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OLIGOCHAETA

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Originally appearing in Volume V05, Page 799 of the 1911 Encyclopedia Britannica.
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OLIGOCHAETA.—AS contrasted with the other subdivisions of the Chaetopoda, the Oligochaeta may be thus defined. Setae c very rarely absent (genus Achaeta) and as a rule not so large or so numerous in each segment as in the Polychaeta, and different in shape. Eyes rarely 13. present and then rudimentary. Prostomium generally small, sometimes pro-longed, but never bearing tentacles or processes. Appendages of body reduced to branchiae, present only in four species, and to the ventral copulatory appendages of Alma and Criodrilus. CIitelIum always present, extending over two (many limicolous forms) to forty-five segments (Alma). Segments of body numerous and not distinctive of species, being irregular and not fixed in numbers. In terrestrial forms dorsal pores are usually present; in aquatic forms a head pore only. Anus nearly always terminal, rarely dorsal, at a little distance from end of body. Suckers absent. Nervous system rarely (Aeolosoma) in continuity with epidermis. Vascular system always present, forming a closed system, more complicated in the larger forms than in the aquatic genera. Several specially large contractile trunks in the anterior segments uniting the dorsal and ventral vessels. Nephridia generally paired, often very numerous in each segment, in the form of long, much-coiled tubes with intracellular lumen. Gonads limited in number of pairs, testes and ovaries always present in the same individual. Special sacs developed from the intersegmental septa lodge the developing ova and sperm. Special gonad ducts always present. Male ducts often open on to exterior through a terminal chamber which is variously specialized, and sometimes with a penis. Generative pores usually paired, sometimes single and median. Spermathecae nearly always present. Alimentary canal straight, often with appended glands of complicated or simpler structure; no jaws. Eggs deposited in a cocoon after copulation. Development direct. Reproduction by budding also occurs. Fresh-water (rarely marine) and terrestrial. The Oligochaeta show a greater variety of size than any other group of the Chaetopoda. They range from a millimetre or so (smaller species of Aeolosoma) to 6 ft. or even rather more (Microchaeta rappi, &c.) in length. Setae.—The setae, which are always absent from the peristomial segment, are also sometimes absent from a greater number of the a, Penial seta of Perichaeta cey- d, Seta of Lumbricus. lonica. e, Seta of Criodrilus. b, Extremity of penial seta of f, g, Setae of Bohemilla comata. Acanthodrilus (after Horst). h, i, j, Setae of Psammoryctes bar- c, Seta of Urochaeta (Perier). batus(ftojafterVezhdovsky). anterior segments of the body, and have completely disappeared in Achaeta cameranoi. When present they are either arranged in four bundles of from one to ten or even more setae, or are disposed in continuous lines completely encircling each segment of the body. This latter arrangement characterizes many genera of the family Megascolicidae and one genus (Periscolex) of the Glossoscolicidae. It has been shown (Bourne) that the "perichaetous " condition is probably secondary, inasmuch as in worms which are, when adult, " perichaetous " the setae develop in pairs so that the embryo passes through a stage in which it has four bundles of setae, two to each bundle, the prevalent condition in the group. Rarely there is an irregular disposition of the setae which are not paired, though the total number is eight to a segment (fig. to), e.g. Pontoscolex. The varying forms of the setae are illustrated in fig. 11. Structure.—The body wall consists of an epidermis which secretes a delicate cuticle and is only ciliated in Aeolosoma, and in that genus only on the under surface of the prostomium. The epidermis contains numerous groups of sense cells; beneath the epidermis there is rarely (Kynotus) an extensive connective tissue dermis. Usually the epidermis is immediately followed by the circular layer of muscles, and this by the longitudinal coat. Beneath this again is a distinct peritoneum lining the coelom, which appears to be wanting as a special layer in some Polychaetes (Benham, Gilson). The muscular layers are thinner in the aquatic forms, which possess only a single row of longitudinal fibres, or (Enchytraeidae) two layers. In the earthworms, on the other hand, this coat is thick and composed of many layers. The clitellum consists of a thickening of the epidermis, and is of two forms among the Oligochaeta. In the aquatic genera the epidermis comes to consist entirely of glandular cells, which are, however, arranged in a single layer. In the earthworms, on the other hand, the epidermis becomes specialized into several layers of