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, Coelentera, Protozoa, Tunicata, Echinodermata (qq.vf), 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), Nemertines (q.v.), Chaetognatha (see Sagitta), Gephyrea (see Annelida), Rotifera (q.vf, Discophora (see Leech), Chaetopoda.
The Chaetopoda are divided into Cligochaeta and Polychaeta, which have been shortly treated of, together with Discophora and Gephyrea, in the article Annelida (<7.v.). The leech and its kindred (Discophora) have been more fully described in another article (Leech, q.vj). The present article will treat of the earthworm and its immediate allies. The earthworm belongs to the order Oligochaeta, which also includes a number of freshwater forms; these latter Cligochaeta have been distinguished as “Limicolae” from the earthworms or “TermcolaeP There are, however, no structural peculiarities of any importance which absolutely 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 implies an increased thickness of the muscular layers, and by the thickness of the intersegmental septa, particularly in the anterior region of the body. All these structural modifications, however, are so obviously connected with the density of the medium in which they live tlιat they cannot be held to be of primary importance. On the other hand, there is no deep-seated anatomical character which distinguishes earthworms from the freshwater Cligochaeta. An article on earthworms must therefore necessarily include an account of the aquatic and mud-inhabiting Cligochaeta.
The Cligochaeta range in size from a few lines to several feet in length; the large earthworm from the Cape Colony (Microchaeta 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 Cligochaeta 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 perishable bodies, nothing is known respecting the distribution of the Oligochaeta in past time.
The most prominent characteristic of the Oligochaeta, as of the Chaetopoda 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 disposition of the setae. The mouth 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 Oligochaeta {Urochaeta, Thamnodrilus). BodyWall.— In all Oligochaeta three layers can be distinguished 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 caelom.
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. Among the ‘ ‘ Limicolae” {e.g., in Limnodrilus, Rhynchelmvs, Enchytraeidae) 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 copulation. The clitellum is universal among the Oligochaeta. 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.
Setae. — These are universally found in the Oligochaeta, but differ in their shape, as well as in their number and arrangement, in different families. In the majority of forms the setae are disposed in four longitudinal rows; the setae in each of these rows may be comparatively numerous {Naidomorpha), or may be limited to two Lumbriculidae 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 Urochaeta (fig. 1, D) and Diachaeta each segment has also eight setae, but these are disposed 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 Anachaeta by Vejdovsky (15) as abortive setae; their presence in Urochaeta 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 Perichaetidae (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 setae 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.
Caelom.— The Oligochaeta, like other Annelids, have a caelom 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, which in the Archi∙ annelida suspend the gut from the body-wall, are largely absent in the Oligochaeta, 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 which suspends the ventral blood-vessel from the intestine.
The caelom communicates with the exterior by the nephridia and ducts of the reproductive organs and by certain dorsally-placed pores. These latter are sometimes present only on the head segment {Criodrilus and many aquatic genera), sometimes on the body segments also {Enchytraeida) ; .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 caelom 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 Kükenthal (16) show that these cells are concerned with the excretory function.
Nervous System.— This consists of (1) a pair of cerebral ganglia connected by a circumaesophageal ring with a chain of ventral ganglia arranged in pairs—a pair to each segment; (2) a system of small 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 Chordata (including the Vertebrata) have sprung. The discovery of Eisig (“Die Capitelliden,” Naples Monographs), that this lateral cord is on both sides connected with segmentally aιτanged sense-organs in the Capitellidae, is an additional argument for considering this lateral system as the homologue of the lateral line in fishes.
