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ANATOMY

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Originally appearing in Volume V19, Page 366 of the 1911 Encyclopedia Britannica.
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ANATOMY.—Proboscis and Proboscidian Sheaih.—The organ most characteristic of a Nemertine is without doubt the proboscis. With very few exceptions (Malacobdella, Akrostomum, where it. has fused A. B below; B, from above. m, Mouth. Br, Brain-lobes. div, Intestinal diverticula. In, Longitudinal nerve stems. pr, Proboscis. ps, Proboscidian sheath. p.o., Opening for proboscis. with the mouth into a single exterior opening), there is a terminal opening, the rhynchostome (subterminal in Valencinia), at the fore-most tip of the body, out of which the proboscis is seen shooting backwards and forwards, sometimes with so much force that both its interior attachments are severed and it is entirely expelled from the body. It then often retains its vitality for a long time, apparently crawling as if it. were itself a worm, a phenomenon which Is at least partially explained by the extraordinary development of nervous tissue, equally distributed all through the walls of the proboscis, and either united into numerous longitudinal nerve-stems (Drepanophorus, Amphiporus) or spread out into a uniform and comparatively thick layer (Cerebratulus, sp.). This very effective and elaborate innervation, which has been directly traced to the brain, whence strong nerves (generally two) enter the proboscis, renders it exceedingly probable that the most important functions of the proboscis are of a sensiferous, tactile nature. In Nemertines the everted proboscis is retracted in the same way as the tip of a glove finger would be if it were pulled backwards by a thread situated in the axis and attached to the tip. The comparison may be carried still further. The central thread just alluded to is represented in the Nemertean proboscis by that portion which is never everted, and the tip of the glove by the boundary between the evertible and non-evertible portion of the proboscis—a boundary which in the Metanemertini is marked by the presence of a pointed or serrated stylet. This stylet is thus situated terminally when the proboscis has reached its maximum eversion. It adds a decisively aggressive character to an organ the original significance of which, as we have seen, was tactile. This aggressive character has a different aspect in several genera which are destitute of a central stylet, but in which the surface that is turned outwards upon eversion of the proboscis is largely pro- P. vided with nematocysts, sending the urticating rods of different sizes in all directions. In others this surface is beset with thick, glandular, adhesive papillae. The comparison with the glove-finger is in so far insufficient as the greater portion of the non-avertible half of the proboscis is also hollow and clothed by glandular walls. Only at the very hinder-most end does it pass into the so-called retractor-muscle (fig. 2), which is attached to the wall of the space, or rhynchocoel, in. which the proboscis moves about. This retractor-muscle, indeed, serves to pull back with great rapidity the extruded proboscis, and, is aided in its action by the musculature of the head. The extrusion itself depends en- FIG. 3.—Anterior portion tirely upon contraction of the muscular of the body of a Nemerwalls of the space just mentioned, the tine. rhynchocoel. As it is (I) closed on all Br, Brain-lobes. sides, and (2) filled with a corpuscular N, Lateral nerves. fluid, the contractions alluded to send PS, Proboscidian sheath. this fluid to impinge against the anterior Pr, Proboscis. portion, where the proboscis, floating in P.o., Exterior opening its sheath, is attached with it to the through which the muscular tissue of the head (fig. 3). proboscis is everted Partial extrusion lessening the resist- or rhynchostome. ance in this region inevitably follows, Oesophagus and and when further contractions of the mouth shown by walls of the sheath ensue total ex- dotted lines. trusion is the consequence. It is worthy of notice that in those Nemertines which make a very free use of their proboscis, and in which it is seen to be continually protruded and retracted, the walls of the proboscidian sheath are enormously muscular. On the other hand, they are much less considerably or even insignificantly so in the genera that are known to make a rather sparing use of their proboscis. The rhynchocoel is formed by a split which appears in the mesoblast surrounding the epiblastic pit which is the fore-runner of the proboscis. It does not seem to be coelomic. The proboscis, which is thus an eminently muscular organ, is composed of two or three, sometimes powerful, layers of muscles—one of longitudinal and one or two of circular fibres. In the posterior re-tractor the longitudinal fibres become united into one bundle, which, as noticed above, is inserted in the wall of the FIG. 4. FIG. 5. sheath. At the circular inser- Fins. 4, 5.