NEMERTINES 



87 



CT 



Hoplonemertines it is smaller and rounded ; in Malacobdella and 

 Akroslomum. 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 lu- 

 men being composed of 

 smaller ciliated cells, 

 the outer thicker one 

 containing numerous 

 granular cells and hav- 

 ing a more glandular 

 character. Outside the 

 wall of the oesophagus a 

 vascular space has been 

 detected (11) which is 

 in direct continuity 

 with the longitudinal 

 blood-vessels. In cer- 

 tain cases, however, the 

 walls of the ossophagus 

 appear to be very closely 

 applied to the muscular 

 body-wall, and this vas- 

 cular space thereby con- 

 siderably reduced. 



The posterior portion 

 of the intestine is speci- 

 ally characterized by the 

 appearance of the intes- 

 tinal diverticula hori- 

 zontally and symmetric- 

 ally placed right and 

 left and opposite to each 

 other. Sometimes this re- 

 gion, into which the oeso- 

 phagus leads, stretches 

 forwards under the 

 oesophagus (Hoplone- 

 mertines) for a certain 

 distance, anteriorly ter- 

 minating by a cul-de- 

 sac. Cases of asym- 

 metry or irregularity in 

 the arrangement of the 

 caeca, though sometimes 

 occurring, are not nor- 

 mal. At the tip of the 



CT 



IM 



*' A 



LBu 



Fig. 17. 



tail, where the growth FIGS. 15-17. Diagrammatic sections to show dis- 

 of the animal takes position of internal organs in Cariiulla (Palxo- 

 nlapp HIP PIPPA arp al aemerlea), fig. 15, Sehizmemertea, fig. 16, and 

 ice, tne caeca are al- H( , plmeT ^. rUa ; flg . 17. c , cellular portion of 

 integument; B, basement membrane; A, circu- 

 lar muscular layer; A', longitudinal do.; A", 

 second circular (in Carinella) ; A'", second longi- 

 tudinal (in Schizonemertea) ; If, nerrous layer ; 

 i.V, lateral nerves ; PS, cavity of proboscidian 

 sheath (the sheath itself of varying thickness); 

 P, proboscis ; /, intestine ; LBc, lateral blood- 

 vessel ; DBc, dorsal do. ; CT, connective I issue. 



ways eminently regular. 

 So they are throughout 

 the whole body in most 

 of the Hoplonemertines. 

 In Carinella, they are 

 generally deficient and 

 the intestine straight ; 

 in young specimens of this species, however, they occur, though less 

 regular and more in the form of incipient foldings by which the 

 digestive surface is increased. The inner surface of the intestinal 

 cseca is ciliated, the caeca themselves are sometimes especially in 

 the hindermost portion of the body of a considerably smaller lumen 

 than the intermediate genital spaces ; sometimes, however, the 

 reverse is the case, and in both cases it is the smaller lumen that 

 appears enclosed between and suspended by the transverse fibres 

 constituting the muscular dissepiments above mentioned. 



The anus is situated terminally, the muscular body-wall through 

 which the intestine must find its way outwards probably acting in 

 this region the part of a sphincter. The lateral nerve stems mostly 

 terminate on both sides in closest proximity to the anus ; in certain 

 species, however, they interfuse by a transverse connexion above 

 the anus. The longitudinal blood-vessels do the same. The 

 question has been raised whether the regular intestinal caeca of 

 Nemertines might not be compared with those intestinal diverticula 

 of the embryo Amphioxus which ultimately become the mesoblastic 

 somites of the adult (8). This view would be a further extension 

 of the views concerning the coelom first propounded by Huxley. 



(/) Circulatory Apparatus. This consists of three longitudinal 

 trunks, a median and two lateral ones. They are in direct con- 

 nexion with each other both at the posterior and at the anterior 

 end of the body. At the posterior end they communicate together 

 by a T-shaped connexion in a simple and uniform way. Anteriorly 

 there is a certain amount of difference in the arrangement. Whereas 

 in the Hoplonemertines an arrangement prevails as represented in 



fig. 18, the lateral stems in the Schizonemertines, while entirely 



uniform all through the posterior portion of the body, no longer 



individually exist in the oesophageal region, but 



here dissolve themselves into a network of vascu- 



lar spaces surrounding this portion of the di- 



gestive tract (11). The median dorsal vessel, 



however, remains distinct, but instead of con- 



tinuing its course beneath the proboscidian 



sheath it is first enclosed by the ventral muscu- 



lature of this organ, and still farther forwards 



it even bulges out longitudinally into the cavity 



of the sheath. Anteriorly it finally communi- 



cates with the lacunae just mentioned, which 



paratus in the ant 

 rior body-region of 

 a Hoplonemertine. 