The nervous system of Aeolosoma is probably degenerate; it consists 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 Archiannelwla (see below), is imbedded in the epidermis throughout its whole extent. In the higher forms, in fact in all the remaining Oligochaeta, 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 Oligochaeta possess, in addition to the corpusculated fluid of the cce1om, a system of closed vessels which in the higher forms attains to a very highly developed condition. This vascular system contains in nearly all the Oligochaeta, as in the Polychaeta and Hirudinea, a red-coloured fluid, which has been proved to owe its coloration to haemoglobin, and in which are suspended corpuscles. Aeolosoma has a colourless pseudhaemal fluid. In the lower forms the walls of the blood-vessels are excessively 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 Aeolosoma and Cteηodrilus. 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 toit, which arc regularly arranged in pairs. In the Enchytraeidae 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 Oligochaeta there is reticulum of blood capillaries developed in the walls of the alimentary 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 Lumbriculidae, 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 (Urochaeta, 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 investigations of D’Udekem, Claparède, and recently of Horst. In Lumbricus 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 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 subnervian 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 alimentary 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 subnervian vessel is absent in PontodrUus 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 Oligochaeta 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 Aeolosoma represents a primitive condition is shown by the investigations of Vejdovsky (15) into the development of Rhynchelmis. In this worm the dorsal vessel is at first only visible in the anterior region of the body, where it lies upon the oesophagus below the peritoneal covering of the latter; posteriorly it communicates with a blood sinus surrounding the intestine; this stage exactly corresponds to the adult Aeolosoma, and the blood is also colourless; subsequently the dorsal vessel becomes connected with the ventral by a few lateral trunks; this stage is retained in the Enchylraeiclae. Moreover, in the Chloraemidae a similar condition exists (Horst, Zool. Anzeig. , viii. p. 12), and in the larval Terebellα. Excretory Organs.— It has been recently shown by Vejdovsky (15) that the Oligochaeta, like Polygordius, many Polychaeta, Gephyrea, αηHHirudineα, possess temporary larval excretory tubules as well as the definitive nephridia. These organs have been found in Rhynchelmis, Nais, Chaetogaster, and Aeolosoma. 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, Chaetogaster, and Aeolosoma 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 Phιtellus 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 Perichaeta, &c. ). In the limicolous Oligochaeta 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. Ctenodril·us 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 Claparède, intra- cellular; in this particular the nephridia of the Oligochaeta differ from those of the Polychaeta, where the walls of the duct are made up of rows of cells, the lumen therefore being inter-cellular. While in the greater number of Oligochaeta the lumen of the tubule is simple and unbranched, in Chaetogaster 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 il Limicolae") 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 immediately after the funnel. The terminal section of the nephridium, the “contractile” vesicle, is more marked among earthworms than in the “Limicolae " ; 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 diverticulum. 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 invaginated from the ectoderm.
In Λcαηthodrilus multiporus Beddard found that the number of nephridial pores in the anterior region of the body to each segment was more than 100. In Typhaeus an almost identical arrangement exists, and in Perichaeta (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, uninterrupted by the septa. In Acanthodrilus the network of each segment is independent. In this genus, as in Perichaeta, there are numerous ciliated funnels in each segment.
The relation of the nephridia of the Chaetopoda 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 Chaetopoda, and many Hirudinea and Gephyrea, the larvae, like those of the Oligochaeta, possess excretory organs, which are constructed on the type of the nephridia in Platyhelminths. This is at any rate the case with Polygordius, the Chaetopod larva, and the larva of Echiuηιs ; in all these types the nephridia are paired branched tubes, which open separately on to the exterior; they have, however, no internal openings, the flagellate cells of the Platvhelminth, with their single flagellum and funnel-shaped perforation, being absent. The larval excretory organs of the Hirudinea, like these of the Oligochaeta 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 abovecited 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 Hirudinea 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 (t.e., the larval nephridia) in such diverse types as Mollusca, Hirudinea, 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 Chaetopoda, and its rudimentary character in the embryo Oligochaeta. It follows from this that the permanent nephridia of the Chaetopoda 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 Platyhelminth ancestors, and (2) the fact that the larval excretory organs of Polygordius, Polychaeta, and Oligochaeta and Hirudinea are connected with the permanent system or at least are not in any way replaced by the permanent excretory system; in the Oligochaeta 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 Terebella. Vejdvosky (15) has recorded in Anachaeta bohemica 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 Poηtobdella the nephridia (Bourne, Q. J. Min. Sti., 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 segments of the Capitellidae, which are connected together, is also, like the other facts referred to, in harmony with the supposition that the excretory system of ∖ λ ∖ e 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 ora pair of orifices on to the exterior. Secondary external apertures are then formed, which are irregular in their disposition (these actually occur in certain Platyhelmin ths), and more or less numerous; this condition is largely retained in Acanthodrilus and Perichaeta; the secondary external apertures as well as the internal apertures then become reduced in number and metamerically arranged; this condition occurs in Poηtobdella and Terebella, and to a very limited extent in Anachaeta. The connexion between the nephridial system of succession segments then disappears, and the characteristic Annelid excretory system is arrived at.
