Rhopalura giardiiis of distinct sexes. Either males or females are found in oneAmphiura. Two kinds of females, flattened unsegmented, and cylindrical segmented forms, are known. They consist of a ciliated ectodermal layer enclosing an endodermal mass of eggs, between which is a fibrillar layer usually considered to be of a muscular nature. The cylindrical female gives rise to eggs which develop, probably exclusively, into males. The flattened female produces eggs from which females alone arise, though the origin of the two forms of this sex is not well ascertained. The males contain spermatozoa which fertilise the eggs of the cylindrical female, whereas the ova of the flat form probably develop parthenogenetically.
fig47Fig. 47.—Rhopalura giardiiMetschn. (from the brittle-starAmphiura squamata). ♂, Full-grown male (× 800); ♀1, flattened form of female (× 510); ♀2, cylindrical female (× 510). (After Julin.)
Fig. 47.—Rhopalura giardiiMetschn. (from the brittle-starAmphiura squamata). ♂, Full-grown male (× 800); ♀1, flattened form of female (× 510); ♀2, cylindrical female (× 510). (After Julin.)
Fig. 47.—Rhopalura giardiiMetschn. (from the brittle-starAmphiura squamata). ♂, Full-grown male (× 800); ♀1, flattened form of female (× 510); ♀2, cylindrical female (× 510). (After Julin.)
Trichoplax.[116]—This anomalous animal has only been found in aquaria, originally in the marine aquarium at Graz bySchulze. It has the appearance of a large, flattened, ciliatedAmoeba(1.5-3 mm. in diameter), but is distinguished by its structure. The upper surface is composed of a flattened epithelium. The lower surface is made up of cylindrical ciliated cells, which pass imperceptibly into the branched cells, embedded in a hyaline matrix, which compose the middle layer of the body. No distinct organs, and beyond simple fission, no mode of reproduction, have been observed. One species,T. adhaerens, is known, but has never been met with in a free state.
Salinella.[117]—This is another aquarium-animal, found by Frenzel in the Argentine, in an artificial saline solution with which he filled some aquaria. It measures .2 mm. in length, and has a somewhat flattened, barrel-shaped appearance. A single layer of ciliated cells bounds a central cavity opening at each end. Fission, and conjugation followed by encystment, have been observed. One form,S. salve, is known from salines taken from Cordova.
BY
LILIAN SHELDON
Staff Lecturer in Natural Science, Newnham College, Cambridge.
NEMERTINEA
INTRODUCTORY—EXTERNAL CHARACTERS—ANATOMY—CLASSIFICATION—DEVELOPMENT—HABITS—REGENERATION—BREEDING—GEOGRAPHICAL DISTRIBUTION—LAND, FRESH-WATER, AND PARASITIC FORMS—AFFINITIES
The Nemertinea form a compact group, the affinities of which have not been at present clearly determined. Several species were mentioned and described in the works of various naturalists during the latter half of the eighteenth century, though their anatomy was not understood until considerably later. The first mention of any member of the group was made by the Rev. W. Borlase in hisNatural History of Cornwall, published in 1758. He gives a short description and a rough figure ofLineus marinus. From that time the increase in the knowledge of the group was very gradual. New species were from time to time described, but few of the descriptions could boast of much completeness, and many erroneous views were held until comparatively recent years. The group was very variously classified, but the general arrangement in early times seems to have been to unite it with the Planarians. Valuable contributions to the history of the development were made in 1848 and the few subsequent years by Desor,[118]Gegenbaur,[119]Krohn,[120]and Leuckart and Pagenstecher[121]; and more recently by Metschnikoff[122]and Salensky.[123]
Nemertines for the most part closely resemble one another in all essential points, though they differ considerably in size, colour, and external details. They vary in length from less than an inch to thirty yards, this extreme size being attained byLineus marinus.
fig48Fig. 48.—Lineus marinusMont., from the living specimen in the coiled condition. Plymouth. × 1.a, Anterior end;b, posterior end.
Fig. 48.—Lineus marinusMont., from the living specimen in the coiled condition. Plymouth. × 1.a, Anterior end;b, posterior end.
Fig. 48.—Lineus marinusMont., from the living specimen in the coiled condition. Plymouth. × 1.a, Anterior end;b, posterior end.
fig49Fig. 49.—L. marinus, from the same specimen as Fig. 48, in the expanded condition.a, Anterior end;b, posterior end.
Fig. 49.—L. marinus, from the same specimen as Fig. 48, in the expanded condition.a, Anterior end;b, posterior end.
Fig. 49.—L. marinus, from the same specimen as Fig. 48, in the expanded condition.a, Anterior end;b, posterior end.
Nemertines are common on the British coasts; about forty species have been recorded from this area. On turning over a stone on a sandy or muddy shore in a pool left by the receding tide, there may often be seen a coiled mass, having the appearance of a uniform slimy string twisted into a complicated knot. If it be carefully removed, the ends can generally be made out, one bluntly rounded and the other slightly tapering (Fig. 48,aandb). Occasionally there may be seen attached to the blunter end a fine thread, which moves about freely. This thread may, by an instantaneous movement, be drawn into the body, no trace of its existence being left except at the tip of the head, where a small pore is visible; this is the orifice through which it was withdrawn. Shortly afterwards the thread may be again shot out, the process being instantaneous and often accomplished withgreat force. This thread (Fig. 50,p) is the proboscis, a very important and characteristic organ in Nemertines.
