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posteriorly along the walls of the canal, between the epithelium and the muscles of the muscular bulb. At the back of the
muscular bulb, they again relinquish the oesophageal walls, doubling round the bulb, towards the oesophageal ring (Pl. IV,
fig. 16, 17 m.). » I have been unable to determine whether they are connected with this collar or not. From these nerves,
numerous branches in the oesophageal walls issue and, frequently, anastomoze. In the oesophageal walls, nerve-cells occur,
frequently several together, more particularly close to one of the nerves Pl. IV, fig. 13 & 14 n.c). The epithelium of this
canal has a remarkable structure (Pl. IV, fig. 8). The cells are much elongated; at their inferior extremity they have a
slender prolongation by means of which they are united to nerve-fibres; the nuclei are long, slender and, usually, thickest in the
external extremity.
The foregoing description refers, specially, to the structure of M. giganteum but it also applies in essentials, to
the structure of most of the other species. In some of them however, especially that of M. graffi, the structure varies
somewhat. In this species, the oesophageal ring is much less developed, and is surrounded by only a few cells; a little apart,
at the posterior extremity of the muscular bulb there is, however, a ring of cells surrounding the oesophagus. Nerve-cells
occur, extremely sparingly, between the oesophageal and tentacular rings. But, on the other hand, the tentacular ring is
largely surrounded by numerous cells (Pl. IV. 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-system, is originally derived from the oesophageal ring.
The ventral nerve-cord has an oblong form, generally, and is short and concentrated (Pl. I, fig. 8,
Pl. V, fig. 2); in M. graffi, however, it is more elongated (Pl. V, fig. 3); it is provided with a double neurilemma
sheath, and is constructed of ganglionic cells and fibrinous 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 nearly similar thickness as they themselves posess. Between these
commissures, other transverse commissures are situated; these are thick and thin alternately, but none of them however, are, by a
long way, so thick as the first mentioned (Pl. V, fig. 2 t.cm and c.m). The intermediate nerve communicates with the
commissures by means of alternating branches. Between, and on the outside of the commissures, the ganglionic cells are
situated in several distinct groups. Pl. V, fig. 9, partially illustrates these groups. The ganglionic cells vary very much in
size; some of them are large, whilst others of them are quite small. In M. giganteum, a pair of large cells is especially
prominent, situated in regular order between each of the thick transverse commissures (Pl. V, fig. 2). The prolongations of
these cells form a chiasma (Pl. V, fig. 11 & 14), passing thus to the opposite side. The prolongations of most of the cells
pass through the transverse commissures into the nerves of the opposite side, some of the cells, however, send their
prolongations directly into the nerves of the same side ;P1. V, fig. 16 b). Most of them are unipolar. I have not, distinctly,
observed multipolar cells. Exclusive of the oesophageal commissures, there are 11 pairs of nerves issuing form the ventral cord.
There are 5 pairs of large lateral nerves (Pl. V, fig. 2 n.st’ — n.st5) and 6 pairs of small ones; the first pair, of these last
mentioned, have their origin between 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 backward«, vide PI. I, lig. 8 n’—n6, Pl. V, fig. 3, n’—n6.
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, and he is scarcely correct when he states that the ganglionic cells are confined to the
places where, especially large nerves are given oft’ . I would prefer to say that the cells are, principally, situated in the
spaces between the large nerves, and send their prolongations into the thick transverse commissures which occur here. I
find, however, that the segmentation may he traced, especially, in the regular placement of the nerves; these are given off
at uniform intervals, alternately, one thin and one thick.
The peripheric nerves. Regarding the distribution of the nerves, I refer my readers to Pl. I. fig. 8 and Pl. II, fig. 10,
II, 12, 13. Pl. I, fig. 8 illustrates the distribution of the entire nervous system. In Pl. II, fig. 10, we see the distribution of the
first of the large nerves on the left-hand side; the exterior branches send a large number of branchlets to the margin of the
body, and they there connect with the epidermic cells. A stouter branch proceeds towards each cirrus and forms here a
complex ramification (fig. 10, d) sending a bunch of nerves into the cirrus (fig. 10 and fig. 13). The nerve g.n, and the posterior of
the two corresponding nerves 011 the opposite and posterior side, run to the parapodial ganglia and connect them with the
ventral cord. The nerve d (absent in fig. 10) provides the muscles surrounding the hook-apparatus with nerve fibres. The
distribution is seen illustrated in fig. 11 (n1, n2, n3). The smaller nerves (situated between the large ones) anastomoze, usually, 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.1)
]) If glands, they must be analogous to — perhaps homologous with — the glands in the feet of some Crustaceans vide Dr K. Blanc.
Contr. a l’hist. nat. des Asselotes hetérépodes. Ree. zool. Suisse, T. I. 1884.
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