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...
