bearing at its free upper end a stiff bristle and running out at its base
into a nerve-fibre; (3) concrement-cells, which produce intercellular
concretions, so-called otoliths.
By means of
vibrations or shocks
transmitted through the
water, or by displacements
in the balance or
position of the animal,
the otoliths are caused
to impinge against the
bristles of the sensory
cells, now on one side,
now on the other, causing
shocks or stimuli which
are transmitted by the
basal nerve-fibre to the
central nervous system.
Two stages in the development
of the otocyst
can be recognized, the
first that of an open pit
on a freely-projecting
knob, in which the otoliths
are exposed, the
second that of a closed
vesicle, in which the otoliths
are covered over.
Further, two distinct
types of otocyst can be
recognized in the Hydromedusae:
that of the Leptolinae, in which the entire organ is
ectodermal, concrement-cells and all, and the organ is not a tentaculocyst;
and that of the Trachylinae, in which the organ is a
tentaculocyst, and the concrement-cells
are endodermal,
derived from the endoderm
of the modified tentacle, while
the rest of the organ is ectodermal.
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Modified after Linko, Traveaux Soc. Imp. Nat., St. Petersbourg, xxix. | |
|
Fig. 30.—Section of a Statocyst and Ocellus of Tiaropsis diademata; cf. fig. 29. | |
| ex, | Ex-umbral ectoderm. |
| sub, | Sub-umbral ectoderm. |
| c.c, | Circular canal. |
| v, | Velum. |
| st.c, | Cavity of statocyst. |
| con, | Concrement-cell with otolith. |
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|
Modified after O. and R. Hertwig, Nervensystem und Sinnesorgane der Medusen, by permission of F. C. W. Vogel. | |
|
Fig. 31.—Section of a Statocyst of Mitrocoma annae. | |
| sub, | Sub-umbral ectoderm. |
| c.c, | Circular canal. |
| v, | Velum. |
| st.c, | Cavity of statocyst. |
| con, | Concrement-cell with otolith. |
In the Leptolinae the otocysts are seen in their first stage in Mitrocoma annae (fig. 31) and Tiaropsis (figs. 29, 30) as an open pit at the base of the velum, on its sub-umbral side. The pit has its opening turned towards the sub-umbral cavity, while its base or fundus forms a bulge, more or less pronounced, on the ex-umbral side of the velum. At the fundus are placed the concrement-cells with their conspicuous otoliths (con) and the inconspicuous auditory cells, which are connected with. the sub-umbral nerve-ring. From the open condition arises the closed condition very simply by closing up of the aperture of the pit. We then find the typical otocyst of the Leptomedusae, a vesicle bulging on the ex-umbral side of the velum (figs. 32, 33). The otocysts are placed on the outer wall of the vesicle (the fundus of the original pit) or on its sides; their arrangement and number vary greatly and furnish useful characters for distinguishing genera. The sense-cells are innervated, as before, from the sub-umbral nerve-ring. The inner wall of the vesicle (region of closure) is frequently thickened to form a so-called “sense-cushion,” apparently a ganglionic offshoot from the sub-umbral nerve-ring. In many Leptomedusae the otocysts are very small, inconspicuous and embedded completely in the tissues; hence they may be easily overlooked in badly-preserved material, and perhaps are present in many cases where they have been said to have been wanting.
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After O. and R. Hertwig, Nervensystem und Sinnesorgane der Medusen, by permission of F. C. W. Vogel. | |
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Fig. 34.—Tentaculocyst (statorhabd) of Cunina solmaris. n.c, Nerve-cushion; end, endodermal concrement-cells; con, otolith. |
In the Trachylinae the simplest condition of the otocyst is a freely projecting club, a so-called statorhabd (figs. 34, 35), representing a tentacle greatly reduced in size, covered with sensory ectodermal epithelium (ect.), and containing an endodermal core (end.), which is at first continuous with the endoderm of the ring-canal, but later becomes separated from it. In the endoderm large concretions are formed (con.). Other sensory cells with long cilia cover a sort of cushion (n.c.) at the base of the club; the club may be long and the cushion small, or the cushion large and the club small. The whole structure is innervated, like the tentacles, from the ex-umbral nerve-ring. An advance towards the second stage is seen in such a form as Rhopalonema (fig. 36), where the ectoderm of the cushion rises up in a double fold to enclose the club in a protective covering forming a cup or vesicle, at first open distally; finally the opening closes and the closed vesicle may sink inwards and be found far removed from the surface, as in Geryonia (fig. 37).
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|
After O. and R. Hertwig, Nervensystem und Sinnesorgane der Medusen, by permission of F. C. W. Vogel. | |
|
Fig. 35.—Tentaculocyst of Cunina lativentris. | |
| ect, | Ectoderm. |
| n.c, | Nerve-cushion. |
| end, | Endodermal concrement-cells. |
| con, | Otolith. |
The ocelli are seen in their simplest form as a pigmented patch of ectoderm, which consists of two kinds of cells—(1) pigment-cells, which are ordinary indifferent cells of the epithelium containing pigment-granules, and (2) visual cells, slender sensory epithelial cells of the usual type, which may develop visual cones or rods at their free extremity. The ocelli occur usually either on the inner or outer sides of the tentacles; if on the inner side, the tentacle is turned upwards and carried over the ex-umbrella, so as to expose the ocellus to the light; if the ocellus be on the outer side of a tentacle, two nerves run round the base of the tentacle to it. In other cases ocelli may occur between tentacles, as in Tiaropsis (fig. 29).
Fig. 36.—Simple tentaculocyst of Rhopalonema velatum. The process carrying the otolith or concretion hk, formed by endoderm cells, is enclosed by an upgrowth forming the “vesicle,” which is not yet quite closed in at the top. (After Hertwig.) |
The simple form of ocellus described in the foregoing paragraph may become folded into a pit or cup, the interior of which becomes filled with a clear gelatinous secretion forming a sort of vitreous