Most Nemertines are marine; they are mostly indifferent to climate and to the nature of the soil on which they live.
A few forms live on land (e.g.Tetrastemma agricola,[124]Geonemertes palaensis,[125]andG. chalicophora[126]) or in fresh water (e.g.Tetrastemma aquarum dulcium[127]andT. lacustre[128]) in various parts of the globe. There are also parasitic forms; the best known of which isMalacobdella.[129]A pelagic form,Pelagonemertes,[130]has been described by Moseley.
fig50Fig. 50.—Side view of head ofCerebratulus(Micrura)tristisHubr., showing the everted proboscis. Naples. × 2. Drawn from a spirit specimen.c.s, Cephalic slit;m, mouth;p, proboscis.
Fig. 50.—Side view of head ofCerebratulus(Micrura)tristisHubr., showing the everted proboscis. Naples. × 2. Drawn from a spirit specimen.c.s, Cephalic slit;m, mouth;p, proboscis.
Fig. 50.—Side view of head ofCerebratulus(Micrura)tristisHubr., showing the everted proboscis. Naples. × 2. Drawn from a spirit specimen.c.s, Cephalic slit;m, mouth;p, proboscis.
External Characters.—A typical Nemertine possesses an elongated worm-like body (Fig. 49), which is usually thrown into numerous close coils (Fig. 48). In section it may be either round or more or less flattened, with the lateral edges in some cases quite thin and almost fin-like. One or two broad, flattened, and leaf-shaped forms are known, but such a condition is exceptional, and the forms in which it occurs have probably assumed it owing to the adoption of special modes of life.
In the ordinary forms the posterior end of the body is pointed either bluntly or sharply. The head is somewhat broader than the rest of the body, and often assumes a spatulate form. Eyes (Fig. 51,e) are usually present either in one or several pairs, or in symmetrically-arranged groups on each side of the head. The mouth (Fig. 58,m) is situated near the front end of the body on the ventral surface, and is usually rendered conspicuous by being surrounded by thick tumid lips. It varies in form from being slit-like to elliptical. At the anterior end of the body asmall terminal pore occurs; this is the external opening of the proboscis (Fig. 51,p.p).
Nemertines are often very diversely and brilliantly coloured, the hues most commonly found being white, yellow, green, deep purple, and various shades of red and pink. The ventral surface is usually paler in colour than the dorsal, and the latter is often marked by longitudinal and transverse stripes (Fig. 59) in contrasting colours.
The whole animal is enveloped in a layer of mucus, which sometimes becomes hardened to form a tube, and this may be still further strengthened by an admixture of particles of sand or earth.
The body is capable to a great extent of contraction and extension, a Nemertine many inches long being apt, when irritated or alarmed, to contract itself to the length of not more than half an inch. Hence, unless the animal is kept and carefully watched, a very erroneous idea may be conceived as to its size.
Anatomy.—The body-wall consists of several layers (Fig. 52), which in a typical highly-developed Nemertine are as follows:—
1. An external epidermic layer (ep), consisting of ciliated cells, among which are placed numerous unicellular glands. These glands probably secrete the mucus in which the Nemertine is usually enveloped; their contents when in the body are very highly refracting. The epidermis rests on a basement membrane (b.m).
2. The two or three muscular layers, arranged as either an external circular and an internal longitudinal, or an inner and an outer circular separated by a longitudinal layer, or, as in the figure (c.mandl.m), two longitudinal separated by a circular layer.
fig51Fig. 51.—Amphiporus lactifloreusJohnst., drawn from the living specimen, from the dorsal surface. Plymouth. × 2.e, Eyes;g, generative organs;n.g, nerve ganglion;p.p, proboscis pore;p, proboscis.
Fig. 51.—Amphiporus lactifloreusJohnst., drawn from the living specimen, from the dorsal surface. Plymouth. × 2.e, Eyes;g, generative organs;n.g, nerve ganglion;p.p, proboscis pore;p, proboscis.
Fig. 51.—Amphiporus lactifloreusJohnst., drawn from the living specimen, from the dorsal surface. Plymouth. × 2.e, Eyes;g, generative organs;n.g, nerve ganglion;p.p, proboscis pore;p, proboscis.
3. A fairly thick connective-tissue layer often found between the epidermis and the muscles, into which latter it gradually merges (s.t).
The Digestive System.—The mouth is placed on the ventral surface near the anterior end of the body (Figs. 53, 58,m). It leads into a straight oesophagus (Fig. 53,oes), whence passes off the intestine (int), which is continued as a straight non-convoluted tube to the anus (a), situated terminally at the posterior end of the body. The intestine is thrown out throughout the greater part of its course into paired lateral pouches.
fig52Fig. 52.—Diagrammatic transverse section of a Nemertine (Schizonemertea) through the middle region of the body.b.m, Basement membrane;c.m, circular muscle layer;d.b, dorsal blood-vessel;ep, epidermis;g, generative organs;int, intestine;l.b, lateral blood-vessel;l.m, longitudinal muscle layers;n.c, lateral nerve-cord;n.l, nerve plexus;p, proboscis;p.s, proboscis sheath;s.t, subcutaneous layer.
Fig. 52.—Diagrammatic transverse section of a Nemertine (Schizonemertea) through the middle region of the body.b.m, Basement membrane;c.m, circular muscle layer;d.b, dorsal blood-vessel;ep, epidermis;g, generative organs;int, intestine;l.b, lateral blood-vessel;l.m, longitudinal muscle layers;n.c, lateral nerve-cord;n.l, nerve plexus;p, proboscis;p.s, proboscis sheath;s.t, subcutaneous layer.
