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AND MORPHOLOGY]
143
HYDROMEDUSAE


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.

EB1911 Hydromedusae - Statocyst and Ocellus of Tiaropsis diademata.jpg

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.
EB1911 Hydromedusae - Statocyst of Mitrocoma annae.jpg

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.

EB1911 Hydromedusae - Statocyst of Phialidium.jpg

Modified after O. and R. Hertwig, Nervensystem und Sinnesorgane der Medusen, by permission of F. C. W. Vogel.

Fig. 32.—Section of a Statocyst of Phialidium.

ex, Ex-umbral ectoderm.
sub, Sub-umbral ectoderm.
v, Velum.
st.c Cavity of statocyst.
con, Concrement-cell with otolith.
EB1911 Hydromedusae - Statocyst of Octorchis.jpg

Modified after O. and R. Hertwig, Nervensystem und Sinnesorgane der Medusen, by permission of F. C. W. Vogel.

Fig. 33.—Optical Section of a Statocyst of Octorchis.

con, Concrement-cell with otolith.
st.c Cavity of statocyst.
EB1911 Hydromedusae - Tentaculocyst of Cunina solmaris.jpg

After O. and R. Hertwig, Nervensystem und Sinnesorgane der Medusen, by permission of F. C. W. Vogel.

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).

EB1911 Hydromedusae - Tentaculocyst of Cunina lativentris.jpg

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).

EB1911 Hydromedusae - Simple tentaculocyst of Rhopalonema velatum.jpg

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