Alimentary Tract.— The alimentary canal of all the Oligochaeta is a straight tube running from mouth to anus, but even in the lowest forms is specialized into different regions. A pharynx, oesophagus, and intestine can be recognized universally; the pharynx is formed by the stomodaeal invagination of the epiblast, while the terminal section of the intestine is formed by the proctodaeal invagination; the rest of the alimentary canal is hypo-blastic. In Aeolosoma the pharynx is restricted to the first segment of the body, a condition which is seen in the embryonic stages of 2 1 v". er .0ij⅛oc‰Eto, 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, but afterwards becomes narrower; the whole alimentary canal is ciliated. In the higher types the pharynx occupies several serments, and is preceded by a buccal cavity, the epithelium of which is not ciliated; in many Oligochaeta (e.g., Enchytraeus) the pharynx is protrusιble. It is frequently furnished with glands, which are of two kinds, and probably not morphologically comparable. In many L·nchytraeidae and Baidomorpha certain of the anterior segments contain glands attached to the anterior faces of the intersegmental septa; these have been termed septal glands. In Anachaeta there are on!y two pairs of these glands, but four in Pachydrilus ; the glands ol each side of the body are connected by a continuous longitudinal duct which opens into the pharynx. Similar glands appear to occur in most Lumbricidae 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 Enchytraeidae, 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 iiι 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-chaeta, Diachaeta, and Acanthodrilus multiporus possess a pair of 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 Urochaeta they are branched and open on to the surface of the body on the one hand, and into the caelom by several funnels (Beddard). It is possible that they are the homologues of the salivary glands in the Eìwhytraeidae.
The oesophagus is ciliated in the lower forms, but among earthworms cilia appear to be limited to that section of the oesophagus which lies behind the gizzard. The intestine, however, appears to be ciliated in all the Oligochaeta. There is thus a gradual diminution in the ciliation of the alimentary tract in passing from the lower to the higher forms. In Aeolosoma the whole canal from mouth 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 Enchytraeidae, while the rest of the alimentary tract is ciliated. Among earthworms the cilia are partially wanting in the oesophagus, while the intestine is lined with a ciliated epithelium.
In some Oligochaeta the oesophagus 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 “LimicolaeP 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 oesophagus, as in Mais. In Lumbricus, as well as in many other genera of earthworms, the gizzard occupies two segments, and the septum dividing these segments has disappeared, or is at most represented by a few bands of muscle, which bind down the gizzard to the bodywall. On the other hand, in some earthworms as well as in the Naidomorpha 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 Moniligaster there are four or five. In Trigaster and Moniligaster 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 pairs of glandular diverticula, which arc known as ‘ ‘ calciferous glands” or “glands of Morren.” Those glands produce a calcareous secretion.
The intestine is wider than the oesophagus, but has much the same structure; in Aeolosoma 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, i)∙. A typhlosole does not exist in Pontodrilus nor in any known limicolous form. In Perichaeta the intestine is furnished with one or more pairs of short caeca; in Megascolex and other genera there are series of compact glands opening into the intestine.
Reproductive Organs.— The Oligochaeta are, so far as is known, invariably hermaphrodite; in this particular they differ from the , Polychaeta, where as a rule the testes and ovaries are found in l distinct individuals. Among the Polychaeta, however, Protula and other Serpulidae are hermaphrodite. The reproductive organs consist of testes and ovaries, with their ducts, and spermathecae, which are filled with spermatozoa during copulation. The reproductive 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 L·umbricus 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 wero first discovered bv Herin<r. In the maioritv of earthworms (Acantho~ drilus, Eudrilus, Penchaeta, &c.) there is an identical number of testes occupying the same segments. It is probable but not yet proved that in Uroehaeta 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 Typhaeus, where the seminal reservoirs and vasa deferentia are single, there is but one testis on each side placed in the 10th segment.