Fig. 52.—Diagrammatic transverse section of a Nemertine (Schizonemertea) through the middle region of the body.b.m, Basement membrane;c.m, circular muscle layer;d.b, dorsal blood-vessel;ep, epidermis;g, generative organs;int, intestine;l.b, lateral blood-vessel;l.m, longitudinal muscle layers;n.c, lateral nerve-cord;n.l, nerve plexus;p, proboscis;p.s, proboscis sheath;s.t, subcutaneous layer.
The alimentary canal is lined throughout by a ciliated epithelium. The oesophagus has, in addition to this layer, an outer thick coat of large granular cells, which probably have a glandular function.
Proboscis.—The most characteristic organ of the Nemertines is the proboscis (Figs. 50, 53, 54). For many years its disposition and function were misunderstood, and it was supposed to be a portion of the digestive system. The proboscis, which lies dorsal to the alimentary canal, opens at the extreme anterior end of the body by a small pore (Figs. 51, 53, 58). When retracted it is sometimes considerably folded, and lies in a long pouch or sheath. To the walls of this sheath it is attached round its anteriorend; and strong muscles unite its posterior extremity to the sheath a short distance from the posterior end of the latter.
The proboscis seems to be exclusively a tactile and protective and defensive organ, for which functions it is eminently fitted by the great ease and rapidity with which it is everted or thrust out from the body. It consists of two distinct regions (Fig. 54,g.pandm.p). In the retracted state the anterior part is a hollow tube with very thick muscular walls made up of several layers. At the base of this part in many of the Nemertines there is situated a sharp-pointed spine projecting forward into the lumen, and several smaller stylets situated in a pair of vesicles close to the base of the central spine. The position of the spines in the everted proboscis is shown in Fig. 57. The posterior part of the proboscis is also a tube, but instead of being muscular, its walls are glandular. This posterior glandular part is never everted.
fig53Fig. 53.—Diagrammatic drawing of a Nemertine from the dorsal surface to show the position of some of the principal organs.a, Anus;c.s, cephalic slit;g, generative organs;int, intestine with its lateral diverticula;m, mouth;n.c, lateral nerve-cord;n.g, nerve ganglion;oes, oesophagus;p, proboscis;p.p, proboscis pore;p.s, proboscis sheath.
Fig. 53.—Diagrammatic drawing of a Nemertine from the dorsal surface to show the position of some of the principal organs.a, Anus;c.s, cephalic slit;g, generative organs;int, intestine with its lateral diverticula;m, mouth;n.c, lateral nerve-cord;n.g, nerve ganglion;oes, oesophagus;p, proboscis;p.p, proboscis pore;p.s, proboscis sheath.
Fig. 53.—Diagrammatic drawing of a Nemertine from the dorsal surface to show the position of some of the principal organs.a, Anus;c.s, cephalic slit;g, generative organs;int, intestine with its lateral diverticula;m, mouth;n.c, lateral nerve-cord;n.g, nerve ganglion;oes, oesophagus;p, proboscis;p.p, proboscis pore;p.s, proboscis sheath.
The eversion is effected by a turning inside out of the anterior part of the proboscis (Fig. 54). The process whereby the proboscis is retracted has been very aptly compared to the effect which would be produced by the inversion of the finger of a glove, accomplished by pulling a string attached to its tip on the inside, the anterior muscular part being comparable to the finger and the glandular part to the string. It is thus obvious that in the everted condition the stylet will form the anterior tip of theproboscis, and will there be in a position for offence or defence (Fig. 57,s).
Nervous System.[131]—The brain is composed of two ganglionic masses (Fig. 53,n.g) lying at the anterior end of the body, one on each side of the proboscis, and united by commissures passing round it (Fig. 55,d.candv.c). Each ganglionic mass is often partially divided into a dorsal and ventral lobe (n.g.dandn.g.v). From the brain a pair of cords pass off backwards along the sides of the body (n.c); these cords, which have no ganglionic swellings, in some forms unite with one another above the anus. Anteriorly nerves are given off from the brain to the eyes and front part of the head (a.n). A nerve to the proboscis is given off from the commissure which unites the two halves of the brain dorsal to the proboscis (d.n).
fig54Fig. 54.—Diagrammatic representation of the proboscis, (A) in the retracted condition, (B) in the everted condition.g.p, Glandular portion of the proboscis;m, muscle attaching the proboscis to its sheath;m.p, muscular portion of the proboscis;p.pinA, proboscis pore;p.pinBrepresents the position of the proboscis pore in the retracted condition of the proboscis;p.s, proboscis sheath.
Fig. 54.—Diagrammatic representation of the proboscis, (A) in the retracted condition, (B) in the everted condition.g.p, Glandular portion of the proboscis;m, muscle attaching the proboscis to its sheath;m.p, muscular portion of the proboscis;p.pinA, proboscis pore;p.pinBrepresents the position of the proboscis pore in the retracted condition of the proboscis;p.s, proboscis sheath.
Fig. 54.—Diagrammatic representation of the proboscis, (A) in the retracted condition, (B) in the everted condition.g.p, Glandular portion of the proboscis;m, muscle attaching the proboscis to its sheath;m.p, muscular portion of the proboscis;p.pinA, proboscis pore;p.pinBrepresents the position of the proboscis pore in the retracted condition of the proboscis;p.s, proboscis sheath.