cells, all of which are glandular. It is therefore obviously much thicker than the clitellum in the limicolous forms. The position of the clitellum, which is universal in occurrence, varies much as does the number of component segments. As a rule—to which, however, there are exceptions—the clitellum consists of two cr three segments only in the small aquatic Oligochaeta, while in the terrestrial forms it is as a general rule, to which again there are exceptions, a more extensive, sometimes much more extensive, region. In the Oligochaeta there is a closer correspondence between external metamerism and the divisions of the coelom than is apparent in some Chaetopods. The external segments are usually definable by the setae ; and if the setae are absent, as in the anterior segments A, Lumbricus: 9, TO, segments containing spermathecae, the orifices of which are indicated; 14, segment bearing oviducal pores; 15, segment bearing male pores; 32, 37, first and last segments of clitellum. B, Acanthodrilus: cp, orifices of spermathecae; ?, oviducal pores; a', male pores; on 17th and 19th segments are the apertures of the atria. C, Perichaeta: the spermathecal pores are between segments 6 and 7, 7 and 8, 8 and 9, the oviducal 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 setae by strokes. of several Geoscolicidae, the nephridiopores indicate the segments; to each segment corresponds internally a chamber of the coelom which is separated from adjacent segments by transverse septa,which are only unrecognizable in the genus Aeolosoma and in the head region of other Oligochaeta. In the latter case, the numerous bands of muscle attaching the pharynx to the parietes have obliterated the regular partition by means of septa. Nephridia.—The nephridia in this group are invariably coiled tubes with an intracellular lumen and nearly invariably open into the coelom by a funnel. There are no renal organs with a wide inter-cellular lumen, such as occur in the Polychaeta, nor is there ever any permanent association between nephridia and ducts connected with the evacuation of the generative products, such as occur in Alciope, Saccocirrus, &c. In these points the Oligochaeta agree with the Hirudinea. They also agree in the general structure of the nephridia. It has been ascertained that the nephridia of Oligochaeta are preceded in the embryo by;a pair of delicate and sinuous tubes, also found in the Hirudinea and Polychaeta, which are larval excretory organs. It is not quite certain whether these are to be regarded as the remnant of an earlier excretory system, replaced among the Oligochaeta by the subsequently developed paired structures, or whether these " head kidneys are the first pair of nephridia precociously developed. The former view has been extensively held, and it is supported by the fact that in Octochaetus the first segment of the body has a pair of nephridia which is exactly like those which follow, and, like them, persists. On the other hand, in most Oligochaeta the first segment has in the adult no nephridium, and in the case of Octochaetus the existence of a " head kidney " antedating the subsequently developed nephridia of the first and other segments has neither been seen nor proved to be absent. In any case the nephridia which occupy the segments of the body generally are first of all represented by paired structures; the " pronephridla," in which the funnel is composed of but one cell, which is flagellate. This stage has at any rate been observed in Rhynchelmis and Lumbricus (in its widest sense) by Vezhdovsky. It is further noticeable that in Rhynchelmis the covering of vesicular cells which clothes the drain-pipe cells of the adult nephridium is cut off from the nephridial cells themselves and is not a peritoneal layer surrounding the nephridium. Thus the nephridia, in this case at least, are a part of the coelom and are not shut off from it by a layer of peritoneum, as are other organs which lie in. it, e.g. the gut. A growth both of the funnel, which becomes multicellular, and of the rest of the nephridium produces the adult nephridia of the genera mentioned. The paired disposition of these organs is the prevalent one among the Oligochaeta, and occurs in all of twelve out of the thirteen families into which the group is divided. Among the Megascolicidae, however, which in number of genera and species nearly equals the remaining families taken together, another form of the excretory system occurs. In the genera Pheretime, Megascolex, Dichogaster, &c., each segment contains a large number of nephridia, which, on account of the fact that they are necessarily smaller than the paired nephridia of e.g. Lumbricus, have been termed micronephridia, as opposed to meganephridia; there is, however, no essential difference in structure, though micronephridia are not uncommonly (e.g. Megascolides, Octochaetus) unprovided with funnels. It is disputed whether these micronephridia are or are