Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/343

N E M E K T I N E 8 329 must be looked upon as local longitudinal accumulations of nervous tissue in what was in more primitive ancestors a less highly dif ferentiated nervous plexus, situated in the body-wall in a similar way to that which still is found in the less highly organized Crelenterates. Such a nervous plexus indeed occurs in the body- wall of all Schizonemertines (7), sometimes even as a compara tively thick layer, situated, as are the nerve stems, between the external longitudinal and the circular muscles (fig. 9). In Cari nclla, 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 c:ises 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 Carinclla is FIG. 12. The brain of a Xemertine, with its lobes and a fr ain continued in commissures. f>.y., nerves to sensory apparatus; front of it, whereas p : N -< nel 7 c * fov , Pboscis ; tag, nerves for CESO- ,, r, , . phagus; L. A., lateral nerve stems, in the Schizonemer tines the inner vation of the anterior extremity of the head, in front of the brain, takes the form of more definite and less numer ous branching stems. The presence of this plexus in connexion vith the central stems, sending out nervous filaments amongst the muscles, explains the absence, both in Palreo- and Schizo nemertines, of separate and distinct peripheral nerve stems spring ing from the central stems innervating the different organs and body-regions, the only exceptions being the nerves for the pro boscis, those for the sense organs in the head, and 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 Hoplonemcrtea, 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 Hoplonemertines 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 (Drcpanophorus} ; in a genus of Schizonemertines (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 (6). 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 (esophagus, afterwards becoming constricted off, as well as two ingrowths from the epiblast, contribute towards its formation, at least as far as both Hoplo- and Schizonemertines are concerned. As to the Pal&onemcrtca, their embryology has not yet been studied, and in the most primitive genus, Carinclla, 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, Carinclla inexpcctata, 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 Schizoncmertca, in which a separate posterior brain-lobe receives a similar ciliated canal, and in which the cesophageal 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 Schizonemertines are in direct continuity of tissue with the upper pair of principal lobes, cease to have this intimate connexion in the Hoplonemertca ; and, although still constituted of (1) a ciliated duct, opening out exter nally, (2) nervous tissue surrounding it, and (3) histological ele ments distinctly different from the nervous, and most probably directly derived from the cesophageal outgrowths, they are never theless 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. Furthermore, 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 con nexions, has considerably increased. With the significance of these parts we are still insufficiently acquainted. There appear to be analogous organs amongst Platyelminthes, but a careful comparative study is wanted. A partial comparison has been hazarded (8) with the anterior cesophageal outgrowths in Balanocjlossus and Antyliioxus, and for the Schizonemertines arguments have been adduced (6) 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 hemoglobin. The hemoglobin would, by its pre-eminent properties of fixing oxygen, serve to fur nish 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 or delicately ramifying blood-vessels, and that live in mud and under stones, where the natural supply of freshly oxygenated sea-water is practically limited. Whether in the Hoplo nemertines, where the blood fluid is often provided with hoemo- globiniferous disks, the chief functions of the side organs may not rather be a sensory one must be further investigated. The exterior opening of the duct has been several times alluded to. In the Hoplouemertines 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 Valcncinia there is nothing but a circular opening without furrow. In all Schizonemertines there is on each side of the head a longitudinal slit of varying -p. -,0 p; -, * length but generally considerable depth, in the bottom of which the dark red brain FlGS - 13 > 14. Lateral views 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. 10 and 13). With respect to the sense organs of the Nemertines, we find that eyes are of rather constant occurrence, although many Schizonemer tines living in the mud appear to be blind. The more highly organized species have often very numerous eyes (Ampkiporus, Drcpanophorus), 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 Hoplonemcrtea. 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 papilla? are some times observed in small number at the tip of the head (11) ; some times 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. (e) 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 Hoplonemertca, somewhat more backward and behind the brain in the other Nemertines. In most Schizonemertines it is found to be an elongated slit with corrugated borders ; in the XVII. 42 mertine (fig. 13) with longitudinal slit, and of a Ho[jlonemertine (fig. 14) with transverse groove and furrows.