_T2l 



posteriori}' alons:; tlu' walls of the canal, ln'twri'ii tlu' t'pitlu-liiim ami thr musck's of tlu' iiniscnlar luill). At llu' liack of the 

 muscular hull), tlu-y a^aiii rcliii(|uish llu- cH-sopha<:;cal walls, doubliii<:( rouiul the bulb, towards tlu- ocscjphagcal ring (PL IV, 

 licr. i6, 17 ni. . .1 ha\c been unabk- to determine whether the\- are connected with this collar or not. From these nerves, 

 numerous bi'anches in the oesophat^t'al walls issue antl, fre(|LR'ntly, anastomoze. hi the oesophageal walls, nerve-cells occur, 

 freiiuently several tooetlu'r, more particularh' close to one of the nerves iPl. ]\', hg. 13 & 14 n.c). Tlu- e])itlu]ium of this 

 canal has a remarkable structure (PI. I\', lig. 8". The cells are much elongated; at their inferior e.\treinit_\- the}- have a slen- 

 der prolongation by means of which they are united to nerve-fibres; the nuclei are long, slender and, usualh-, thickest in the 

 external extremity. 



The foregoing descripti(_)n reters, specially, to the structure of M. gig.mteuni but it also applies in essentials, to 

 the structure of most of the other species. In some of them however, especiall_\- that of M. graffi, the structure varies 

 somewhat. In this species, the oesophageal ring is much less developed, and is surrounded b\- onh- a few cells; a little apart, 

 at the posterior extremity of the muscular Itulb there is, however, a ring of cells surrounding the oesophagus. Nerve-cells 

 occur, extremely sparingl}-, between the oesophageal and tentacul.ir rings. But, on the other hand, the tentacular ring is 

 largely surrounded b}- numerous cells (PI. W . fig. 6, 7). It appears, indeed, as if the tentacular nerve-ring is developed at 

 the expense of the oesophageal ring. I am inclined to consider that this ring, and probably also the entire proboscideal 

 nerve-s}-stem, is originally derived from the oesophageal ring. 



The ventral ner\'e-cord has an oblong form, generall}", and is short and concentrated (PI. I, fig. 8, 

 PI. \\ fig. 2); in M. graffi, however, it is more elongated (PI. \', fig. 3); it is provided with a double neurilemma 

 sheath, and is constructed of ganglionic cells and fibrillous cords; these cords are two longitudinal commissures with 

 an intermediate nerve passing between them. The longitudinal commissures communicate, at their anterior and posterior 

 extremities, by transverse commissures of nearh' similar thickness as the)' themselves posess. Between these commis- 

 sures, other transverse commissures are situated; these are thick and thin alternateh', but none of them however, are, by a 

 long way, so thick as the first mentioned (PI. V, fig. 2 t.cm antl cm). The intermediate nerve communicates with the 

 commissures bj- means of alternating branches. Between, and on the outside of the commissures, the ganglionic cells are 

 situated in several distinct groups. PI. \'. fig. 9, partially illustrates these groups. The ganglionic cells vary \'er}' much in 

 size; some of them are large, whilst others of them are (]uite small, In M. giganteum, a pair of large cells is especiall}' 

 prominent, situated in regular order between each of the thick transverse commissures (PI. V, fig. 2). The prolongations of 

 these cells form a chiasma (PI. V, fig. 11 & 14), passing thus to the opposite side. The prolongations of most of the cells 

 pass through the transverse commissures into the ner\'es of the opposite side, some of the cells. liowe\'cr, send their prolong- 

 ations directly into the nerves of the same side (PI. V, fig. 16 b). Most of them are unipolar. I have not, distinctly, ob- 

 served multipolar cells. Exclusive of the oesophageal commissures, there are 1 1 pairs of nerves issuing form the ventral cord. 

 There are 5 pairs of large lateral nerves iPI. \^ fig. 2 n.st' — n.st''') and (> pairs of small ones; the first pair, of these last 

 mentioned, have their origin lietween the oesophageal commissures and the first pair of large nerves, and have a forward 

 direction; the next succeeding 4 pairs originate, one between each of the large nerves; and the ulterior pair have their origin 

 between the ulterior-posterior pair of large nerves, and are directed backwards, vide PI. I, fig. 8 n' — n'', PI. \\ fig. 3, n' — n*'. 

 In M. graffi the nerves of this 6th pair are united close to their origin. The segmentation of the ventral cord is not so 

 easily observed as Beard describes, ani_l he is scarcely correct when Ik- states that the ganglionic cells are confined ito the 

 places where, especially large nerves are given oft':. I would prefer to say that the cells are, principall}', situated in the 

 spaces between the large nerves, and send their prolongations into the thick trans\'erse commissures which occur here. I 

 find, however, that the segmentation may he traced, especially, in the regular placement of the nerves; these are given oft" 

 at uniform intervals, alternate!}', one thin and one thick. 



The peripheric nerves. Regarding the distribution of the nerves, I refer m}' readers to PI. I, fig. 8 and PI. II, fig. lO, 

 11,12,13. PI. I, fig. 8 illustrates the distribution of the entire nervous system. In PI. II, fig. 10, we see the distribution of the 

 first of the large nerves on the left-hand side; the exterior Ijranches send a large number of branchlets to the margin of the 

 bod\-, and the}- there connect with the epidermic cells. A stouter branch proceeds towartls each cirrus and forms here a com- 

 plex ramification (fig. 10, d) sending a bunch of nerves into the cirrus (fig. 10 and fig. 13). The ner\'e g.n, and the posterior of 

 the two corresponding nerves on the opposite and posterior side, run to the parapodial ganglia ami connect them with the vent- 

 ral cord. The nerve d (absent in fig. 10) provides the muscles surrounding the hook-apparatus with nerve fibres. The distri- 

 bution IS seen illustrated in fig. II (n', n^, n^). The smaller nerves (situated between the large ones) anastomoze, usuall}', with 

 the large nerves in their front (fig. 10, n). The distribution of one of these small nerves is shown in fig. 12. 



The parapodial ganglia.. I have been in great doubt with respect to the real nature of these organs. Several 

 things in their structure (e.g in M. graffi) seemed to indicate a glandulous nature, whilst other things indicated a nervous nature.') 



') If glands, they must be analogous to — perhaps homologous with — the glaiuls in the feet of some Crustaceans vide Dr K. Bl.anc. 

 Contr. a I'hist. nat. des Asselotes hetérépodes. Ree. zool. Suisse, T. I. 1884. 