not connected together in each segment and from segment to segment. In any case they have been shown in three genera to develop by the growth and splitting into a series of original paired pronephridia. A complex network, however, does occur in Lybiodrilus and certain other Eudrilidae, where the paired nephridia possess ducts leading to the exterior which ramify and anastomose on the thickness of the body wall. The network is, however, of the duct of the nephridium, possibly ectodermic in origin, and does not affect the glandular tubes which remain undivided and with one coelomic funnel each. The Oligochaeta are the only Chaetopods in which undoubted nephridia may possess a relationship with the alimentary canal. Thus, in Octochaetus multiporus a large nephridium opens anteriorly into the buccal cavity, and numerous nephridia in the same worm evacuate their contents into the rectum. The anteriorly-opening and usually very large nephridia are not uncommon, and have been termed peptonephridia." Gonads and Gonad Ducts.—The Oligochaeta agree with the leeches and differ from most Polychaeta in that they are hermaphrodite. There is no exception to this generalization. The gonads are, more-over, limited and fixed in numbers, and are practically invariably attached to the intersegmental septa, usually to the front septum of a segment, more rarely to the posterior septum. The prevalent number of testes is one pair in the aquatic genera and two pairs in earthworms. But there are exceptions; thus a species of Lamprodrilus has four pairs of testes. The ovaries are more usually one pair, but two are sometimes present. The segments occupied by the gonads are fixed, and are for earthworms invariably X, XI, or one of them for the testes, and XIII for the ovaries. The position varies in the aquatic Oligochaeta. The Oligochaeta contrast with the Polychaeta in the general presence of outgrowths of the septa in the genital segments, which are either close to, or actually involve, the gonads, and into which may also open the funnels of the gonad ducts. These sacs contain the developing sperm cells or eggs, andare with very few exceptions universal in the group. The testee are more commonly thus involved than are the ovaries. It is indeed only among the Eudrilidae that the enclosure of the ovaries in septal sacs is at all general. Recently the same thing has been recorded in a few species of Pheretima (=Perichaela), but details are as yet wanting. We can thus speak in these worms of gonocoels, i.e. coelomic cavities connected only with the generative system. These cavities communicate with the exterior through the gonad ducts, which have nothing to do with them, but whose coelomic funnels are taken up by them in the course of their growth. There are, however, in the Eudrilidae, as already mentioned, sacs envolving the ovaries which bore their own way to the exterior, and thus may be termed coelomoducts. These sacs are dealt with later under the description of the s ermathecae,which function they appear to perform. The gonad ducts are male and female, and open opposite to or, rarely, alongside of the gonads, whose products they convey to the exterior. The oviducts are always short trumpet-shaped tubes and are some-times reduced (Enchytraeidae) to merely the external orifices. It is possible, however, that those oviducts belong to a separate morphological category, more comparable to the dorsal pores and to abdominal pores in some fishes. The sperm ducts are usually longer than the oviducts; but in Limicolae both series of tubes opening by the funnel into one segment and on to the exterior in the following segment. While the oviducts always open directly on to the exterior, it is the rule for the sperm ducts to open on to the exterior near to or through certain terminal chambers, which have been variously termed atrium and pro-state, or spermiducal gland. The distal extremity of this apparatus is sometimes eversible as a penis. Associated with these glands are frequently to be found bundles or pairs of long and variously modified setae which are termed penial setae,to distinguish them from other setae sometimes but not always associated with rather similar gland swhich are found anteriorly to these, and often in the immediate neighbourhood of the spermathecae; the latter are spoken of as genital setae. Spermathecae. — These structures appear to be absolutely distinctive of the Oligochaeta, unless the sacs which contain sperm and open in common with the nephridia of Saccocirrus (see HAPLODRILI) are similar. Spermathecae are generally present in the Oligochaeta and are absent only in comparatively few genera and species. Their position varies, but is constant for the species, and they are rarely found behind the gonads. They are essentially spherical, pear-shaped or oval sacs opening on to the exterior but closed at the coelomic end. In a few Enchytraeidae and Lumbriculidae the spermathecae open at