Among the “Lknicolae" there is rarely (Phreoryctes) more than a single pair of testes, which may be in the 5th (Naidomorpha), 9th (LumbricuUdae), 10th (Tubifex), or 11th (Enchytraeus, &c.) segment.
(δ) Seminal Reservoirs.— The spermatozoa are not, however, developed within the testes, but in special receptacles, the seminal reservoirs (seminal vesicles, vesiculae seminales). These structures frequently enclose the testes, with which they have been confounded 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 terrestris, &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 Allolobophora foetida 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 trabeculae, 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 Uroehaeta, Typhaeus, Titanus, and Diachaeta there appear to be only a single pair, which are of great length. In Diachaeta (Benham) they occupy 26 segments.
In many of the il Limicolaef as was first proved by Lankester in Tubifex, seminal reservoirs are found; and where their development has been traced they appear to originate as outgrowths of the septa. In some of the simpler forms, e.g., Chaetogaster, seminal reservoirs are not developed, but the testicular products float freely in the perivisceral cavity, where they undergo their development.
(c) Vasa Deferentia. — It is a rule without any exceptions among the Oligochaeta that the spermatozoa are carried out of the body by special ducts, which perform only this function. In this respect the Oligochaeta are in marked contrast to the7¾h/cÄaeto,where the ripe spermatozoa arc either set free by a rupture of the body-wall, or are conveyed to the exterior by means of the nephridia, which, however, may be slightly modified in relation to this function. These ducts are termed vasa deferentia; they invariably open freely into the body cavity, and only sometimes (e.g., Lumbricus) acquire a secondary relation with the testes by way of the seminal reservoirs; they are developed independently of the testes. In the Hirudinea 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 Clepsine the vasa deferentia are developed independently of the testes.
In their simplest form (in many Enchytraeidae) 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 aperture is situated in the following segment. Very generally in those Oligochaeta 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 large glandular cells. Among a large number of the lower Oligochaeta 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 complication is, however, introduced in the form of a glandular terminal organ, which opens on to the exterior by one extremity, and communicates 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 large 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 and partly in the 9th segment; the vas deferens, which is much coiled, lies in the same segment; it is continuous 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. Chaetogaster diaphanus (Vejdovsky, 15) has the same general disposition of the vasa deferentia, but the atrium is divided into a glandular “vesicula seminalis,” into which opens the vas deferens, and a distal non-glandular portion, which can be everted during copulation.
The reproductive ducts of the Tubificidae are still further complicated in their structure. In Tubifex rivulorum 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 Chaetogaster ; an additional structure is, however, developed in the form of a group of unicellular glands, collectively termeα me 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 ρrotrusible penis; this is surrounded by a second invagination of the integument, which forms a penis sheath and part of the penis proper. The glandular part of the atrium (vesicula seminalis) is ciliated, but cilia are wanting in the penis. In Telmatodrilus (Eisen) the prostates are very numerous, and arranged in pairs; in Psammoryetes (Vejdovsky) and in Hernitubifex (Eisen) the upper part of the vesicula seminalis, into which open the vas deferens and the prostate, forms a globular chamber distinct from the remainder of the atrium.
Another type of efferent apparatus is found in the Lumbriculidae. In Stylodrilus there is a single pair of atria, which have much the same stnιcture as in Stylαria, 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 lower “LimicolaeP 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 il Limicolae." Uroehaeta and Typhaeus 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 llth); 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, Perichaeta), or one or two segments farther back (Microchaeta), 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 Oligoehaeta, except that the funnels are usually much plicated. In Lumbricus, Uroehaeta, &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 interioι , 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, and 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 Perichaeta, Acaηthodrilus, &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 convoluted tube of uniform diameter; (2) in Perichaeta, &c., they have a racemose lobulated appearance. It is probable that these structures are homologous. The prostates of Perichaeta (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 Acanthodrïlas, &c., 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 Perichaeta the glandular cells have become segregated into groups, while the lumen of the gland is branched. In at least one species of Perichaeta both these differences from Acanthodrilus are less marked.
