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several species. On the ventral side they, usually, are somewhat more developed than on the dorsal side, whilst in some
places, c. g. the dorsal side of the body-margin, they almost disappear. J have sometimes found the protoplasm of these
cells to be striated, as described by Beard, but that the cili;e are directly connected with the nucleus« by means of these
threads is, I think, very problematical indeed. Inside of the epidermic cells, especially on the ventral side of M. giganteum,
there are, frequently, larger cells seen containing large nuclei and nucleoli. From these cells, a prolongation may, frequently,
be seen issuing towards the epidermic cells; whether these prolongations are connected with the epidermic cells, consequently
nerve-cells, or if all of them penetrate through to the surface, I have been unable to determine. Some ot them are, in any
case, gland-cells, because I have distinctly detected excretory openings in them. Several such large cells are found situated
among the epidermic cells. In the margin of the body, I have found these cells to be in correspondence with nerve-fibres.
I have observed nothing resembling the sense-organs, or chitinous hollow rods, described by Beard. 3) The cutis varies in
thickness; it is especially thick upon the dorsum, in most species e. g. M. glabrum, M. carpenteri. In M. giganteum it is,
on the contrary, more developed on the ventrum. I have, also, observed here, in the cutis, unipolar nerve-cells whose
prolongations are directed towards the ventral muscular layer. 4) The ectodermal muscular layer is situated beneath the cutis.
Its development varies considerably in the different species, and it appears as if it, usually, was more developed in the stouter
less migratory species. I have not observed such a regular arrangement of concentric muscular rings and radial fibres as is
described by Graff in M. cerriferum.
Nervous System.
This is, in the Myzostoma, quite in accordance with the usual type, common to Annelids and Arthropods. It
consists of an oesophageal ring with cerebral ganglia, (connected with this ring, there is a proboscideal — stomato-gastric
-nerve-system) and a short ventral nerve-cord. It, altogether, appears to be greatly differentiated, and the central parts have
in a great degree gagne la profondeur«. A thick muscular layer separates the ventral cord from the ectoderm.
The oesophageal ring’) with the cerebral ganglia, is, relatively, only slightly developed; the ganglia have no
special sheath but lie entrenched in the connective tissue. The ring-commissures are, on the contrary, provided with a double
neurilem ma-sheath, and are connected with the ventral cord at the anterior extremity, between the first pair of peripheric
nerves (Pl. I, fig. 9, O.cm.). In their inferior part, they are rather slender, and are not connected with ganglionic cells; these
parts are very long. In the superior part, they are, on the contrary, usually, surrounded by groups of ganglionic cells whilst
there are, also, two small ganglia within the neurilemma-sheath one on each side, which contain a few cells (Pl. Ill, fig. 6, a.,
8, Pl. I, fig. 9, a.). The surrounding cells, usually, occur, as well, in front of, as to the back of, the oesophageal ring.
They are arranged in different groups, vide Pl. Ill, fig. 5—7 which show 3 sections out of a series taken from this
part. In the posterior part of the proboscis, cells are found placed nearer towards the ventral side, where still further back
they unite so as to form a cellular ring round the proboscis independently of the oesophageal commissures (Pl. I, fig. 9, s. c.
Pl. III. fig. 7). From the commissures, a pair of nerves issue (Pl. I, fig. 9, n", Pl. III, fig, 6, n.) towards the ventral side, their
purpose being, I think, to bring the ventral cells into correspondence with the commissures. I have found multipolar cells
in the posterior parts of the cellular ring (Pl. Ill, fig. 10), which laterally and dorsally is more developed and
voluminous; this part must be considered, therefore, to be analogous to the usual cerebral ganglia of the Annelids, and the
ventral parts to be originally sprung from the dorsal cerebral part. The prolongations of the multipolar cells in the posterior
cerebral parts of the cellular ring are directed towards the oesophageal and gastric epithelium. I am, therefore, disposed
to consider this portion of the cellular ring as sympathetic. Several nerves issue towards the gastric epithelium Pl. IV. fig.
19, 11., fig. 23, n.. n.’). One pair of nerves passes posteriorly along the stomach (PL I, fig. 9, n.s; Pl. IV, fig, i& 16, n.s).
I consider these nerves to be nervi sympathici. I have not examined their further course, as I have not been able to
determine how they are connected with the oesophageal collar.
The proboscideal nerve system is well developed. Towards the anterior extremity of the proboscis, 3 pairs of
nerves issue from the oesophageal ring (PL I, fig. 9, c.m.’, c.m.", c.m.’"). These nerves connect the oesophageal ring
with the tentacular nerve-ring situated in the proboscis, in front of the muscular bulb. From a series of sections, it can be
seen that there are many nerve-cells situated between the oesophageal ring and the tentacular ring — vide Pl. III, fig. 3 -6.
The cells, as they approach this ring, become considerably reduced in number, and round about the ring there are, usually,
extremely few — if any at all — to be found. These cells communicate, by means of minute nerve-branches, with the 3 pairs
of longitudinal nerves around which they are especially grouped. The tentacular nerve-ring is provided with a
neurilemma-sheath, and inside of this I found no ganglionic cells. From this ring, nerves issue, one to each of the tentacles, also a great
many to the anterior part of the proboscideal canal; and further, four stouter oesophageal nerves issue from it, passing
M The description of a ring given by Graff is not correct, such a ring does not really exist, as may be gathered from the above remarks.
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