In two out of the three groups into which the Nemertines are divided, the lateral nerve-cords are in connexion with a network or plexus of nerves lying between the muscular layers of the body-wall (Fig. 52,n.l), and in some forms constituting a comparatively thick layer. In these two groups there are no definitenerve branches except the anterior ones to the head. In the third group of Nemertines the lateral nerve-cords lie within the muscular layers of the body-wall, and in this case paired nerve branches are given off at definite intervals throughout the whole length of the body. These branches divide up among the organs to which they pass, and no nerve plexus is present.
The lateral cords vary in position in different cases. Sometimes they lie laterally, at others the cords tend to approximate to one another in the median dorsal or in the median ventral line, though in every case they remain distinctly separated.
Sense Organs.—Sense organs are usually present in the form of eyes arranged at the sides of the head (Fig. 51,e), sometimes as a single pair and sometimes in one or more groups on each side. The structure of the eyes varies from a simple pigment spot to an organ which receives a special nerve-supply from the brain, and possesses a refracting body answering to a lens, and behind this a pigment layer and a layer of rods. Some forms are devoid of all traces of eyes.
fig55Fig. 55.—Diagram to show the relations of the nervous system, circulatory system, and proboscis sheath in the anterior end of the body in theHoplonemertea, modified from M‘Intosh.a.n, Nerves to anterior part of body and eyes;d.c, dorsal commissure;d.n, median dorsal nerve;d.v, dorsal vascular trunk;l.v, lateral vascular trunk;n.c, lateral nerve-cord;n.g.d, dorsal lobe of nerve ganglion;n.g.v, ventral lobe of nerve ganglion;p.p, proboscis pore;p.s, proboscis sheath;v.c, ventral commissure;v.s, vascular ring or collar.
Fig. 55.—Diagram to show the relations of the nervous system, circulatory system, and proboscis sheath in the anterior end of the body in theHoplonemertea, modified from M‘Intosh.a.n, Nerves to anterior part of body and eyes;d.c, dorsal commissure;d.n, median dorsal nerve;d.v, dorsal vascular trunk;l.v, lateral vascular trunk;n.c, lateral nerve-cord;n.g.d, dorsal lobe of nerve ganglion;n.g.v, ventral lobe of nerve ganglion;p.p, proboscis pore;p.s, proboscis sheath;v.c, ventral commissure;v.s, vascular ring or collar.
Fig. 55.—Diagram to show the relations of the nervous system, circulatory system, and proboscis sheath in the anterior end of the body in theHoplonemertea, modified from M‘Intosh.a.n, Nerves to anterior part of body and eyes;d.c, dorsal commissure;d.n, median dorsal nerve;d.v, dorsal vascular trunk;l.v, lateral vascular trunk;n.c, lateral nerve-cord;n.g.d, dorsal lobe of nerve ganglion;n.g.v, ventral lobe of nerve ganglion;p.p, proboscis pore;p.s, proboscis sheath;v.c, ventral commissure;v.s, vascular ring or collar.
A pair of simple auditory capsules has been found in some of theHoplonemertea, where they occur as small vesicles on the brain.
The whole surface of the body appears to be remarkably sensitive. In a few forms small tufts of tactile hairs are said to be present in the region of the head, while in others thereare a few long hairs scattered sparsely among the cilia of the epidermis.
Frontal Organ.—In many Nemertines there is present at the anterior tip of the head a disc-shaped group of cells bearing long hairs or bristles. On this disc open the secreting ducts of a number of gland cells lying in the head. It seems possible that this frontal organ may function as an organ of taste.
Side Organs.—In the Carinellidae there is a pair of circular epithelial patches lying one on each side of the body in the region of the excretory pore. The cells composing them are richly ciliated and provided with a plentiful nerve-supply. The function of these epithelial patches is not known, but it has been suggested that they may be auditory organs.
Cephalic Slits and Cerebral Organs.—In most Nemertines there is a peculiar pair of organs (Figs. 50, 53,c.s), situated in the head and in close connexion with the brain. The function of these organs is not known. Hubrecht has suggested that they may be respiratory, while Bürger[132]conjectures that they may be organs which are used for discriminating the condition of the surrounding medium. In an external examination of the head, the cephalic slits may usually be seen as a pair of lateral furrows or pits. Their form and direction vary considerably; they may take the form of shallow circular depressions, or they may lie longitudinally and be slit-like in shape (Fig. 50), or the slit may lie at right angles to the long axis of the body and be beset with short transverse furrows. In some forms these slits are merely superficial depressions, but in others they are continued into ciliated ducts, which pass inwards and penetrate into special lobes, consisting of glandular tissue and ganglion cells, in close connexion with the brain. These lobes are called the cerebral organs.
In many forms the nervous system is charged with haemoglobin, which gives to it a bright red colour.
Circulatory or Blood-Vascular System.—The circulatory system consists of three main longitudinal vessels, a median dorsal and a pair of lateral ones. These are connected together posteriorly by a transverse trunk, and also throughout the whole length of their course by branches, which are given off at regular intervals. Anteriorly the three longitudinal vesselseither all unite and form a collar (Fig. 55,v.s) round the oesophagus, or they break up into a number of lacunar or open spaces in free communication with one another.
