The _statocysts_ present in general the structure of either a knob or a closed vesicle, composed of (1) indifferent supporting epithelium: (2) sensory, so-called auditory epithelium of slender cells, each 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. [Illustration: 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.] [Illustration: 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.] [Illustration: 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.]