86 



NEMERTINES 



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 

 Coelenterates. Such a nervous plexus indeed occurs in the body- 

 wall of all Schizouemertines (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 Gari- 

 nella, 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 fig. 12.— The brain of a Nemertine, with its lobes and 

 again continued in commissures. S.N.^ nei*ves to sensory apparatus; 

 front of it, whereas '':"■' nci-ves for proboscis; uasr, nerves tor ceso- 



.-,_ CI T." phagus; L.iv., lateral nen'e stems. 



m the Sohizonemer- 



tines the innervation 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 

 with the central stems, sending out nervous filaments amongst 

 the muscles, explains the absence, both in Palaeo- and Schizo- 

 uemertines, 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 Hoplmiemertea, 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 Hoplonemertiues 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 (DrepaTiophoriis) ; in a genus of Schizouemertines 

 (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 dorsaUy (6). 



In addition to the nerves starting from the brain-lobes just now 

 especially mentioned, there is a donble 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, (6) nerve 

 lobes in immediate connexion with the nervous ti.ssue of the brain, 

 and (e) ciliated ducts penetrating into the latter and communicating 

 with the former. Embryology shows that originally these difierent 

 parts are separately started, and only ultimately become united 

 into one. Two lateral outgrowths of the.foremost portion of the 

 cesophagus, afterwards becoming constricted off, as well as two 

 ingrowths from the epiblast, contribute towards its formation, at 

 least as far as both Hoplo- and Schizouemertines are concerned. 

 As to the Palaioiiemertea, their embryology has not yet been studied, 

 and 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 inexpedata, 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 io 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 Schizonemertea, in which 

 a separate posterior brain-lobe receives a similar ciliated canal, and 

 in which the cBSophageal outgrowths have made their appearance 

 and are coalesced with the nerve-tissue in the organ of the adult 



fj\ 



animal. The histological elements of this portion remain distinct 

 both by transmitted light and in actual sections. 



These posterior brain-lobes, which in all Schizouemertines are in 

 direct continuity of tissue with the uj)per pair of principal lobes, 

 cease to have this intimate connexion in the Eoplonemertea ; 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 oesophageal 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 stiU insufSciently 

 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 

 oesophageal outgrowths in Balanoglossus and Amphioxus, and for 

 the Schizouemertines 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 aU 

 these forms is strongly charged with haemoglobin. The haemoglobin 

 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- 

 nemertiues, where the blood fluid is often provided with haemo- 

 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 Hoplonemertines it is generally situated towards the 

 middle of a lateral transvbrse groove on either side of the head, as 

 was noticed for Carinella, and as is also 

 present in Folia. 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 Valeneinia there is nothing but 

 a circular opening without furrow. In all 

 Schizouemertines there is on each side of 

 the head a longitudinal slit of varying -p. -^^ 

 length but generally considerable depth, 

 in the bottom of which the dark red brain ^'p|- H'.i*' 

 is very plainly visible by transparency. 

 These slits are continued into the ciliated 

 duet, 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 Schizouemer- 

 tines living in the mud appear to be blind. The more highly 

 organized species have often very numerous eyes {AmpAiporus, 

 JDrepanophorus), 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 

 Hoplmiemertea. 



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 papfllse 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 ffoplonemertea, somewhat more backward and behind 

 the brain in the other Nemertines. In most Schizouemertines it is 

 found to be an elongated slit with corrugated borders ; in the 



4ii!fy!p, 



Fig. 14. 



■Lateral views 

 of bead of a Schizone- 

 raertine (fig. 13) with 

 longitudinal slit, and of 

 a Hoplonemertine (fig. 14) 

 with transverse gi'oove 

 and furrows. 