The blood is usually colourless, but in some cases the corpuscles are coloured red by haemoglobin.
fig56Fig. 56.—Excretory system of Nemertines.A,Drepanophorus spectabilisQtrf., part of one of the lateral vessels encircled by branches of the excretory organ, × 585;e, main canal of the excretory system:B,D. crassusQtrf., a terminal branch of the excretory system, × 585;f, ciliated flame:C,Malacobdella grossaO. F. Müll., entire animal, slightly magnified, showing the excretory system (black) and the vascular system;e.a, excretory aperture;d.v, dorsal vessel;l.v, lateral vessel. (From Bürger.)
Fig. 56.—Excretory system of Nemertines.A,Drepanophorus spectabilisQtrf., part of one of the lateral vessels encircled by branches of the excretory organ, × 585;e, main canal of the excretory system:B,D. crassusQtrf., a terminal branch of the excretory system, × 585;f, ciliated flame:C,Malacobdella grossaO. F. Müll., entire animal, slightly magnified, showing the excretory system (black) and the vascular system;e.a, excretory aperture;d.v, dorsal vessel;l.v, lateral vessel. (From Bürger.)
Fig. 56.—Excretory system of Nemertines.A,Drepanophorus spectabilisQtrf., part of one of the lateral vessels encircled by branches of the excretory organ, × 585;e, main canal of the excretory system:B,D. crassusQtrf., a terminal branch of the excretory system, × 585;f, ciliated flame:C,Malacobdella grossaO. F. Müll., entire animal, slightly magnified, showing the excretory system (black) and the vascular system;e.a, excretory aperture;d.v, dorsal vessel;l.v, lateral vessel. (From Bürger.)
Excretory System.—Max Schultze[133]found inTetrastemma obscurum, on the outer side of, but near to the lateral blood-vessels, a pair of canals. He observed ciliary movements in the canals, but could not discover flame cells. Further contributions to our knowledge of the excretory system were made by Semper,[134]von Kennel,[135]Hubrecht,[136]and Oudemans.[137]The latter states that the excretory system consists of a pair of canals situated laterally near the anterior end of the body. Each canal communicates with the exterior by one or more ducts having lateral regularly-arranged apertures. In some cases he was unable to make out any communication with the vascular system, but in othersa direct communication, by means of open connexions with the lacunar blood spaces, is said to occur.
Silliman[138]inTetrastemma aquarum dulciumdescribes the excretory vessels as ending in numerous capillary branches, at the blind terminations of which cilia are present. He states that there is no important difference between the excretory systems of Rhabdocoeles and Nemertines.
Bürger,[139]as the result of recent investigations on the excretory system in Nemertines, finds that the minute branches end in flame-cells (Fig. 56, B) lying on and among the blood-vessels, but having no open connexion with them.
Generative System.—The Nemertines are for the most part dioecious, only a few certainly hermaphrodite species having been described, e.g.Tetrastemma("Borlasia")kefersteiniiMar.[140]
The generative products in both cases are contained in sacs (Figs. 52, 53,g) which lie in the lateral region of the body between the pouches of the alimentary canal. The ova and spermatozoa are conveyed to the exterior by short ducts. Most species are oviparous, though a few viviparous species are known (e.g.Prosorhochmus claparedii).
Classification.—Nemertines were divided by M. Schultze[141]into:—
1.Enopla, in which the proboscis is armed with stylets.
2.Anopla, in which the proboscis is unarmed.
Although this classification was fairly correct as far as it went, since many other distinctive features were correlated with the presence or absence of armature in the proboscis, still there are several primitive forms belonging to theAnopla, which possess characters such as render it necessary to class them together in a separate group.
For this reason Hubrecht divided the Nemertinea into three Orders—Hoplonemertea,Schizonemertea,Palaeonemertea; the first of these Orders corresponding with theEnopla, and the other two with theAnopla.
Order I. Hoplonemertea.
The proboscis is armed. The epidermis rests on a thick layer of connective tissue plentifully supplied with glands, below which is a prominent basement membrane. The muscular layers of the body are two in number, an outer circular and an inner longitudinal. The nerve-trunks lie within the muscular layers of the body and give off regularly-arranged branches. There is no nerve plexus. Each of the cephalic slits generally opens by a pore situated in the centre of a transverse groove, which is beset along one side by a row of shorter grooves at right angles to it. The apparatus consists of a ciliated duct surrounded by nerve tissue, and passing into lobes of tissue which are connected with the brain by thick nerve-cords. The mouth opens rather far forward in front of the brain. The intestinal pouches are symmetrically arranged. Auditory organs are said to exist in some forms, consisting of vesicles containing otoliths. The vascular trunks are connected anteriorly by closed vessels and not by lacunar spaces.
fig57Fig. 57.—Anterior end of the everted proboscis (Hoplonemertea).g.p, Glandular portion of the proboscis;l.s, lateral sacs containing stylets;m.p, muscular portion of the proboscis;s, stylet;s.b, granular basal portion of stylet.
Fig. 57.—Anterior end of the everted proboscis (Hoplonemertea).g.p, Glandular portion of the proboscis;l.s, lateral sacs containing stylets;m.p, muscular portion of the proboscis;s, stylet;s.b, granular basal portion of stylet.
Fig. 57.—Anterior end of the everted proboscis (Hoplonemertea).g.p, Glandular portion of the proboscis;l.s, lateral sacs containing stylets;m.p, muscular portion of the proboscis;s, stylet;s.b, granular basal portion of stylet.
