The sensory cells are slender epithelial cells, often with a cilium or stiff protoplasmic process, and should perhaps be regarded as the only ectoderm-cells which retain the primitive ciliation of the larval ectoderm, otherwise lost in all Hydrozoa. The sense-cells form, in the first place, a diffuse system of scattered sensory cells, as in the polyp, developed chiefly on the manubrium, the tentacles and the margin of the umbrella, where they form a sensory ciliated epithelium covering the nerve-centres; in the second place, the sense-cells are concentrated to form definite sense-organs, situated always at the margin of the umbrella, hence often termed "marginal bodies." The possession of definite sense-organs at once distinguishes the medusa from the polyp, in which they are never found. The sense-organs of medusae are of two kinds--first, organs sensitive to light, usually termed _ocelli_ (fig. 29); secondly, organs commonly termed _otocysts_, on account of their resemblance to the auditory vesicles of higher animals, but serving for the sense of balance and orientation, and therefore given the special name of _statocysts_ (fig. 30). The sense-organs may be _tentaculocysts_, i.e. modifications of a tentacle, as in Trachylinae, or developed from the margin of the umbrella, in no connexion with a tentacle (or, if so connected, not producing any modification in the tentacle), as in Leptolinae. In Hydromedusae the sense-organs are always exposed at the umbrellar margin (hence _Gymnophthalmata_), while in Scyphomedusae they are covered over by flaps of the umbrellar margin (hence _Steganophthalmata_). [Illustration: 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.e, Cavity of statocyst. con, Concrement-cell with otolith.]