a, Nerve ring, a´, Radial nerve. b, Tentaculocyst. c, Circular canal. e, Radiating canal, g´´. Ovary. h, Peronia or cartilaginous process ascending from the cartilaginous margin of the disk centripetally in the outer surface of the jelly-like disk; six of these are perradial, six interradial, corresponding to the twelve solid larval tentacles, resembling those of _Cunina_. k, Dilatation (stomach) of the manubrium. l, Jelly of the disk. p, Manubrium. t, Tentacle (hollow and tertiary, i.e. preceded by six perradial and six interradial solid larval tentacles). u, Cartilaginous margin of the disk covered by thread-cells. v. Velum.] The ectoderm furnishes the general epithelial covering of the body, and the muscular tissue, nervous system and sense-organs. The external epithelium is flat on the ex-umbral surface, more columnar on the sub-umbral surface, where it forms the muscular tissue of the sub-umbrella and the velum. The nematocysts of the ectoderm may be grouped to form batteries on the tentacles, umbrellar margin and oral lappets. In places the nematocysts may be crowded so thickly as to form a tough, supporting, "chondral" tissue, resembling cartilage, chiefly developed at the margin of the umbrella and forming streaks or bars supporting the tentacles ("Tentakelspangen," _peronia_) or the tentaculocysts ("Gehörspangen," _otoporpae_). [Illustration: After O. Maas in _Results of the "Albatross" Expedition_, Museum of Comparative Zoology, Cambridge, Mass., U.S.A. FIG. 27.--_Stomotoca divisa_, one of the _Tiaridae_ (Anthomedusae).] The muscular tissue of the Hydromedusae is entirely ectodermal. The muscle-fibres arise as processes from the bases of the epithelial cells; such cells may individually become sub-epithelial in position, as in the polyp; or, in places where muscular tissue is greatly developed, as in the velum or sub-umbrella, the entire muscular epithelium may be thrown into folds in order to increase its surface, so that a deeper sub-epithelial muscular layer becomes separated completely from a more superficial body-epithelium.