and others of sight. Some species possess both, but, as a general rule, among Medusa, where organs of hearing are present, those of sight are wanting, and vice versa. It may seem extraordinary that there should be such differences of opinion as to these organs. The earlier naturalists, however, had but imperfect microscopes, and probably often examined specimens in a bad state of preparation. As regards the alternative between the view that they served as eyes and that which regarded them as ears, it must, moreover, be remembered that as long as we merely know that there was a capsule containing a transparent body, the function might well be doubtful.

The auditory organs of the jelly-fishes were first recognized as such by Kölliker.* They are ranged round the umbrella, and vary considerably in number, ranging up to sixty in Cunina, eighty in Mitrocoma, and as many as six hundred in Equorea.

There are three types. In the first, the auditory organ is an open pit, lined with cells. The majority of those on the outer side contain an otolithe, while a row

on the opposite side are strapshaped, their free ends terminating in auditory hairs, which reach to the cells containing the otolithes, while their inner ends are continuous with fibres from the inner nerve-ring.

In such an auditory organ as that of Ontorchis (Fig. 51), the otolithes present a very deceptive resemblance to the lenses of an eye.

* "Ueber die Randkörper der Quallen," Frorieps Neue Not., 1843.

Fig. 52.-Auditory organ of Phialidium (after Hertwig). d', Epithelium of the upper surface of the velum; d, epithelium of the under surface of the velum; hh, auditory hairs; h, auditory cells; np, nervous cushion; nr', nerve-ring; r, circular canal at the edge of the velum.

The second type is more advanced, the vesicle being closed, and the otolithes fewer in number, the Eucopidæ, indeed, having only one.*

In the third type, that of the Trachymedusa, the

Fig. 53.-Auditory organ of Rhopalonema, still showing a small orifice (after Hertwig). hk, Modified tentacle; o, auditory organ.

auditory organs are modified tentacles. They form a club-shaped body, with a central endodermal axis, and

* Hertwig considers that the supposed hairs shown by Hensen in his figure of the ear of Eucope are really the edges of auditory canals.

bearing at the apex one or more sometimes spherical, sometimes prismatic, otolithes. In some cases the organ becomes enclosed in a cup, which in Geryonia closes at the top.

In another family of the Hydromedusa, the Oceanidæ, these organs are absent, and appear to be replaced by certain pigment spots at the base of the tentacles, which, however, from their structure are considered to be rudimentary organs of vision, and will be described in the chapter on eyes.

ga

sle

Fig. 54.-Sense-organ of Pelagia

(after Hertwig). o, Group of crystals; sk, sense-organ; sf,

fold of the skin; ga, gastro-vas

cular channel.

Some species have, in addition, other organs, obviously of sense, but the function of which is still far from clear. Fig. 54 represents one of these curious sense-organs in Pelagia, after Hertwig. It is in the form of a somewhat bent finger, is situated in a deep fold of the umbrella, contains a branch of the gastrovascular canal, and is filled at the tip with a group of solid, shining, rod-like crystals.

The auditory organ in worms and molluscs consists of a closed vesicle, containing one or more otolithes, and lined with nerve-cells, which are, in the higher groups, connected at their base with the auditory nerve, and bear setæ at the other end. De Quatrefages was the first who established clearly the existence of auditory organs in worms.

In the Mollusca, the existence of an organ of hearing in some Gasteropods was justly inferred by Grant from

MOLLUSCA-ANNELIDES.

87

the fact that one species, Tritonia arborescens, emits certain sounds, doubtless intended to be heard by its fellows.

The ciliæ contained in the auditory vesicle are sometimes short, and scattered over

the general surface, as in Unio (Fig. 55); sometimes long and borne on papillary projections, as in Carinaria and Pterotrachea* (Fig. 56), where also there are certain special cells, supposed to act as buffers or dampers. The otolithe is sometimes single, and nearly spherical, as in Acephala

and Heteropoda, and consists of calcareous matter with an organic base; in the Gasteropods, Pteropods, and

Fig. 56.-Auditory organ of Pterotrachea Friderici (after Claus). Na, Auditory nerve; c, central cells; d, supporting plate; b, outer circle of auditory cells; a, ciliated cells.

some Annelides (Arenicola, Amphicora) they are * Claus., 66 Ueber den Acoust. App. im Gehörorgane der Heteropoden," Arch. für Mic. Anat., 1878.