the distal extremity into the oesophagus, which is a fact difficult of explanation. Among the aquatic Oligochaeta and many earthworms (the families Lumbricidae, Geoscolicidae and a few other genera) the spermathecae are simple structures, as has been described. In the majority of the Megascolicidae each sac is provided with one or more diverticula, tubular or oval in form, of a slightly different histological character in the lining epithelium, and in them is invariably lodged the sperm. The spermathecae are usually paired structures, one pair to each of the segments where they occur. In many Geoscolicidae, however, and certain Lumbricidae and Perichaelidae, there are several, even a large number, of pairs of very small spermathecae to each of the segments which contain them. In the Eudrilidae there are spermathecae of different morphological value. In figs. 12 and 13 are shown the spermathecae of the genera Hyperiodrilus and Heliodrilus, which are simple sacs ending blindly as in other earthworms, but of which there is only one median opening in the thirteenth segment or in the eleventh. In Heliodrilus the blind extremity of the spermatheca is enclosed in a coelomic sac which is in connexion with the sacs envolving the ovaries and oviducts. In Hyperiodrilus the whole spermatheca is thus included in a corresponding sac, which is of great extent. In such other genera of the family as have been examined, the true spermatheca has entirely disappeared, and the sac which contains it In Hyperiodrilus alone remains. This sac has been already referred to as a coelomoduct. Its orifice on to the exterior is formed by an involution *Iv xu1 [n xI (as it appears) of the epidermis, and that it performs the function of a spermatheca is shown by its containing spermatozoa, or, in Stuhlmannia, a spermatophore. In Polytoreutus, also, spermatophores have been found in these spermathecal sacs. We have thus the replacement of a spermatheca, corresponding to those of the remaining families of Oligochaeta, and derived, as is believed, from the epidermis, by a structure performing the same function, but derived from the mesoblastic tissues, and with a cavity which is coelom. Alimentary Canal.—The alimentary canal is always a straight tube, and the anus, save in the genera Criodrilus and Dero, is completely terminal. A buccal cavity, a pharynx, an oesophagus and an intestine are always distinguishable. Commonly among the terrestrial forms there is a gizzard, or two gizzards, or a larger number, in the oesophageal region. There is no armed protrusible pharynx, such as exists in some other Chaetopods. This may be associated with mud-eating habits; but it is not wholly certain that this is the case; for in Chaetogaster and Agriodrilus, which are predaceous worms, there is no protrusible pharynx, though in the latter the oesophagus is thickened through its extent with muscular fibres. The oesophagus is often furnished with glandular diverticula, the glands of Morren," which are often of complex structure through the folding of their walls. Among the purely aquatic families such structures are very rare, and are represented by two caeca in the genus Limnodriloides. It is a remarkable fact, not yet understood, that in certain Enchytraeidae and Lumbriculidae the spermathecae open into the oesophagus as well as on to the exterior. The only comparable fact among other worms is the Laurer's canal or genitointestinal canal in the Trematoda. The intestine is usually in the higher forms provided with a typhlosole, in which, in Pontoscolex, runs a ciliated canal or canals communicating with the intestine. It is possible that this represents the syphon or supplementary intestine of Capitellidae, which has been shown to develop as a groving of the intestine ultimately cut off from it. The intestine has a pair of caeca or two or three pairs (but all lie in one segment) in the genus Pheretima and in one species of Rhinodrilus. In Typhoeus and Megascolex there are corn-Fro. 13.—Female reproductive system plex glands appended to of Hyperiodrilus.—X III, XIV,thirteenth the intestine. and fourteenth segments. In Benhamia caecifera sp, Spermatheca. ov, Ovary. and at least one other sp', Spermathecal sac r.o, Egg sac. earthworm there are involving the last. od, Oviduct. numerous caeca, one pair to each segment. Classification.—The classifications of Adolf Eduard, Grube and Claparede separated into two subdivisions the aquatic and the terrestrial forms. This scheme, opposed by many, has been reinstated by Sedgwick. The chief difficulty in this scheme is offered by the Moniligastridae, which in some degree combine the characters of both the suborders, into neither of which will they fit accurately. The following arrangement is a compromise: Group I. A phaneura.