—Proboscis with stylet, tion of the proboscis in front " reserve " sacs and muscular bulb of the brain the muscular fibres of a Hoplonemertine. Fig. 4 re-belonging to the anterior ex- tracted; fig. 5 everted. tremity of the body and those connected with the proboscis are very intimately interwoven, forming a strong attachment. The short tube between this circular insertion and the rhynchostome is called the rhynchodaeum. The proboscis broken off and expelled is generally reproduced, the posterior ribbon-like end of this reproduced portion again fusing with the walls of the sheath. There is reason to suppose that, when a wound is inflicted by the central stylet, it is envenomed by the fluid secreted in the posterior proboscidian region being at the same time expelled. A reservoir, a duct and a muscular bulb in the region (fig. 4) where the stylet is attached serve for this purpose. The significance of two or more (in Drepanophorus very numerous) small sacs containing so-called " reserve " stylets resembling in shape that of the central dart is insufficiently known. The muscular walls of the rhynchocoel, which by their transverse contractions serve to bring about eversion of the proboscis in the way above traced, are attached to the musculature of the head just in front of the ganglionic commissures (fig. 3). In nearly all Nemertines the rhynchocoel extends backwards as far as the posterior extremity, just above the anus; in Carinella it is limited to the anterior body-region. The corpuscles floating in the fluid it contains are of definite a, Anus. ov, Ovaries. n, Nephridia. shape, and in Cerebralulus urticans they are deep red, possibly from the presence of haemoglobin. They are usually larger than the blood corpuscles. Internally the muscular layers are lined by an epithelium. In the posterior portion this epithelium in certain Heteronemertea has a more glandular appearance, and sometimes the interior cavity is obliterated by cell-proliferation in this region. Superiorly the sheath either closely adheres to the muscular body-wall, with which it may even be partly interwoven, or it hangs freely in the connective tissue which fills the space between the intestine and the muscular body-wall. Cutaneous System.—Externally in all species a layer of ciliated cells forms the outer investment. In it are, moreover, enclosed unicellular glands pouring their highly refracting contents, of a more or less rod-like shape, directly to the exterior. They appear to be the principal source of the mucus these animals secrete. In most Heteronemertines these elements are separated by a thin homogeneous basement membrane (fig. 8) from the following—that is, from a layer in which longitudinal muscular fibres are largely inter-mixed with tortuous glands, which by reason of their deeper situation communicate with the exterior by a much longer and generally very narrow duct. The pigment is also principally localized in this layer, although sometimes it is present even deeper down within the musculature. The passage from this tegumentary layer to the subjacent longitudinal muscular one is gradual, no membrane separating them. In Carinella, Cephalothrix, Polia and the Metanemertines the two tegumentary layers with their different glandular elements are fused into one; a thick layer of connective tissue is situated beneath them (instead of between them) and keeps the entire cutaneous system more definitely separate from the muscular (figs. 7, 8). Musculature and Connective Tissue.—The muscular layers by which the body-wall is constituted have been very differently and to some sxtent confusingly described by the successive authors on Nemertean anatomy. There is sufficient reason for this confusion. The fact is that not only have the larger subdivisions a different arrangement and even number of the muscular layers, but even within the same genus, nay, in the same species, well-marked differences occur. e Bm long long Fins. 7-9.—The layers of the body-wall in Carinella (fig. 7), the Metanemertini (fig. 8) and the Heteronemerlini (fig. 9). c, Cellular tissue of the integument; Bm, basement membrane; tire. I, outer circular, and long., longitudinal layer of muscular tissue; circ. 2, long. I, additional circular and longitudinal layers of the same; nl, nervous layer. Increase in size appears sometimes to be accompanied by the development of a new layer of fibres, whereas a difference in the method of preparation may give to a layer which appeared homogeneous in one specimen a decidedly fibrous aspect in another. Nevertheless there are three principal types under which the different modifications can be arranged. One of them is found in the two most primitively organized genera, Carinella and Cephalothrix, i.e. an outer circular, a longitudinal and an inner circular layer of muscular fibres (fig. 7). The second is common to all the Heteronemertines, as well as to Polia and Valencinia, and also comprehends three layers, of which, how-ever, two are longitudinal, viz. the external and the internal one, there being a strong circular layer between them (fig. 9). To the third type all the Metanemertini correspond; their muscular layers are only two, an external circular and an internal longitudinal one (fig. 8). The Heteronemerlini thus appear to have developed an extra layer of longitudinal fibres internally to those which they inherited from more primitive ancestors, whereas the Metanemertini are no longer in possession of the internal circular layer, but have on the contrary largely developed the external circular one, which has dwindled away in the Heteronemerlini. The situation of the lateral nerve-stems in the different genera with respect to the muscular layers lends definite support to the interpretation of their homologies here given and forms the basis of Burger's