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). [Illustration: 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 _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). 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 body. The distal portion of the vitreous body may project from the cavity of the cup, forming a non-cellular lens as in _Lizzia_ (fig. 28). Beyond this simple condition the visual organs of the Hydromedusae do not advance, and are far from reaching the wonderful development of the eyes of Scyphomedusae (_Charybdaea_). [Illustration: After O. and R, Hertwig, _Nervensystem und Sinnesorgane der Medusen_, by permission of F. C. W. Vogel. FIG. 37.--Section of statocyst of _Geryonia_ (_Carmarina hastata_).