—T his group is referred by A. Sedgwick to the Archiannelida. It is, however, though doubtless near to the base of the Oligochaetous series, most nearly allied in the reproductive system to the Oligochaeta. It contains but one family, Aeolosomatidae.. There are three pairs of spermathecae situated in segments III-V, a testis in V and an ovary in VI. There are a clitellum and sperm ducts which though like nephridia have a larger funnel and a less complexly wound duct. This family consists of only one well-known genus, Aeolosoma, which contains several species. They are minute worms with coloured oil drops (green, olive green or orange) contained in the epidermis. The nervous system is em-bedded in the epidermis, and the pairs of ganglia are separated as in Serpula, &c.; each pair has a longish commissure between its two ganglia. The intersegmental septa are absent save for the division of the first segment. The large prostomium is ciliated ventrally. The setae are either entirely capillary or there are in addition some sigmoid setae even with bifid free extremities. This genus also propagates asexually, like Ctenodrilus, which may possibly belong to the same family. Asexual reproduction universal. Group II. Limicolae.—With a few exceptions the Limicolae are, as the name denotes, aquatic in habit. They are small to moderate-sized Oligochaeta, with a smaller number of segments than in the Terricolae. The alimentary canal is simple and a gizzard or oesophageal diverticula rarely developed. The vascular system is simple with as a rule direct communication between dorsal and ventral vessels in each segment. Nerve cord lies in coelom; brain in first segment or prostomium in many forms. Clitellmo generally onlytwo or three segments and more anterior in position than in Terricolae. Nephridia always paired and without plexus of blood capillaries. Spermatheca rarely with diverticula; sperm ducts as a rule occupying two segments only, usually opening by means of an atrium. Sperm sacs generally occupying a good many segments and with simple interior undivided by a network of trabeculae. Ova large and with much yolk. Asexual reproduction only in Naids. Egg sacs as large or nearly so as sperm sacs. Testes and ovaries always free. The following families constitute the group, viz. Naididae, Enchytraeidae, Tubificidae, Lumbriculidae, Phreoryctidae, Phreodrilidae, Alluroididae, the latter possibly not referable to this group. Group III. Moniligastres.—Moderate-sized to very large Oligochaeta, terrestrial in habit, with the appearance of Terricolae. Generative organs anterior in position as in Limicolae. Sperm ducts and atria as in Limicolae; egg sacs large; body wall thick; vascular system and nephridia as in Terricolae. Only one family, Moniligastridae. Group IV. Terricolae.—Earthworms, rarely aquatic in habit. Of small to very large size. Clitellum commonly extensive and more posterior in position than in other groups. Vascular system complicated without regular connexion between dorsal and ventral vessels, except in anterior segments. Nephridia as a rule with abundant vascular supply. Testes, and occasionally ovaries, en-closed in sacs. Sperm sacs generally limited to one or two segments with interior subdivided by trabeculae. Sperm ducts traverse several segments on their way to exterior. They open in common with, or near to, or, more rarely, into, glands which are not certainly comparable to the atria of the Limicolae. Egg sacs minute and functionless (?). Eggs minute with little yolk. Nephridia some-times very numerous in each segment. Spermathecae often with diverticula. Earthworms are divided into the following families, viz. Megascolicidae, Geoscolicidae, Eudrilidae, Lumbricidae. As an appendix to the Oligochaeta, and possibly referable to that group, though their systematic position cannot at present be deter-mined with certainty, are to be placed the Bdellodrilidae (Discodrilidae auct.), which are small parasites upon crayfish. These worms lay cocoons like the Oligochaeta and leeches, and where they depart from the structure of the Oligochaeta agree with that of leeches. The body is composed of a small and limited number of segments (not more than fourteen), and there is a sucker at each end of the body. There are no setae and apparently only two pairs of nephridia, of which the anterior pair open commonly by a common pore on the third segment after the head, whose segments have not been accurately enumerated. The intervening segments contain the genitalia, which are on the Oligochaeta plan in that the gonads are independent of their ducts and that there are special spermathecae, one pair. The male ducts are either one pair or two pairs, which open by a common and complicated efferent terminal apparatus furnished with a protrusible penis. The ganglia are crowded at the posterior end of the body as in leeches, and there is much tendency to the obliteration of the coelom as in that group. Pterodrilus and Cirrodrilus bear a few, or circles of, external processes which may be branchiae; Bdellodrilus and Astacobdella have none. The vascular system