earthworms into three groups:—(1) Preclitellians [e.g., Lumbricus), where the male pores arc situated in front of the cliteilum ; (2) Intraclitcllians (e.g., Eudrilus), where the male pores are within the clitellum ; and (3) Postclitellians {e.g., Pericheela), where the male pores open behind the clitellum. Advancing knowledge has shown this classification to be untenable, for two principal reasons: first, because it separates some species of Acanthodrilus which are postclitellian from others which are intraclitellian, and it separates the iutraclitellian Megaseolex from the postclitellian Pcrichseta, between which genera there are numerous points of affinity ; and, secondly, because this classification is based on the assumption that earthworms can be considered as a group apart from other Oligochmta, whereas it is now impossible to draw any line of division between any two such groups.
The simplest forms of Oligochieta are the genera JEolosoma and Ctcnodrilus. JEolosoma has a "head," consisting of one segment which contains the pharynx. The nervous system consists of cerebral ganglia, which are placed in the first segment, and retain connexion with the epiblast; the apparent absence of a ventral nervous cord is in all probability to be looked upon as evidence of degeneration. The body possesses considerable traces of the primitively continuous ciliation; upon the head are a pair of ciliated pits. In all these particulars, as well as in the characters of the vascular system, JEolosoma agrees with Ctcnodrilus; the latter form has, however, a ventral nerve cord, which, like the cerebral ganglia, is lodged in the epidermis. In most of these points Alolosoma and Ctcnodrilus resemble larval forms of the higher Oligocholia. The reproductive organs of Ctcnodrilus are not known, but those of JEolosoma are constituted on the Oligochsetous plan. These two genera are also closely allied to the Archiannelida, of which a short description is appended, as they are not treated elsewhere in this work.

JEolosoma therefore retains certain Archiannelid characters, but is to be referred to the Oligochseta on account of—(1) the limitation of the reproductive glands to two segments and the presence of special efferent testicular ducts ; (2) the paired arrangement of the setse bundles, which agree in their structure with those of other Oligochieta ; and (3) the relative complication of the nephridia, as compared with Archiannelida.
The affinities of Ctcnodrilus are not so plain ; but, in the absence of any knowledge respecting the generative organs, it is impossible to refer them definitely to the Oligochseta or to the Polychasta.
Archiannelida.—This subclass includes certain small marine Archian-worms which were formerly placed among the Polychseta. Hatchek nelida. originally created the group for the reception of Polygordius and Protodrilus; and recently Jottinger has placed Histriodrilus^ (an animal formerly referred to the Eiscophora) in the same group. The name Archiannelida implies that these forms stand at the base of the Annelida, and that they represent most nearly the common ancestral form from which both the Polychseta and Oligochseta have been derived. Their structure, which bears out this supposition, suggested the name.
Polygordius is found on the northern and southern coasts of Europe; it is a small slender worm, varying in length (according to the species) from 30 m. to 1 decim. The segmentation is hardly marked externally. The head segment is divided as in the Chsetopoda generally into a prostomium and a peristomial ring; the prostomium gives rise to two tentacles. On either side of the prostomium is a ciliated pit, which is of special interest, as it occurs on the one hand among the NEMERTINES (q.v.), and on the other among certain Polychseta (e.g., Ophelidse), and in the lowly organized Oligochset JEolosoma. Cilia are found in certain species in the neighbourhood of the mouth and on the anal segment.
The ciliation of the body is more marked in Protodrilus, where there is a continuous ventral groove lined by cilia; there are rings of cilia also on each segment. The partial ciliation of Polygordius is to be derived from this by reduction. Certain Oligochseta retain traces of the primitive ciliation of the body (e.g., JEolosoma).
2 The correct name of this worm is Histriobdella. The altered termination is to express the change in the view regarding its affinities.
There are no traces of setse in Polygordius or its allies. The segmentation of the body, although hardly marked externally, is very clearly marked internally ; the body-cavity is divided by suc-cessive septa into a series of segments ; there is no such fusion of the anterior segments as is met with among the Polychseta and Oligochmta, and the segments are less differentiated among them-selves than is usually the case in the higher Chsetopoda. The head segment contains the pharynx, which is limited to this segment; the nephridia, which consist of simple tubes (probably but not certainly formed of a chain of "drain-pipe" cells, as in Oligochseta) are found in all the segments except the first and last. The nephridia do not lie freely within the coelom, but in the thickness of the parietal peritoneum ; the funnel only just opens into the cavity of the segment preceding that which bears the external pore; that is to say, it is chiefly contained within the thickness of the septum.