classification. In Carinella, Cephalothrix and Polia, as well as in all Metanemertines, the basement membrane of the skin already alluded to is particularly strong and immediately applied upon the muscular layers. In the Heteronemertines there is a layer in which the cutaneous elements are largely represented below the thin basement membrane (fig. 8), between it and the bulk of the outer longitudinal muscles. The difference in the appearance of the basement membrane sometimes wholly homogeneous, sometimes eminently fibrillar—can more especially be observed in differently preserved specimens of the genus Polia. The connective tissue of the integument and basement membrane imperceptibly merges into that which surrounds the muscular bundles as they are united into denser and definite layers, and this is especially marked in those forms (Akroslomum) where the density of the muscular body-wall has considerably diminished, and the connective tissue has thus become much more prominent. It can then at the same time be observed, too, that the compact mass of connective tissue (" reticulum," Barrois) which lies between the muscular body-wall and the intestine is directly continuous with that in which the muscular layers are embedded. Nuclei are everywhere present. The omnipresence of this connective tissue tends to exclude the formation of any perivisceral body cavity in Nemertines. In Polia the connective tissue enclosed in the external muscular layer is eminently vacuolar—all the intermediate stages between such cells in which the vacuole predominates and the nucleus is peripheral and those in which the granular protoplasm still entirely fills them being moreover present. In addition to the musculature of the proboscis and proboscidian sheath, longitudinal muscular fibres are found in the walls of the oesophagus, whilst transverse ones are numerous and united into vertical dissepiments between the successive intestinal caeca, thus bringing about a very regular internal metamerization. The genital products develop in intermediate spaces similarly limited by these dissepiments and alternating with the digestive caeca. Nervous System and Sense Organs.—The nervous system of Nemertines presents several interesting peculiarities. As central organs we have to note the brain-lobes and the longitudinal lateral cords which form one continuous unsegmented mass of fibrous and cellular eo nerve-tissue. The fibrous nerve-tissue is more dense in the higher differentiated, more loose and spongy in the lower organized Al- P•L forms; the cellular nerve-tissue is FIG. to FIG. II similarly less compact in the forms FIGS. to, II —Brain and that are at the base of the scale. lateral organ of a Schizonemer-No ganglionic swellings whatever tine (fig. to) and a Hoplooccur in the course of the longi- nemertine (fig. II). eo, Exterior tudinal cords. The brain must be opening; u.l, superior brain-looked upon as the anterior thick- lobe; p.l., posterior brain-lobe. eni ng of these cords, and at the same time as the spot where the two halves of the central nerve system intercommunicate. This is brought about by a double commissure, of which the ventral portion is considerably thicker than the dorsal, and which, together with the brain-lobes, constitutes a ring through which both proboscis and proboscidian sheath pass. The brain-lobes are generally four in number, a ventral and a dorsal pair, respectively united together by the above-mentioned commissures, and moreover anteriorly interfusing with each other, right and left. In Carinella this separation into lobes of the anterior thickenings of the cords has not yet commenced, the ventral commissure at the same time being extremely bulky. There is great probability that the central stems, together with the brain, must be looked upon as local longitudinal accumulations of ner- vous tissue in what was in more primitive ancestors a less highly differentiated nervous plexus, situated in the body-wall in a similar way to that which still is found in the less highly organizedCoelen- PN terates. Such a nervous plexus indeed occurs in the body-wall of all Heteronemertines, sometimes even as a comparatively thick layer, situated, as are the nerve stems, between the external longitudinal and the circular muscles (fig. 9). In Carinella, where the longitudinal nerve-stems are situated exteriorly to the muscular layers, this plexus, although present, is much less dense, and can more fitly be compared to a network with wide meshes. In both cases it can be shown to be in immediate continuity with the coating of nerve-cells forming part of the longitudinal cords. It stretches forward as far as the brain, and in Carinella is again continued in front of it, whereas in the Heteronemertines the innervation of the anterior extremity of the head, in front of the brain, takes the form of more definite and less numerous branching stems. The presence of this plexus in connexion with the central stems, sending out nervous filaments amongst the muscles, explains the absence, in Pro-, Meso- and Heteronemertines, of separate and distinct peripheral nerve stems springing from the central stems innervating the different organs and body-regions, the only exceptions being the I eo L.N. L.N. 12.