is as in the lower Oligochaeta. There are two chitinous jaws in the buccal cavity, a dorsal and a ventral, which are of specially complicated structure in Cirrodrilus. HIRUDINEA: The leeches are more particularly to be compared with the Oligochaeta, and the following definition embraces the main features in which they agree and disagree with that group. Setae are only present in the genus Acanthobdella. Eyes are present, but hardly so complex as in certain genera of Polychaetes. The appendages of the body are reduced to branchiae, present in certain forms. A clitellum is present. The segments of body are few (not more than thirty-four) and fixed in number. The anus is dorsal. One or two (anterior and posterior) suckers always present. Nervous system always in coelom. Coelom generally reduced to a system of tubes, sometimes communicating with vascular system; in Acanthobdella and Ozobranchus a series of metamerically arranged chambers as in Oligochaeta. Nephridia always paired, rarely (Pontobdella) forming a network communicating from segment to segment; lumen of nephridia always intracellular, funnels pervious or impervious. Alimentary canal sometimes with protrusible proboscis; never with gizzard or oesophageal glands; intestine with caeca as a rule. Jaws often present. Testes several pairs, rarely one pair, continuous with sperm ducts; ovaries, one pair, continuous with oviducts; generative pores single and median. No separate spermathecae or septal chambers for the development of the ova and sperm. Eggs deposited in a cocoon. Development direct. No asexual generation. Fresh-water, marine and terrestrial. Parasitic or carnivorous. In external characters the Hirudinea are unmistakable and not to be confused with other Annelids, except perhaps with the Bdellodrilidae, which resemble them in certain particulars. The absence of setae—save in Acanthobdella, where five of the anterior segments possess each four pairs of setae with reserve setae placed close behind them (fig. 14), and the presence of an anterior and posterior sucker, produce a looping mode of progression similar to that of a Geometrid larva. The absence of setae and the great secondary annulation render the mapping of the segments a subject of some difficulty. The most reliable test appears to be the nerve ganglia, which are more distinct from the intervening connectives than in other Annelids. In the middle of the body, where the limits of the somites can be checked by a comparison with the arrangement of the nephridia and the gonads, and where the ganglia are quite distinct and separated by long connectives, each ganglion is seen to consist of six masses of cells enclosed by capsules and to give off three nerves on each side. This corresponds to the usual presence (in the Rhynchobdellidae) of three annuli to each segment. Anteriorly and posteriorly separate ganglia have fused. The brain consists not only of a group of six capsules corre•• sponding to the archicerebrum of the Oligochaeta, but of a further mass of cells surrounding and existing below the alimentary canal, which can be analysed into five or six more separate ganglia. The whole mass lies in the seventh or .eighth segment. At the posterior end of the body there are likewise seven separate ganglia partially fused to form a single ganglionic mass, which innervates the segments lying behind the anus and corresponding to the posterior sucker. So that a leech in which only twenty-seven segments are apparent by the enumeration of the annuli, separate ganglia, nephridia, lines of sensillae upon the body, really possesses an additional seven lying behind that which is apparently the last of the series and crowded together into a minute space. The annuli into which segments are externally divided are so deeply incised as to render it impossible to distinguish, as can be readily done in the Oligochaeta as a rule, the limits of an annulus from that of a true segment. As remarked, the prevalent number of annuli to a segment is three in the Rhynchobdellidae. But in that group (Cystobranchus) there may be as many as eight annuli. In the Gnathobdellidae the prevailing number of annuli to a segment is five; but here again the number is often increased, and Trocheta has no less than eleven. The reason for this excessive annulation has been seen in the limited number of segments (thirty-four) of which the body is composed, which are laid down early and do not increase. In the Oligochaeta, on the other hand, there is growth of new segments. It is important to notice that the metameric plan of growth of Chaetopods is still preserved. The nephridia are like those of the Oligochaeta in general structure; that is to say, they consist of drain-pipe cells which are placed end to end and are perforated by their duct. The internal funnel varies in the same way as in the Oligochaeta in the number of cells which form it. In Clepsine (Glossiphonia) there are only three cells, and in Nephelis five to eight cells. In Hirudo the funnel is not pervious and is composed of a large number of cells. Externally, the nephridium opens by a vesicle, as in many Oligochaetes whose lumen is intercellular. In Pontobdella and Branchellion the nephridia form a network extending from segment to segment, but there is only one pair of funnels in each segment. Slight differences in form have been noted between nephridia of different segments; but the Hirudinea do not show the marked differentiation that is to be seen in some other Chaetopods; nor do the nephridia ever acquire any relations to the alimentary canal. Coelom.