The principal British genera and species[142]are:—
Amphiporus bioculatusM‘Int.,A. dissimulansRiches,A. hastatusM‘Int.,A. lactifloreusM‘Int.,A. pulcherJohnst.Drepanophorus rubrostriatusHubr. (=A. spectabilisQtrf.).Tetrastemma ambiguumRiches,T. candidumO. F. Müll.,T. dorsaleAbildg.,T. flavidumEhrenb.,T. immutabileRiches,T. melanocephalumJohnst.,T. nigrumRiches,T. robertianaeM‘Int.,T. vermiculatumQtrf.Prosorhochmus claparediiKeferstein.Nemertes carcinophilaKöll.,N. gracilisJohnst.,N. neesiiOerst.Malacobdella grossaO. F. Müll.
Amphiporus bioculatusM‘Int.,A. dissimulansRiches,A. hastatusM‘Int.,A. lactifloreusM‘Int.,A. pulcherJohnst.
Drepanophorus rubrostriatusHubr. (=A. spectabilisQtrf.).
Tetrastemma ambiguumRiches,T. candidumO. F. Müll.,T. dorsaleAbildg.,T. flavidumEhrenb.,T. immutabileRiches,T. melanocephalumJohnst.,T. nigrumRiches,T. robertianaeM‘Int.,T. vermiculatumQtrf.
Prosorhochmus claparediiKeferstein.
Nemertes carcinophilaKöll.,N. gracilisJohnst.,N. neesiiOerst.
Malacobdella grossaO. F. Müll.
Order II. Schizonemertea.
The proboscis is unarmed. The epidermis is separated from the layer of connective tissue by a thin basement membrane, hence the glands in the connective tissue are more deeply situated and have long ducts. The muscular layers are three in number, an outer and an inner longitudinal layer between which lies a layer of circular muscles. The lateral nerve-cords lie between the outer longitudinal and the circular muscle layers. They are connected throughout the body by a nerve plexus, the only definite nerve branches given off being those to the brain, oesophagus, and proboscis. The cephalic slits are a pair of deep longitudinal grooves at the sides of the head. From each groove a canal passes inwards into a posterior brain-lobe. The mouth opens behind the brain, and is an elongated slit bounded by corrugated lips. Auditory organs have not been observed. The longitudinal vascular trunks are connected anteriorly by lacunar spaces, and not by closed vessels.
fig58Fig. 58.—Head end ofCerebratulus marginatusRen., from the ventral surface. Drawn from a spirit specimen. Naples. × 1.c.s, Cephalic slit;m, mouth;p.p, proboscis pore.
Fig. 58.—Head end ofCerebratulus marginatusRen., from the ventral surface. Drawn from a spirit specimen. Naples. × 1.c.s, Cephalic slit;m, mouth;p.p, proboscis pore.
Fig. 58.—Head end ofCerebratulus marginatusRen., from the ventral surface. Drawn from a spirit specimen. Naples. × 1.c.s, Cephalic slit;m, mouth;p.p, proboscis pore.
Principal British genera and species:—
Lineus bilineatusRen.,L. lacteusMont.,L. marinusMont. (=L. longissimusGunnerus),L. gesserensisO. F. Müll. (=L. obscurusDesor andL. sanguineusM‘Int.).Borlasia elizabethaeM‘Int.Cerebratulus angulatusO. F. Müll.,C. fuscusM‘Int.,C. pantherinusHubr.Micrura aurantiacaGrube,M. candidaBürger,M. fasciolataEhrenb.,M. purpureaJ. Müll.Meckelia asulcataM‘Int.
Lineus bilineatusRen.,L. lacteusMont.,L. marinusMont. (=L. longissimusGunnerus),L. gesserensisO. F. Müll. (=L. obscurusDesor andL. sanguineusM‘Int.).
Borlasia elizabethaeM‘Int.
Cerebratulus angulatusO. F. Müll.,C. fuscusM‘Int.,C. pantherinusHubr.
Micrura aurantiacaGrube,M. candidaBürger,M. fasciolataEhrenb.,M. purpureaJ. Müll.
Meckelia asulcataM‘Int.
Order III. Palaeonemertea.
The proboscis is unarmed. The epidermis and connective tissue form one layer, below which is the basement membrane. The muscular layers are three in number, two circular separated by a longitudinal layer. The nerve-cords lie altogether externalto the muscular layers, and are connected together throughout by a plexus. No nerve branches are given off. The brain is not divided into lobes. The cephalic slits are only represented by a shallow depression on each side of the head, and no canals have been observed leading from them. The intestine is straight, and the pouches are usually absent or rudimentary. The circulatory system is largely made up of lacunar spaces, the closed system being but little developed.
Principal British genera and species:—
Carinella annulataMont.,C. linearis(Mont., MS.) M‘Int.,C. macintoshiBürger (Fig. 59),C. polymorphaRen.Cephalothrix bioculataOerst.,C. linearisRathke.Valencinia lineformisM‘Int.
Carinella annulataMont.,C. linearis(Mont., MS.) M‘Int.,C. macintoshiBürger (Fig. 59),C. polymorphaRen.
Cephalothrix bioculataOerst.,C. linearisRathke.
Valencinia lineformisM‘Int.
fig59Fig. 59.—Carinella macintoshiBürger, drawn from the living specimen, slightly contracted. Plymouth. Considerably magnified.a, Anterior end;b, posterior end.
Fig. 59.—Carinella macintoshiBürger, drawn from the living specimen, slightly contracted. Plymouth. Considerably magnified.a, Anterior end;b, posterior end.
Fig. 59.—Carinella macintoshiBürger, drawn from the living specimen, slightly contracted. Plymouth. Considerably magnified.a, Anterior end;b, posterior end.