—The brain of a Nemertine, with its lobes and commissures. S.N., Nerves to sensory apparatus. P.N., Nerves for proboscis. vag, Nerves for oesophagus. L.N., Lateral nerve-stems. nerves for the proboscis, those for the sense organs in the head and the natural supply of freshly oxygenated sea-water is practically the strong nerve pair (n. vagus) for the oesophagus. At the same time it renders more intelligible the extreme sensitiveness of the body-wall of the Nemertines, a local and instantaneous irritation often resulting in spasmodic rupture of the animal at the point touched. In the Metanemertini, where the longitudinal stems lie inside the muscular body-wall, definite and metamerically placed nerve branches spring from them and divide dichotomously in the different tissues they innervate. A definite plexus can here no longer be traced. In certain Metanemertines the lateral stems have been noticed to unite posteriorly by a terminal commissure, situated above the anus, the whole of the central nervous system being in this way virtually situated above the intestine. In others there is an approximation of the lateral stems towards the median ventral line (Drepanophorus) ; in a genus of Heteronemertines (Langia), on the other hand, an arrangement occurs by which the longitudinal stems are no longer lateral, but have more or less approached each other dorsally. In addition to the nerves starting from the brain-lobes just now especially mentioned, there is a double apparatus which can hardly be treated of in conjunction with the sense organs, because its sensory functions have not been sufficiently made out, and which will therefore rather be considered along with the brain and central nervous system. This apparatus is usually known under the name of the lateral organs. To it belong (a) superficial grooves or deeper slits situated on the integument near the tip of the head, (b) nerve lobes in immediate connexion with the nervous tissue of the brain, and (c) ciliated ducts penetrating into the latter and communicating with the.former. Embryology shows that originally these different parts are separately started, and only ultimately become united into one. Two lateral outgrowths of the foremost portion of the oesophagus, afterwards becoming constricted off, as well as two ingrowths from the epiblast, contribute towards its formation, at least as far as both Meta- and Heteronemertines are concerned. As to the Mesonemertini, in the most primitive genus, Carinella, we do not find any lateral organs answering to the description above given. What we do find is a slight transverse furrow on each side of the head, close to the tip, but the most careful examination of sections made through the tissues of the head and brain shows the absence of any further apparatus comparable to that described above. Only in one species, Carinella inexpectata, a step in advance has been made, in so far as in connexion with the furrow just mentioned, which is here also somewhat more complicated in its arrangement, a ciliated tube leads into the brain, there to end blindly amidst the nerve-cells. No other intermediate stages have as yet been noticed between this arrangement and that of the Heteronemertini, in which a separate posterior brain-lobe receives a similar ciliated canal, and in which the oesophageal outgrowths have made their appearance and are coalesced with the nerve-tissue in the organ of the adult animal. The histological elements of this portion remain distinct both by transmitted light and in actual sections. These posterior brain-lobes, which in all Heteronemertines are in direct continuity of tissue with the upper pair of principal lobes, cease to have this intimate connexion In the Metanemertini; and, although still constituted of (1) a ciliated duct, opening out externally, (2) nervous tissue surrounding it, and (3) histological elements distinctly different from the nervous, and most probably directly derived from the oesophageal outgrowths, they are nevertheless here no longer constantly situated behind the upper brain-lobes and directly connected with them, but are found sometimes behind, sometimes beside and sometimes before the brain-lobes. Further-more, they are here severed from the principal lobes and connected with them by one or more rather thick strings of nerve-fibres. In some cases, especially when the lobes lie before the brain, their distance from it, as well as the length of these nervous connexions, has considerably increased. These curious neuro-glandular pits (fig. I), absent in the Mesonemertine and one or two aberrant species, have been shown to possess large glandular cells at their base which secrete a mucus. The development of these organs, which in the Protonemertine are but grooves in the epidermis, not far removed from the similar cephalic slits of many Turbellaria, reaches its height in Drepanophorus. Here the pits split into two, one part ending in a sac lined with sensory epithelium, and embedded in nervous tissue, the other projecting backwards as a long, glandular, blind canal. The exit of these organs takes many shapes, of value in systematic work. Their function is still little understood. Two lateral, shallow pits occur on the side of the body about the level of the hinder end of the proboscis in some species of the genus Carinella, which are termed side-organs. These are capable of being everted, and are probably sensory in function (fig. 20, 17). For the Heteronemertines arguments have been adduced to prove that here they have the physiological significance of a special respiratory apparatus for the central nervous tissue, which in all these forms