—The coelom of the Hirudinea differs in most genera from that of the Oligochaeta and Polychaeta. The difference is that it is broken up into a complex sinus system. The least modified type is shown by Acanthobdella, a leech, parasitic upon fishes, in which transverse sections (see figs. 15 and 16) show the gut, the nervous system, &c., lying in a spacious chamber which is the coelom. This coelom is lined by peritoneal cells and is divided into a series of metameres by septa which correspond to the segmentation of the c, Coelom. g, Nerve cord. c.ch, Coelomic epithelium (yellow- m, Intestine. cells). mc, Circular muscle. cg, Glandular cells. ml, Longitudinal muscle. cl, Muscle cells of lateral line. vd, Dorsal vessel. cp, Pigment cells. vv, Ventral vessel. ep, Ectoderm. body, the arrangement being thus precisely like that of typical Chaetopoda. Moreover, upon the intestine the coelomic cells are modified into chloragogen cells. In Acanthobdella the testes are, however, not contained in the general coelom, and the nephridia lie in the septa. It is remarkable, in view of the spaciousness of the coelom, that the funnels of the latter have not been seen. Ozobranchus possesses a coelom which is less typically chaetopodous than that of Acanthobdella, but more so than in other leeches. There is a spacious cavity surrounding the gut and containing also blood-vessels, and to some extent the generative organs, and the nervous cord. Furthermore, in the mid region of the body this coelom is broken up by metamerically arranged septa, as in Acanthobdella. These septa are, however, rather incomplete and are not fastened to the gut; and, as in Acanthobdella, the nephridia are embedded vv C7L' in them. In addition to the median lacuna there are two lateral lacunae, one upon each side. These regions of the coelom end at the ends of the body and communicate with each other by means of a branched system of coelomic sinuses, which are in places very fine tubes. Neither in this genus nor in the last is there any communication between coelom and vascular system. In Clepsine (Glossiphonia) there is a further breaking up of the coelom. The median lacuna no longer exists, but is represented by a dorsal and ventral sinus. The former lodges the dorsal, the latter the ventral, blood-vessel. The gut has no coelomic space surrounding it. A complex .9 CT network places these sinuses and the lateral sinuses in communication. Here also the blood system has no communication with the sinus system of the coelom. In Hirudo and the Gnathobdellidae there is only one system of cavities which consist of four principal longitudinal trunks, of which the two lateral are contractile, which communicate with a network ramifying everywhere, even among the cells of the epidermis. The network is partly formed out of pigmented cells which are excavated and join to form tubes, the so-called botryoidal tissue, not found among the Rhynchobdellidae at all. It seems clear from the recent investigations of A. G. Bourne and E. S. Goodrich that the vascular system and the coelom are in communication (as in vertebrates by means of the lymph system). On the other hand, it has been held that in these leeches there is no vascular system at all and that the entire system of spaces is coelom. In favour of regarding the vascular system as totally absent, is the fact that the median coelomic channels contain no dorsal and ventral vessel. In favour of seeing in the lateral trunks and their branches a vascular system, is the contractility of the former, and the fact of the intrusion of the latter into the epidermis, matched among the Oligochaeta, where undoubted blood capillaries perforate the epidermis. A further fact must be considered in deciding this question, which is the discovery of ramifying coelomic tubes, approaching close to, but not entering, the epidermis in the Polychaete Arenicola. These tubes are lined by flattened epithelium and often contain blood capillaries; they communicate with the coelom and are to be regarded as prolongation of it into the thickness of the body wall. Gonads and Gonad Ducts.