In having the form of simple sinuous (not coiled) tubes, and in lying within the peritoneum, the nephridia are in a very archaic condition ; the same thing occurs in the Oapitellidse; in the higher Annelids the nephridia lie within the ccelom, and are usually much coiled and complicated in structure.
The vascular system consists of a dorsal and a ventral trunk, which traverse the thickness of the dorsal and ventral mesenteries. The development of these vessels shows that their cavity is continuous with the blastoccele, and is not a secondary canalization of a solid chain of mesoblast cells, as is so commonly the case in the Annelida. The two trunks communicate in every segment except the last two or three by two lateral vessels, one on each side ; each of these latter gives olf a carcal tube running backwards along the somatic peritoneum. Thus the vascular system undergoes but little modification in different segments. In Protodrilus the blood-vessels have no walls, and therefore permanently retain a condition which is found in the young Polygordius before the adjacent mesoblast cells have become differentiated into an extremely delicate membrane. The condition of the vascular channels in Polygordius represents that of the larval Polychseta and Oligoehseta before the special muscular walls have been formed. JEolosoma, however, is identical with the adult Polygordius in these particulars.
The digestive tube consists of only a single layer of ciliated cells covered by a single layer of peritoneal cells, and is therefore in an embryonic condition as compared with the Chsetopoda and Oligo-cholia. It is, however, specialized into a pharynx, oesophagus, and intestine. The nervous system is extremely simple, and lies in the thickness of the epidermis, thus presenting an embryonic character, which is, however, met with in certain Polychseta.
In Polygordius the cerebral mass is situated in the prostomium, and communicates by a circumcesophageal commissure with a ventral cord which is single and median, and shows no trace of ganglionic enlargements. In Protodrilus there are two ventral cords, while in Histriodrilus there are a series of ganglionic swell-ings. A nervous plexus also exists in the thickness of the longitudinal muscles.
Polygordius is of separate sexes ; Protodrilus is hermaphrodite. The sexual products are developed in all except the first few and last few segments from the lining of the ccelom ; the ova and sper-matozoa are apparently liberated by the rupture of the body-wall. Histriodrilus has special efferent ducts for the ovaries and testes (found in different individuals), the nature of which is not yet settled.
Polygordius leaves the egg as an active larva, first discoverea oy Loven. This larva is shaped like a humming-top, and has a preoral circlet of cilia ; immediately behind this on one side is the mouth, which leads into an alimentary canal opening at the posterior end of the body; at the apical region is an ectodermic thickening.
Literature.—(i) Beddard, " On the Nephridia of AcaiUhodrilus," Proc. Roy. Soc,
1835 ; Ann. Sci. Nat., 18S6 ; (2) Id., "On the Atrium in Earthworm," Zool. Anz.,
188S; (3) Id., "Structure of Eudrilus,"Proc. Zool. Soc, 1SS6; (4) Id., "Nephridia
of Periehseta." Q. J. M. S., 1888 ; (5) Id., " Allurus," ibid.; (6) Id., " Phreoryctes,"
Ann. Nat. Hist., 1888; (7) Benham, "Studies in Earthworms," Nos. i., ii., hi.,
Q. /. M. S., 1886-7; (8) Id., "Brachydrilus," Zool. Anz., 1887; (9) Ilorst, "Notes
on Earthworms," Notes from Leyden Museum, 1SSG-7; (10) Rosa, " Criodrilus,"
AM R. Acad. Sci. Torino, 1SS6"; (11) Id., "Hormogaster," ibid., 1888; (12)
Spencer, "Nephridia of Earthworms," Nature, June 28, 1888; (13) Scharff,
" Ctenodrilus," Q. J. M. S., 1887 ; (14) Fletcher, "Australian Earthworms,"
Proc. L. S. N. S. W., 1S8G; (15) Vejdovsky, System u. Morphol. d. Oligochdten;
(i6)KUkenthal, "Lymphoidzellen d. Aimeliden," Jen. Zeitsch., 1885; (17) Bourne,
"Indian Earthworms," Proc. Zool. Soc, 18S6; (18) Bergh, "Excretionsorgane
d. Wiirmer," Kosmos, 1S85; (19) Id., " Geschlechtsorgane. d. Regenwurmer,"
Zeitsch. f. wiss. Zool., 1886. On Archiannelida see Fraipont, "Polygor-
dius;' Fauna und Flora des Golfen v. Neapel (complete references to previous
papers). (F. E. B.)



Footnotes

The genus A canthodrilus is almost entirely Antarctic in its range. It occurs in Patagonia, S. Georgia, Kergueien's Land, New Zealand, Cape of Good Hope and other parts of Africa, Madagascar, and New Caledonia. Perichxta is characteristic of the Old and New World tropics, particularly of the former; in the Old World it ranges from India to China and Japan, and through the Indian Islands to Australia and New Zealand. Australia has the peculiar genera Megaseolides, Notoscolex, and Cryptodrilus, but the greatest number of peculiar genera are found in the Neotropical region. In Europe the most characteristic genera are o Lumbricus and Allolobophora; these are found in most other parts of the world, but it is possible that they have been accidentally imported.
These figures refer to the "Literature," p. 68L

For an account of tlie function of these glands, as well as of the part which earthworms play as geological agents, see Darwin, Formation of Vegetable Mould, tfce.

Plutellus is, according to Perrier, an exception ; the ovaries are in front of the testes.

The difference between the nephridia and genital ducts in this respect cannot be regarded as being of importance; the nephridia of Polychteta differ from those of Oligochmta in having an intercellular lumen. Lang, moreover, has shown ("Die Polycladen," Naples Monographs) that in some Planarians the reproductive ducts, like the nephridia, may have an intracellular lumen.

2 The correct name of this worm is Histriobdella. The altered termination is to express the change in the view regarding its affinities.






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