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73
1 have described them, here, as belonging to the nervous system. In these organs, I have found two types of structure, one
of them is represented in the structure of M. giganteum, the other in the structure oi M. graffi. In M. giganteum, they
consist of two ganglia containing a large number of cells, and are situated, one on each side of each parapodium. The cells are
multipolar, and they each send a prolongation towards the external extremity of the parapodium. A bundle of fibrils proceeds,
in this way, from each ganglia, into the parapodium — consequently one on each side — towards the extremity (Pl. VI,
fig. i). The nerve fibres unite, here, with the epithelium round the opening in which the chief hook is situated Pl. VI, fig.
2 c.). In M. graffi, however, the ganglia have quite another structure; in it, the two ganglia of each parapodium are united,
and therefore, becomes, a single ganglion only, containing but few large cells — 6—7 cells — but they are perfect giants. The
structure of these ganglia, in M. graffi, is illustrated in Pl. VI, fig. 3. In the inferior extremity, there are, also, a few smaller cells
(fig. 3, n., c.1; fig. 10, n., c.). The giant cells are, usually, multipolar, and most of their prolongations are directed towards the
parapodium The prolongations are distinctly fibrillous; I have not observed any histological difference between them. The
nuclei are large, and oval, with distinct nucleoli (fig. 3, b., fig. 4, b., fig. 11; Pl. VII, fig. 12, 13, 14, 15). A peculiar organ is
situated at the inferior extremity of the ganglion, having the form of a glass bulb-receiver with the long receiver-neck
passing towards the extremity of the parapodium. The globular, upper, extremity of this organ has the structure illustrated
in Pl. VII, fig. 1—7; it consists of several external concentric layers, and an inner, apparently coagulated, substance, vide fig.
2 and 7. This bulb communicates with bundles of fibres issuing from the cells (Pl. VI, fig. 3 and 12, Pl. VIII. fig. 1, 4, 6, 7).
The prolongation is hollow, and contains a substance resembling that of the bulb (Pl. VI, fig. 8, xu.). I have been unable to
determine how this prolongation terminates, or whether it is hollow, quite to its extremity, or not (Pl. VII, fig. 9, xu.). Any
nucleus belonging to this organ, is not, with certainty, observed. Pl. VII, fig. 3, a. illustrates a nucleus that can be seen situated
in the uppermost extremity; it is surrounded by a vacuolar protoplasmic substance, and I think that it belongs to the
organ. PI. \1I, fig. 5> illustrates a similar nucleus that may be seen, situated in a constrictive cavity communicating with
the bulb. I have, occassionally, found nuclei, situated in a similar manner. These nuclei belonged, probably, originally, to cells
from which these organs, by a process of transformation, have sprung, whilst the nuclei receded more and more, into
constrictive cavities (??) Each of these ganglia, in M. graffi, communicate with the ventral cord by 4 nerves, vide Pl. VI, fig. 5
n’, ns, n3 and n4. The nerves n’ and n2 are analogous to those found in M. giganteum. Nerves analogous to n3 and n4, I
have not observed, here, with certainty.
Histology of the nervous system.
The neurilemma-sheath of the nerves and the ventral cord, consists of an outer, and an inner sheath.
The outer sheath consists of a stout, homogeneous membrane, which is very prominent in transverse sections,
owing to its deep staining, and sharp outlines, of which the inner one is the sharpest. This membrane contains no
nuclei, but I have, however, found many such adherent to the exterior side. I think it must be regarded as a
cuticu-lum, derived from the layers of connective-tissue situated on its outer side, vide PI. V, fig. 12. The homogeneous
structure of this membrane appears to vary a little because I have found, in M. graffi, especially in its dorsal portion,
parts that stain different!}’, and some of them somewhat granular, vide Pl. V, fig. 10 and Pl. IX, fig. 2 and 3. Septa issue
from the homogeneous membrane, into the inner neurilemma sheath and inner parts of the ventral nerve-cord. The inner
neurilemm a-sheath fills the spaces between the ganglionic cells, the fibrillar cords, and the outer homogeneous membrane;
it is constructed of layers of connective-tissue which form a reticulation, and provide the fibrillar cords, as well as the
ganglionic cells with membranes. These cellular membranes are continued into, and surround the prolongations. The
connective-tissue, forms thus, sacks in which the cells are situated. Abundance of nuclei occur in the connective-tissue, situated, usually,
in the fibres which form the meshes, and not in the middle of the meshes themselves. Small septa pass into the
fibrillar mass proceeding from the membranes enclosing the longitudinal commissures. In this mass, connective-tissue
nuclei occur sparingly, and usually, only in conjunction with such small septa; the}’ can, especially, be observed where large
nerves are given off, vide Pl. V, fig. 16. The outer, as well as the inner, neurilemma-sheath is continued into the peripheric
nerves, which, also, in the same manner, are provided with double sheaths, as described by Hermann in Ilirudo. By means
of the septa of the inner neurilemma-sheath, the nerves are subdivided into a great many tubes; in these septa,
connective-tissue nuclei occur. The cerebral ganglia are, as above mentioned, unprovided with an}’ special neurilemma-sheath. The cells
have, however, connective-tissue membranes. The surrounding connective-tissue, also, shows a tendency to differentiation,
as it has, especially in certain species, a somewhat lamellar structure. In M. graffi, I have observed that the cells
surrounding the tentacular nerve-ring arc situated in vacuoli, and are thus, provided, with a double membrane, vide Pl. IV,
fig. 6, b. and a. I have not observed muscular fibres embedded in the neurilemma-sheath, but have, however, observed
muscles close to the exterior sides. These muscles are, I believe, secured to the outer sheath, by one of their extremities,
whilst the opposite extremity terminates in the surrounding connective-tissue.
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