A most important monograph by Bürger[143]on Nemertines has just been published, but unfortunately it appeared too late to be adequately considered here. He gives an elaborate account, illustrated by admirable figures, of the present state of our knowledge of this group, and his work will be indispensable to future students of the subject. The older systems of classification are criticised, and the following scheme is adopted in their place:—
Order I. Protonemertini(= part of the Palaeonemertea, e.g.Carinella).—The brain and lateral nerve-cords lie outside the muscle layers in the epithelium or below the basement membrane. The body-wall consists of the following layers: epidermis, basement membrane, circular muscles, and longitudinal muscles. The mouth lies behind the brain. The proboscis is unarmed.
Order II. Mesonemertini(= part of the Palaeonemertea, e.g.Cephalothrix).—The characters of this Order are similar to those of the Protonemertini except that the brain and lateral nerve-cords lie in the muscle layers.
Order III. Metanemertini(= Hoplonemertea).—The brain and lateral nerve-cords lie in the parenchyma of the body internal to the muscle layers. The layers of the body-wall aresimilar to those of the Protonemertini. The mouth lies in front of the brain. The proboscis is armed. At the junction of the fore- and mid-gut a diverticulum is given off which projects forwards beneath the fore-gut and ends blindly in front.
Order IV. Heteronemertini(= Schizonemertea, and the generaEupoliaandValencinia, placed provisionally by Hubrecht in the Palaeonemertea).—The body-wall consists of the following layers: epidermis, thick cutis, and an outer and an inner longitudinal muscle layer separated from one another by a circular muscle layer. The brain and lateral nerve-cords lie between the outer longitudinal and the circular muscle layers. The mouth lies behind the brain. The proboscis is unarmed.
Development of the Nemertinea.—The development of the Palaeonemertea is at present not known: in the Schizonemertea a larval stage occurs; while in the Hoplonemertea the egg develops directly without undergoing any metamorphosis.
There are two forms of larva characteristic of the Schizonemertea, known respectively as Pilidium and the Type of Desor. The Pilidium is hatched early and leads a free-swimming existence, whereas the Type of Desor, though in many respects resembling it, never passes through the free-swimming phase.
fig60Fig. 60.—Diagram of a Pilidium larva. (After Salensky.)c, Tuft of cilia;m, muscle-fibres;mo, mouth, seen through one of the lateral lobes;n, nerve-fibres;n.r, nerve-ring;n.g, nerve ganglion;oes, oesophagus;st, stomach.
Fig. 60.—Diagram of a Pilidium larva. (After Salensky.)c, Tuft of cilia;m, muscle-fibres;mo, mouth, seen through one of the lateral lobes;n, nerve-fibres;n.r, nerve-ring;n.g, nerve ganglion;oes, oesophagus;st, stomach.
Fig. 60.—Diagram of a Pilidium larva. (After Salensky.)c, Tuft of cilia;m, muscle-fibres;mo, mouth, seen through one of the lateral lobes;n, nerve-fibres;n.r, nerve-ring;n.g, nerve ganglion;oes, oesophagus;st, stomach.
The Pilidium (Fig. 60) is a helmet-shaped larva bearing a tuft or spike dorsally, and prolonged downwards laterally into a pair of lobes. The whole larva is covered with cilia, there being a specially strong band round its ventral surface. The dorsal spike is composed of a bunch of strongly developed cilia or of a long flagellum. The alimentary canal consists of a sac constricted intooesophageal and gastric regions (Fig. 60,oesandst). In this condition the larva swims about freely in the water. The helmet-shaped Pilidium-skin forms no part of the future Nemertine, the skin of which is developed as ingrowths from it; these meet one another and unite to form a complete covering round the alimentary canal; the larval skin is then cast off, and by a series of gradual steps the embryo develops into the adult.
Habits.—Nemertines are often found under stones between high- and low-water marks, lying on sandy or muddy bottoms. They are usually in the form of coiled masses, and are generally in a state of quiescence. Hence it is probable that their period of activity is during high-water, and that when left by the receding tide they subside into a resting condition.
The large kinds, such asLineus marinus, seem to be always found living alone, but some of the smaller kinds, notablyTetrastemma dorsaleandProsorhochmus claparedii, have gregarious habits and live in masses, the coils of the different individuals being inextricably mixed.
Some species, such asMicrura purpurea,Amphiporus pulcher, andCerebratulus angulatus, frequent empty bivalve shells, while Nemertines are often found in empty limpet shells adhering to rocks in tidal pools. Other smaller forms resort to no such definite protection, but live among seaweeds; some of these remain naked, while others secrete for themselves tubes of a membranous or gelatinous consistency.Borlasia elizabethaelives in a burrow of clay.
Nemertines are commonly dredged from a depth of six or eight fathoms. They may sometimes be found floating on the surface of the water, and some possess the power of swimming rapidly, propelling themselves by a lateral motion of the tail, the sides of which are in such cases prolonged into a thin fin-like edge. This mode of progression is usually adopted by those which frequent deep water. A pelagic Nemertine (Pelagonemertes) was discovered by Moseley near the southern verge of the South Australian current, being found in a trawl with deep-sea forms from a depth of 1800 fathoms. This animal was leaf-like in shape, bluntly pointed behind and rather square in front.
