The lamina, which lies externally to the endoderm, i.e. on the convex side of the bell, is very much thickened, and forms the umbrella (u.), the substance of which is watery, jelly-like, and as a rule traversed by elastic fibres (e.f.). The umbrella is thickest in the region of the atrium, thinnest at the apparent edge of the bell, close to the circumferential canal. The lamina placed internally to the endoderm, i.e. on the concave surface of the bell, is very thin, like that of the manubrium and tentacles. It forms the sub-umbrella (s.u.). The umbrella and sub-umbrella pass round the circumferential canal, and unite to form the simple supporting lamina of the velum. The ectoderm varies in character in different regions. On the convex surface of the bell the cells are flat, except to the outer side of the circumferential canal, where there is a ridge of columnar ciliated cells, some of which send processes to the outer nerve-ring (n.), which lies at their base. The cells of the outer surface of the velum are cubical.

On the concave surface of the bell they are disposed usually in a single layer, and develope at their bases, as indicated by thicker dashes, circularly disposed muscle-fibres. Similar fibres exist on the inner surface of the velum, but they are wanting, however, to the inner side of the circumferential canal, where the inner nerve-ring (n'.) lies. Scattered ganglion cells in connection with this ring lie in the ectoderm of the sub-umbrella, and supply the muscle-fibres.

The ectoderm of the manubrium developes (1) longitudinally disposed muscle-fibres, indicated by thicker dashes, and (2) at the oral margin nematocysts or thread-cells, which are situated on the oral tentacles when these structures are present.

Tentacles (A: T.) usually spring from the margin of the bell at the ends of the radial canals, sometimes also from the circumferential canal in the inter-radial spaces. They are either tubular and lined by ciliated endoderm cells, or solid with a core of vacuolated endoderm cells arranged in a single row. Their ectoderm cells develope longitudinally arranged muscle-fibres, and nematocysts.

A section taken inter-radially (Fig. n, B.) agrees in most respects with a section taken in the plane of a radial canal (Fig. 11, A.). But in the place of a radial canal it shows a single layer of cubical endoderm cells, forming the so-called gastral lamella (B: g.l.), continuous with the edges of the atrium and of the circumferential canal respectively. In this lamella new radial canals are sometimes developed during the growth of the organism, e. g. in Aequorea.

The eye-spots in an Ocellate Medusa are generally placed at the base of a tentacle on its outer side, sometimes, however, on the inner. The eye has retina cells, supporting cells, basal ganglion cells, and sometimes a cuticular lens. The auditory vesicle in a vesiculate Medusa is developed close to the inner nerve-ring, from a pit of the inner surface of the velum, which projects towards its outer surface. It has otolith cells and sense-cells, derived from the ectoderm of the inner surface of the velum.

A Medusa with a velum is said to be Craspedote. Such Medusae characterize the Crasfiedota, one of the two main sub-divisions of the class Hydrozoa. The other division is known as Acraspeda.

The gonads or generative organs are situated on the manubrium in an ocellate Medusa; at the base of the manubrium or on the course of a radial canal in a vesiculate Medusa. The spermatozoa and ova often arise and invariably ripen in the ectoderm, though they may take their origin in the coenosarc or the blastostyle, and by what is apparently an abbreviation of development, from the endoderm.

Under these circumstances they go through active migrations. In Sertularia pumila the generative zooid is a reduced medusoid gonophore; the ova arise in the endoderm (? from wandering ectoderm cells) of the branch or stem below the spot from which the gonangial bud springs, wander through the endoderm of the blasto-style and gonophore, pierce the supporting lamella, and ripen in the ectoderm of the gonophore. The generative zooid of S. abietina requires investigation.

Hydrozoa, Ray Lankester, Encyclopaedia Brit. (ed. ix.) xii. British Hydroid Zoophytes, Hincks, 2 vols. 1868. Gymnoblastic Hydroids, Allman, Ray Soc. 1871. Id. Hydrozoa, pt. i, Plumularidae, Challenger Reports, vii. 1883. Organistnus der Hydroidpolypen, Hamann, J. Z. xv. 1882. System, etc. der Medusen, Haeckel, Dk. Med. Natwiss. Gesellsch. Jena, i. 1879; ii. 1881; Id. Challenger Reports, iv. 1882. Organismus der Medusen, Hertwig (O. and R.), Jena, 1878. Iid. Nerven-system und Sinnesorgane der Medusen, Leipzig, 1878. Helgolander Leptomedusen, Bohm, J. Z. xii. 1878.

Cordylophora lacustris, Schulze, Leipzig, 1871. Syncoryne Sarsii and Sarsia tubulosa, Id. Leipzig, 1873. Tubularia, Ciamician, Z. W. Z. xxxii. 1879; cf. Conn, Johns Hopkins Univ. Circulars, No. 17, 1882. Eudendrium, Jickeli, M. J. viii. 1882. Limnocodium, Bourne, P. R. S. xxxviii. 1884-85 j Nature, xxxi. 1884-85; Ray Lankester, Q. J. M. xx. 1880; Allman, J. L. S. xv. 1881. Campanularia angulata, Fraipont, A. Z. Expt. viii. 1880. Eucopella Campanularia, von Lenden-feld, Z. W. Z. xxxviii. 1883. Clytia Johnstoni, Klaatsch, M. J. ix. 1884. Medusa of Obelia, Merejkowski, Bull. Soc. Zool. France, viii. 1883.

Nervous system in Hydroid polypes. Jickeli, op. cit. supra. In Siphonophora. Chun, Z. A. iv. 1881: v. 1882; Korotneff, Z. A. v. 1882; Conn and Beyer, Johns Hopkins Biol. Studies, ii. 1883. Physiology. Romanes, Jelly-fish, Star-fish, and Sea-urchins, Internat. Series, 1. 1885.

Intracellular digestion. Metschnikoff, Arb. Zool. Inst. Wien, v. 1883, transl. Q. J. M. xxiv. 1884; Ray Lankester, Q. J. M. xxi. 1881; Krukenberg, Vergleich. Physiol. Studien (2), Abth. ii. p. 139, Water in umbrella of Medusae. Krukenberg, Z. A. iii. 1880; Mobius, Z. A. v. 1882.

Sexual cells, origin, etc. Weismann, Sexual-zellen (male and female) bei Hydromedusen, Jena, 1883; cf. Moseley, Nature, xxix. 1883-84: de Varenne, A. Z. Expt. x. 1882; cf. Bourne, Q. J. M. xxiii. 1883. Female cell. Hartlaub, Z. W. Z. xli. 1884. Male cell. Thallwitz, J. Z. xviil 1885.