The so-called prostates of Acanthodrilus and Pontodrilus agree in their minute structure with the vesicula seminalis of Eudrilus, and they open on to the exterior, like the corresponding structures in Perichaeta, 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 Typhaeus this modification is carried still further: the vas deferens enters the bodywall independently of the vesicula and atrium, and only joins the latter below the epidermis just before its opening on to the exterior. It V ejdovsky 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 Typhaeus. In Acanthodrilus (fig. 5, II) 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 Setae.— It is commonly the case among the Oligoehaeta that the setae upon the clitellum, in the neighbourhood of the spermathecae and the male sexual apertures, undergo a certain amount of modification. Thus in Lumbricus the setae in these regions of the body are longer and more slender than the setae elsewhere (Hering); in Uroehaeta and Thamnodrilus the setae upon the clitellum are ridged. Analogous modifications are found in Nais and other “Limicolae." The function of these setae is probably, as Lankester has suggested, to assist in attaching the worms together during copulation. Before the individual is mature these setae are absent, and the ordinary setae occupy their place; when the clitellum is developed the ordinary setae drop out, and are replaced by the genital setae (Vejdovsky).
In Acanthodrilus, Typhaeus, and some species of Perichaeta the apertures of the atria are furnished with a thin-walled muscular diverticulum in which are found a bundle of extraordinarily long setae; these can be protruded through the sexual orifice and may possibly serve to assist in the transference of the sperm to the spermathecae of another individual. These setae 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 Perichaeta and Acanthodrilus, where the testes are prolonged into numerous digitate 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 and testes. With the exception of Eucli∙ pidrilus and Phreoryctes, the Oligoehaeta possess but a single pair of ovaries, which appear to be invariably placed behind the testes.
(ά) Receptaculum Ovorum. — Corresponding to the seminal reservoirs 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 trabeculae; and 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 receptacula. 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 trumpetshaped 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 Perichaeta the two oviducts open by a common pore situated in the median ventral line of the 14th segment. The apertures of the oviducts are, with the exception of Moniligaster (Horst) and Allιιrus (Beddard, 5), invariably placed in front of the male generative'pores. Among the “limicolous” forms oviducts have been described in Rhynchelmis, Phreoryctes, and Phreαtothrix ; 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 Rhynchelmis and Phreatoihrix the female pore is behind the male pore, while in earthworms (except Moniligaster, Allurus) it is in front. Among the lower Oligoehaeta oviducts are absent; their place is taken in the Enchytraeidae 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 Cyclostomata among fishes, where the abdominal pores act as oviduct (Weber, Zeitsch.f. wiss. Zook, 1887).
The ovaries and oviducts of Eudrilus (Beddard, Horst), differ in many particulars from those of other Oligoehaeta. 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 trabeculae into numerous compartments; directly continuous 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 large 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 Oligochaeta, but is analogous 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 covering 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 Stylariα, 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.
Spermathecae.— 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 copulation 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 oviducts ?), like the nephridia, consist of a funnel, of a more or less elongated tubule, and of a distal vesicular part. The intra cellular duct of the nephridium and the inter cellular duct of the vas deferens may be explained by the different functions which the organs perform. The spermathecae, 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 Claparède that in the “Limicolae” the genital ducts were the homologues of nephridia, but not in the “Terrieolaef 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 setae; 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 setae 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 setae; 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 setae.
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 Oligochaeta form a group which cannot be subdivided into Limieolae and Terricolae as was proposed by Claparède. The genera of Oligochaeta 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., Lumbricidae and Perichaetidae) of earthworms. Since that time, how ever, 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 are situated in front of the clitellum; (2) Iηtraclitellians {e.g., Eudrilus), where the male pores are within the clitellum; and (3) Postclitellians {e.g., Pcrichaeta), 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 intraclitellian Megascokx from the postclitellian Pcrichaeta, 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 Oligochaeta, whereas it is now impossible to draw any line of division between any two such groups.
The simplest forms of Oligochaeta are the genera Aeolosoma and Ctenodrilus. Aeolosoma 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, Aeolosoma agrees with Ctenodrilus ; the latter form has, however, a ventral nerve cord, which, like the cerebral ganglia, is lodged in the epidermis. In most of these points Aeolosoma and Ctenodrilus resemble larval forms of the higher Oligochaeta. The reproductive organs of Ctenodrilus are not known, but those of Aeolosoma are constituted on the Oligochaetous plan. These two genera are also closely allied to the Archiannclida, of which a short description is appended, as they are not treated elsewhere in this λvork.