The power possessed by Nemertines of secreting mucus is very great, their course being often traceable by the tracks which they leave behind them. Many of them glide along with great rapidity, a mode of progression which is probably due to thecilia covering the whole outer skin, and to the extreme contractility of the muscles of the body-wall. In some locomotion is effected by the proboscis; this is protruded and attaches itself by means of its spines to some foreign body, after which the body is drawn up after it. This has been specially observed in a land form,Tetrastemma agricola, discovered by Willemoes-Suhm in the Bermudas. On solid bodies the movement is a kind of crawling action, the head and mouth acting as suckers in much the same way as in many Leeches.
Most Nemertines can be very readily kept in confinement. The chief apparent effect of such a life is a loss of colour, the animal gradually becoming pallid in hue. Owing also to the absence of proper food they diminish very much in size, though even when all food is kept away an animal will sometimes continue to live as long as eighteen months.
Food.—Nemertines are carnivorous in their habits and are very voracious, devouring any prey which comes in their way, whether it be living or dead. No animal food seems to come amiss to them, and they will devour creatures of considerable size. When in contact with its prey, the Nemertine dilates its mouth to a large extent, and the anterior end of the oesophagus is thrust out and engulfs the animal. Chaetopods form a favourite food material, the whole animal being swallowed quite regardless of the hard chitinous bristles and spines with which it is beset. The soft parts are gradually digested, the bristles and other indigestible portions being extruded by the anus. The larger spines often pass out by perforating passages through the wall of the intestine and through the body-wall. The aperture thus formed appears speedily to heal after the foreign body has been extruded.
The carnivorous habits of Nemertines even extend to cannibalism, and when kept in confinement they frequently devour one another. For this reason it is unsafe to keep large and small kinds together, as the small ones speedily disappear, being used as food material by the large. If one be divided into several pieces, the pieces are very rapidly demolished by other individuals.
Regeneration.[144]—This power is, no doubt, of great service to these animals, since injury, or even violent local irritation, often causes complete rupture at the point affected. It seems that thechief power of regeneration is situated in the head, as, if a very short piece be broken off the anterior end of the body, it very rapidly reproduces itself into a new individual. The hind end of the original body often lives for a considerable time, but it does not in most cases appear to possess the power of reproducing a head, and after existing for a time it dies. For a while, however, it so far retains its vital powers that the generative products continue to grow, and actually attain to perfection. Severe wounds also heal very quickly and completely, and all local injuries are speedily repaired.
Owing to the force with which it is shot out, the proboscis is often completely severed from the body, and in such a case the animal grows a new one in an extremely short space of time. The proboscis thus broken off retains its power of movement and contractility for a considerable time, and has been more than once mistaken for a worm. This great vital power is probably due to the great development of nervous tissue, the proboscis being usually richly supplied with nerve plexuses.
One large form,Lineus sanguineus, seems to possess great recuperative powers. It shows a marked tendency to break up into pieces, when not only the head end, but also the other portions develop into perfect animals, each one growing a head and all the organs belonging to it. Thus in this case an animal may multiply by a simple process of transverse fission, and form numerous complete individuals.
Breeding.—The breeding season only appears to cease in the extreme of winter. Different genera and species seem to mature their generative products at different times.
In the armed Nemertines the eggs are deposited separately, and are not connected together except by such accidental mucus as the animal deposits normally; but in the unarmed a special mucous secretion forms a thick investment for the eggs.
M‘Intosh[145]has observed the process of the deposition of the male and female products inNemertes gracilis. He put into a glass vessel a male and female of this species in which the products were apparently ripe. Soon spermatozoa began to issue in wreath-like jets from the body of the male, at first from the middle region of the body, and afterwards anteriorly and posteriorly, until the animal was enveloped in a dense cloud ofspermatozoa. The whole process only lasted a few minutes. When all the spermatozoa had apparently been given out, the female was seen to protrude her head from the sand; she then passed to the side of the vessel and deposited a group of eggs about three inches distant from the spermatozoa.
With only a few exceptions Nemertines are oviparous.Prosorhochmus claparedii,Tetrastemma obscurum, andMonopora viviparahave been observed to contain embryos at certain times of the year. In other forms the eggs are laid when ripe, and development takes place subsequently to their deposition.
Geographical Distribution.—Nemertines have been found in all seas from the arctic to the equatorial regions. Many forms are found in the British Isles both between tide-marks and also at greater depths around our coasts. Some genera seem to be confined to warm climates and others to cold; while others appear to be indifferent to climate, and to subsist equally well under very various degrees of temperature. So far as is known, the land forms are all indigenous to warm countries.
Land Forms.—Land forms, which occur on or in moist earth under stones or decaying vegetable matter, have been discovered and described by Semper,[146]Willemoes-Suhm,[146]and von Graff.[146]
The species found by Semper, and called by himGeonemertes palaensis, lives under damp leaves and the roots of trees on Pelew Island in the North Pacific. It is about 2 inches long, of a reddish-white colour, with narrow, brownish-black, longitudinal stripes on its dorsal surface. It possesses six eyes and very small cephalic slits and cerebral organs. The proboscis is armed, and opens by the mouth instead of by a special pore.
The same peculiarity as to the opening of the proboscis is found inGeonemertes chalicophora, discovered by von Graff in pots ofCorypha australisin the palm-house at Frankfurt-on-Main. He found specimens on and beneath the surface of the earth. As it was only found in pots in which this Australian plant was growing, von Graff thought it almost certain that it was a native of Australia. Those found below the surface of the earth were surrounded by a transparent tube in which particles of earth were embedded. The animal is small, only about two-fifths of an inch in length. The colour is milk-white, with a small quantity of red pigment anteriorly: there are four eyes, and the cephalic slits are absent.