—The gonads and their ducts in the Hirudinea invariably form a closed system of cavities entirely shut off from the coelom in which they lie. There is thus a broad resemblance to the Eudrilidae, to which group of Oligochaeta the Hirudinea are further akin by reason of the invariably unpaired condition of the generative apertures, and the existence of a copulatory apparatus (both of which characters, however, are present occasion-ally in other Oligochaeta). The testes are more numerous than the ovaries, of which latter there are never more than one pair. The testes vary in numbers of pairs. Four (Ozobranchus) to six (Glossiphonia) or ten (Philaemon) are common numbers. In Acanthobdella, however, the testes of each side of the body have grown together to form a continuous band, which extends in front of external pore. Each testis communicates by means of an efferent duct with a common collecting duct of its side of the body, which opens on to the exterior by means of a protrusible penis, and to which is sometimes appended a seminal vesicle. The efferent ducts are ciliated, and there is a patch of cilia at the point where they communicate with the cavity of each testis. The ovaries are more extensive in some forms (e.g. Ozobranchus) than in others, where they are small rounded bodies. The two ducts continuous with the gonads open by a common vagina on to the exterior behind the male pores. This " vagina " is sometimes of exaggerated size. Thus, in Philaemon pungens (Lambert) it has the form of a large sac, into which open by a single orifice the con-joined oviducts. From this vagina arises a narrow duct leading to the exterior. In Ozobranchus the structures in question are still more complicated. The two long ovarian sacs communicate with each other by a transverse bridge before uniting to form the terminal canal. Into each ovarian sac behind the transverse junction opens a slender tube, which is greatly coiled, and, in its turn, opens into a spherical "spermathecal sac." From this an equally slender tube proceeds, which joins its fellow of the opposite side, and the two form a thick, walled tube, which opens on to the exterior within the bursa copulatrix through which the penis protrudes. These two last-mentioned types show features which can be, as it seems, matched in the Eudrilidae. The gonads develop (O. Burger) in coelomic spaces close to nephridial funnels, which have, however, no relation to the gonad ducts. The ovaries are solid bodies, of which the outer layer becomes separated from the plug of cells lying within; thus a cavity is formed which is clearly coelom. This cavity and its walls becomes pro-longed to form the oviducts. A stage exactly comparable to the stage in the leeches, where the ovary is surrounded by a closed sac, has been observed in Eudrilus. In this Annelid later the sac in question joins its fellow, passing beneath the nerve cord exactly as in the leech, and also grows out to reach the exterior. The sole difference is therefore that in Eudrilus the ovarian sac gives rise to a tube which bifurcates, one branch meeting a corresponding branch of the other ovary of the pair, while the second branch reaches the exterior. In the leech the two branches are fused into one. We have here clearly a case of a true coelomoduct performing the function of an oviduct in both leeches and Eudrilidae. The facts just referred to suggest further comparisons between the Hirudinea and Eudrilidae. The large sacs which have been termed vagina are suggestive of the large coelomic spermathecae in Eudrilids, a comparison which needs, however, embryological data, not at present forthcoming, for its justification. It is at least clear that in Ozobranchus this comparison is justifiable; but only probable, or perhaps possible, in the case of Philaemon. In the former, the duct, leading from the ovarian sac, and swelling along its course into the spherical sac, the " spermatheca," is highly suggestive of the oviduct and receptaculum of the Eudrilidae.799 The testes during development become hollowed out and are prolonged into the vasa efferentia. These ducts therefore have not their exact counterparts in the Oligochaeta, unless we are to assume that they collectively are represented by the seminal vesicles of earthworms and the vasa deferentia. It is to be noted that the Hirudinea differ from the Oligochaeta in that the male pore is in advance of the gonads (except in Acanthobdella, which here, as in so many points, approximates to the Oligochaeta), whereas in Oligochaeta that pore is behind the gonads (again with an exception, Allurus). Classification.—The Hirudinea maybe divided into three families : (i.) Rhynchobdellidae.—A protrusible proboscis exists, but there are no jaws. The blood is colourless. Pontobdella, Glossiphonia, &c. (ii.) Gnathobdellidae.—A proboscis absent, but jaws usually present. Blood coloured red with haemoglobin. Hirudo, Nephelis, &c. (iii.) Acanthobdellidae.—Proboscis present, but short. Paired setae of Oligochaetous pattern present in anterior segments. Blood red. Acanthobdella.
End of Article: OLIGOCHAETA
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