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Some other organs, which have also been compared with the segmental organs, are the antennal glands of some
Crustaceans, and the shell-glands of Copepoda and Phyllopoda’) These also agree in having no open communication with the
body-cavity. Before coming to a decision, on this question of the phylogenetic origin of these segmental glands in Myzostomida,
it would be of utmost interest to have elucidated, whether it, really, is the case, as Graff states, that suckers« are not
present in many species, or whether, at all events, traces of these glands do not exist 111 all species.
Hook Apparatus.
The hook-apparati of the Myzostomidae have, I think, an ectodermal origin. They consist of: a) the hooks, (chief
hook, supporting rod, and reserve hooks) with their muscular system, and b) the glandular mass surrounding the hooks. The
hooks are not, as Graff states in his monograph,2) hollow, but consist of two layers; an outer, somewhat homogeneous, layer,
and an inner one composed of a fibrous substance. The outer layer has a yellow colour and is very slightly staining; it is
very thin at the base of the hooks, but increases, gradually, in thickness towards the extremity, and the acuminate extremities
of the hooks become formed, thus, almost exclusively, by this layer, and acquire, also, a more intense yellow colour, especially
in the more developed hooks. The inner fibrous mass consists of colourless fibres, which are thickest in the centre of the
hook and, in transverse sections, exhibit a distinct hexagonal form, vide Pl. VII, fig. 19. Towards the outer layer these
hexagonal fibres become so minute that they are with difficult)’ distinguishable. This fibrous mass is, usually, vividly stained
by colouring reagents. The structure of the supporting rod is similar td that of the hooks, and the manubrial plate is composed
of the homogeneous yellow substance of the outer layer. I have, frequently, observed at the upper extremity of the supporting
rods, large unipolar cells, with their prolongations directed towards the extremity of the rod (Pl. VII, fig. 22, a, b.), but I
have been unable to determine in what manner these prolongations terminate, or what is their physiological function.
The glandular mass surrounding the hooks has, usually, a considerable volume, and this is especially the case
in M. giganganteum; in it, it is divided into two parts, one surrounding the hooks, and the other extending towards the centre
of the bod)r, and in this there are vacuoli communicating with the canal in which the chief hook is situated. The glandular
mass is composed of cells having a variable form and appearance. Most of them are elongated, and have a fibrillous
protoplasm (Pl. VII, fig, 28, a.); these are, I think, the true glandulous cells. Some cells have a more globular form, also furnished
with prolongations (Pl. VII, fig. 28, c.) I have observed in the canal encompassing the chief hook, in some species, large globular
cells, situated in a layer surrounding a layer of smaller elongated cells, vide Pl. VI, fig. 6, c. and h.k. In the glandular
mass surrounding the canal of the chief hook. I have, frequently, observed, especially in M. graffi, deposits, having the form
of small elongated refracting corpuscles. The cavities of the glandular mass communicate with the external sea-water by
means of the canal of the chief hook.
Alimentary Canal.
The alimentary canal may be considered as consisting of 3 parts viz. 1 The proboscisdeai canal with
oesophagus. 2) The stomach. 3) The intestinal branches issuing from the stomach. 4) The rectum and cloacal canal. At the
posterior extremity of the proboscis there is, usually, a very short oesophagus, divided from the stomach by an annular
constriction; but in some species, however, this oesophagus is relatively longer, as Graft, also, states, and this is the case in M.
carpenteri, vide Pl. IX, fig. 18. The epithelium of the oesophagus forms a transition between the epithelium of the posterior
part of the proboscideal canal and that of the stomach; it is not ciliated; large cells are situated in the epithelium as well as
in the subjacent connective-tissue.
The stomach is divided by a muscular annular constriction into two parts; an anterior portion — the true
stomach; and a posterior portion — the intestinal division — from which the lateral branches issue (Pl. IX, fig. 17—20). In
M. carpenteri, the anterior portion is divided into two parts by another, not so prominent, annular fold. The epithelium,
in both portions of the stomach, is cylindrical and ciliated, and the cilia are especially long in the anterior part. No
such rule, as is laid down by Beard (loc. cit. p. 550), regarding the length of the cells can be established. In all parts of
the stomach the cells vary, very much, both in length and in form. In the connective-tissue walls of the stomach, annular
(retractores) as well as radial (dilatatores) muscles occur (Pl. IX, fig. 22). There are, usually, three intestinal branches, but in
M. giganteum, and M. gigas, there are only two. The epithelium of the branches is, generally, cylindrical, but not ciliated.
’) Grobben: Die Antennendrüse der Crustaceen. Arb. Zool. Inst. Wien. Bd. III. 1880. See also with reference to this. — Hock: Report
on the Cirripedia. Zool. Chall. Exp. Part XXVIII, 1884, p. 26.
2) In his Report on the Challenger-Myzostomida (Zool. Chall. Exp. Part. XXVII. 1884’. Graff says however: "It appears also that the column
of the hook and manubrium are not as I formerly thought, hollow, but in many species, at least, solid.«
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