Aeolosoma therefore retains certain Archiannelid characters, but is to be referred to the Oligochaeta 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 setae bundles, which agree in their structure with those of other Oligochaeta ; and (3) the relative complication of the nephridia, as compared with Archiamηelida.
The affinities of Ctenodrilus are not so plain; but, in the absence of any knowledge respecting the generative organs, it is impossible to refer them definitely to the. Oligochaeta or to the Polychaeta. Archiannclida.— This subclass includes certain small marine worms which were formerly placed among the Polychaeta. Hatchek originally created the group for the reception of Polygordius and Protodrilus ; and recently Föttinger has placed Histriodrilus (an animal formerly referred to the Discophora) in the same group. The name Archiannclida 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 Polychaeta and Oligochaeta 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 Chaetopoda 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 (g ’.u), and on the other among certain Polychaeta {e.g., Ophelidae), and in the lowly organized Oligochaet Aeolosoma. 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 Oligochaeta retain traces of the primitive ciliation of the body {e.g., Aeolosoma).
There are no traces of setae 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 successive septa into a series of segments; there is no such fusion of the anterior segments as is met with among the Polychaeta and Oligochaeta, and the segments are less differentiated among themselves than is usually the case in the higher Chaetopoda. 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 Oligochaeta) are found in all the segments except the first and last. The nephridia do not lie freely within the caelom, 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 Capitellidae ; in the higher Annelids the nephridia lie within the coelom, 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 blastocaele, 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 off a caecal tube running backwards along the somatic peritoneum. Thus the vascular system undergoes but little modification in different segments. In Protodrilus the bloodvessels 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 Polychaela and Oligoehaeta before the special muscular walls have been formed. Aeolosoma, 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 Chaetopoda and Oligo-chaeía. 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 Polychaela.
In Polygordius the cerebral mass is situated in the prostomium, and communicates by a circumaesophageal 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 swellings. A nervous plexus also exists in the thickness of the longitudinal muscles.
Polygordius is of separate sexes; ProtodrUus is hermaphrodite. The sexual products are developed in all except the first few and last few segments from the lining of the caelom; the ova and spermatozoa 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 discovereα 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.— (ι) Beddard, “Οη the Nephridia of Acaηthodrilus," Proc. Roy. Soe., 1885; Ann. Sei. Nat., 1886; (2) Id., “0η the Atrium in Earthworm,” Zool. Anz., 1888; (3) Id., “structure of Eudrilus,' ’ Proc. Zool. Soc., 1886; (4) Id., “Nephridia of Perichxta" Q. J. Μ. S., 1888; (5) Id., “Λllurus,'' ibid.; (6) Id., “Plιreoryctesi ’ Ann. Nat. Hist., 1888; (7) Benham, “Studies in Earthworms,” Nos. i., ii., iii., Q. J. Μ. S., 1886-7; (8) Id., “BrachydrilusP Zool. Anz., 1887; (9) Horst, “Notes on Earthworms,” Notes from Leyden Museum, 1886-7; (10) Rosa, “Criodrilus," Atti R. Acad. Sei. Torino, 1886; (11) Id., “Hormogaster,” ibid., 1888; (12) Spencer, “Nephridia of Earthworms,” Nature, June 28, 1888; (13) Scharff, “Ctenodrilus," Q. J. Μ. S., 1887; (14) Fletcher, “Australian Earthworms,” Proc. L. S. N. S. IF., 1886; (15) Vejdovsky, System u. Morphol. d. oligochäten; (16) Kükenthal, “Lymphoidzellen d. Anneliden,” Jen. Zeitsch., 1885; (17)Bourne “Indian Earthworms,” Proc. Zool. Soc., 1886; (18) Bergh, “Excretionsorgane d Würmer," Kosmos, 1885; (19) Id., “Geschlechtsorgane d. Regenwürmer,” ZeUsch. f. wiss. Zool., 1886. 0η Archiannelida see Fraipont, “Polygordius. Fauna und Flora des Golfen r. Neapel (complete references to previous papers).
