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75
fibrils are, however intimately, interlaced with each other and between the longitudinal and transversal nervous-tubes, and have,
thus, a very complex course which, in transverse sections, gives rise to a spongious appearauce. The longitudinal tubes
are, principally, situated in the inner and dorsal part of the longitudinal commissures, where, also, they have a large diameter,
vide Pl. V, fig. 2, 12 and i6. Some tubes, also, pass along the outer side of the commissures where the nerves are given
off, vide Pl. V, fig. 5 and 6. Some of the longitudinal nervous-tubes pass into the Esophageal commissures (Pl. V, fig. 2 and
3) and bring, thus, I think, the oesophageal ring and the cerebral ganglia into communication with the various parts of the
ventral cord. The result of my observations, upon the histological structure of the nervous system of the Myzostomida, may
be summed up in the following laws, which probably, also, to a great extent, apply to the structure of the Annelids and
Arthropods. — 1) Each of the ganglionic cells, whether unipolar or multipolar, has, as a rule, only one true nervous
prolongation. Of these cells there are two types; one type, the prolongation of which passes, directly, into one of the
peripheric nerves and forms nervous tubes; another type, the prolongation of which loosens itself up into the fibrillar reticulation.
From the prolongations passing directly into nerves, fibrils are given off" to the fibrillar reticulation. — 2) Most of the
ganglionic cells of one side of the ventral cord send their prolongations into the peripheric nerves of the opposite side; some of
them send their prolongations to the fibrillar reticulation of the longitudinal commissure of the opposite side. — 3) Some
ganglionic cells send their prolongation, either, into peripheric nerves on the same side as they themselves are situated, or into the fibrillar
reticulation of the commissure on the same side as they themselves are situated. — 4) Each peripheric nerve receives its nervous
tubes, either, directly from the ganglionic cells, or, from the fibrillar reticulation of the longitudinal commissures. — 5) The
longitudinal commissures are composed of a’) longitudinal (and transversal) nervous tubes, of which, some of the longitudinal ones
pass into the oesophageal commissures; the purpose of the longitudinal tubes is. I think, to place the various parts of the
central nervous system in communication with each other; b) the fibrillar reticulation, consisting of slender, intimately
interlaced tubes or fibrils, issuing from the nervous tubes (the prolongations of the cells, the longitudinal tubes, the tubes of the
intermediate nerve) or, from cells whose prolongations loosen up into such fibrils. — 6) Each nerve receives nervous tubes
having communication with the longitudinal tubes, as well as with the fibrillar reticulation. — 7) Branchlets or fibrils are
given off from the nervous tubes to the mass of the commissures; this can be, most distinctly, seen in the tubes of the
intermediate nerves, from which branchlets are given off. alternately, to the commissures. I have, also, distinctly, observed
similar branchlets issuing from the prolongations of the cells.
The development of the nervous system. We know but very little about the development of the nervous
system. Beard describes a functional larval nervous system, which lie believes completely disappears in the later stages*
of larval life and has nothing to do with the formation of the adult nervous system , as he has failed to find any traces of
an osophageal ring in the adult. Judging from the preceding description of the osophageal ring, Beard’s description
cannot be accepted as correct; the median thickening of the epiblast of the apex of the praeoral lobe is, I believe,
the true oesophageal ring and cerebral ganglia, in their initial development. Subsequently comes, the development of the
ventral cord. We have, thus, in the Myzostomida, a development of the nervous system, quite similar to the type common to
the Annelids.
Abnormity in the structure and development of the nervous system. I have frequently observed this.
I have, especially in M. graffi — partly, also in M. gigas — met with specimens in which the ventral nerve-cord has a
remarkably small development, whilst the rest of the nervous system did not appear to be ver)’ abnormal. Pl. II, fig. 7
illustrates the transverse section of such a specimen; b.s. is the trans-sected ventral nerve-cord, posessing a flattened broad
form. The difference between this abnormity and a normal ventral cord, such as is illustrated in Pl. II. fig. 15, is
remarkable. There is but little structure visible in such degenerated cords. Assy m me try in the structure of the nervous system,
especially of the oesophageal ring and the cerebral ganglia, may be, frequently, observed in all species.
Sensory organs.
In the Myzostomidae hitherto known, only the cirri, and the tentacles of the proboscis, can be considered as
special sensor)’ organs. As already stated, these organs are furnished with nerves; from the tentacular nerve-ring, one nerve
issues to each tentacle; and from the complex nerve-ramification, a bundle of nerves passes into each cirrus. From these
nerves, in the cirri as well as, also, in the tentacles, fibrils issue, and connect with the epidermic cells, vide Pl. II, fig. 13 and
Pl. IV, fig. 5. I have been unable to observe the furrow, described by Graff, on the-inferior side of the cirri of M. cirriferum.
I have examined many transverse, as well as longitudinal, sections of cirri, of M. cirriferum, as also of the other species, but
have always found the cirri to be cylindrical, nor could I find any epidermic cells differing from the others, or having any
resemblance, to »the protrusile glutinous cells ( Klebzellen ;) described by Graft.
10*
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