is strongly charged with haemoglobin. The haemoglobin would by .its pre-eminent properties of fixing oxygen, serve to furnish the nerve system, which more than any other requires a constant supply, with the necessary oxygen. Such could hardly be obtained in any other way by those worms that have no special } respiratory apparatus, and that live in mud and under stones where limited. Whether in the Metanemertines, where the blood fluid Is often provided with haemoglobiniferous disks, the chief functions of the side organs may not rather be a sensory one needs further investigation. The exterior opening of the duct has been several times alluded to. In the Metanemertines it is generally situated towards the middle of a lateral transverse groove on either side of the head, as was noticed for Carinella, and as is also present in Polia. Generally a row of shorter grooves perpendicular to the first, and similarly provided with strong cilia, enlarges the surface of these furrows (fig. 14). In Valencinia there is nothing but a circular opening without furrow. In all Heteronemertines there is on each side of the head a longitudinal slit of varying length but generally considerable depth, in the bottom of which the dark red brain is very plainly visible by transparency. These slits are continued into the ciliated duct, being at the same time themselves very strongly ciliated. In life they are commonly rhythmically opened and shut by a wavy movement. They are the head slits (cephalic fissures, " Kopfspalten ") so characteristic of this subdivision (figs. so and 13). With respect to the sense organs of the Nemertines, we find that eyes are of rather constant occurrence, although many Heteronemertines living in the mud appear to be blind. The more highly organized species have often very numerous eyes (Amphiporus, Drepanophorus), which are provided with a spherical refracting anterior portion, with a cellular " vitreous body," with a layer of delicate radially arranged rods, with an outer sheath of dark pigment, and with a separate nerve-twig each, springing from a common or double pair of branches which leave the brain as n. optici, for the innervation of the eyes. Besides these more highly differentiated organs of vision, more primitive eyes are present in others down to simple stellate pigment specks without any refracting apparatus. Organs of hearing in the form of capsules containing otoliths have only been very rarely observed, apparently only in Metanemertini. As to the organ of touch, the great sensitiveness of the body has already been noticed, as well as the probable primary significance of the proboscis. Small tufts of tactile hairs or papillae are sometimes observed in small number at the tip of the head; sometimes longer hairs, apparently rather stiff, are seen on the surface, very sparingly distributed between the cilia, and hitherto only in a very limited number of small specimens. They may perhaps be considered as sensory. Digestive System.—The anterior opening, the mouth, is situated ventrally, close to the tip of the head and in front of the brain in the Metanemertini, somewhat more backward and behind the brain in the other Nemertines. In most Heteronemertines it is found to be an elongated slit with corrugated borders; in the Metanemertines it is smaller and rounded; in Malacobdella and Akrostomum it, moreover, serves for the extrusion of the proboscis, which emerges by a separate dorsal opening just inside the mouth. The oesophagus is the anterior portion of the digestive canal; its walls are folded longitudinally, comparatively thick and provided with longitudinal muscular fibres. Two layers are specially obvious in its walls—the inner layer bordering the lumen being composed of smaller ciliated cells, the outer thicker one containing numerous granular cells and having a more glandular character. Outside the wall of the oesophagus a vascular space has been detected which is in direct continuity with the longitudinal blood-vessels. In certain cases, how-ever, the walls of the oesophagus appear to be very closely applied to the muscular body-wall and this vascular space thereby considerably reduced. The posterior portion of the intestine is specially characterized by the appearance of the intestinal diverticula horizontally and symmetrically placed right and left and opposite to each other. In the Metanemertini there is a curious diverticulum of the intestine which stretches forward in the median line, ventral to the so-called stomach. It is at times sacculated, but its chief interest is that, as Lebedinsky 1 has shown, the tip of the caecum in embryonic life opens to the exterior as the blastopore. This subsequently closes up, and the newly-formed oesophagus and stomach open in the intestine above and behind it. It is a curious feature in Nemertines that the alimentary canal seldom contains traces of food and yet most of these worms are voracious. The food must be digested, absorbed and excreted with great rapidity. There is some evidence that in this group the ectoderm of the oesophagus is chiefly concerned with digestion, whereas the endoderm of the intestine is limited to the absorption of the soluble products. Cases of asymmetry or irregularity in the arrangement of the intestinal caeca, though sometimes occurring, are not normal. At the tip of the tail, where the growth of the animal takes place, the
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