, 



lobes of the brain, pass through the nerve nng 

 together with the proboscidian sheath, and are 

 generally continued in front of the brain as a 

 lacunar space in the muscular tissue, one on each side. 



Special mention must be made of the delicate transverse vessels 

 regularly connecting the longitudinal and the lateral ones. They 

 are metamerically placed, and belong to the same metamer as the 

 digestive cceca, thus alternating with the generative sacs. The 

 blood fluid does not flow in any definite direction ; its movements 

 are largely influenced by those of the muscular body-wall. It is 

 colourless, and contains definite corpuscles, which are round or 

 elliptical, and in many Hoplonemertines are coloured red by haemo- 

 globin, being colourless in other species. The circulatory system 

 of Carinella is considerably different, being more lacnnar and less 

 restricted to definite vascular channels. Two lateral longitudinal 

 lacunae form, so to say, the forerunners of the lateral vessels. A 

 median longitudinal vessel and transverse connecting trunks have 

 not as yet been detected. There are large lacunae in the head in 

 front of the ganglia. 



(g) Nephridia. Although these organs were already very well 

 known to Max Schultze (14), their presence in Nemertines was 

 repeatedly and seriously disputed until Von Kennel (10) definitely 

 proved their existence and gave details concerning their histology. 

 With the exception of a few genera where they have not as yet been 

 discovered (Carinella), one pair of nephridia appears to be very 

 generally present. They essentially consist of a complex coiled 

 tube, one on each side of the oesophagus (fig. 1), communicating 

 with the exterior by a duct piercing the body-wall. The two 

 openings of the nephridia are situated sometimes more towards the 

 ventral, at other times more towards the dorsal side. Even in the 

 larger Schizonemertines these pores are only a few millimetres 

 behind the mouth region. Internal funnel-shaped openings, 

 although sought for, have as yet not been detected. The coiled 

 tubes extend both forwards and backwards of the external opening, 

 by far the greater portion being situated backwards. The anterior 

 coils reach forwards till in the immediate vicinity of the posterior 

 brain-lobes. The coils are tubiform, with an internal lumen, only 

 one layer of rather large cells constituting the walls. These cells 

 are ciliated ; in some transparent species the internal ciliary move- 

 ment can be observed during life. In transverse sections the 

 nephridia can be shown to be generally situated in the region 

 limited by (1) the proboscidian sheath, (2) the upper wall of the 

 intestine, (3) the muscular body-wall. No trace of nephridia is 

 found posterior to the oesophagus. 



(h) Generative System. In the Nemertines the sexes are separate, 

 with only very few exceptions (12) (Tctrastemma hermaphroditica, 

 Marion). The generative products are contained in separate 

 pouches placed metamerically in the way noticed above in treating 

 of the digestive system. They are conveyed outwards along narrow 

 canals, one pair for each metamer piercing the muscular body-wall, 

 and visible on the outside in mature individuals as minute light- 

 coloured specks. The ova and spermatozoa, when mature, present 

 no peculiarities. As the ova are in many species deposited in a 

 gelatinous tube secreted by the body-walls, in which they are 

 arranged (three or more together) in flask-shaped cavities, impreg- 

 nation must probably take place either before or at the very moment 

 of their being deposited. The exact mode has not yet been noticed. 

 Another point not yet sufficiently settled is the oogenesis in 

 Nemertines. In several cases the ova appear to originate directly 

 as the lining of the generative pouches, but the exact part which 

 the mesoblastic connective tissue plays, both with regard to these 

 pouches and to the generative products themselves, remains yet to 

 be settled. 



Prosyrhochmus claparedii is a viviparous form. 



DEVELOPMENT. The embryology of the Nemertines offers Develop. 

 some very remarkable peculiarities. Our knowledge of ment 

 the development of the most primitive forms is very scanty. 

 Of that of Carinella absolutely nothing is known. On 

 Cephalothrix we have observations, in certain respects con- 

 tradictory. Both Schizo- and Hoplonemertea, have been 

 more exhaustively studied, the first, as was noticed above, 



