LIVING MATERIAL

The colonies are profusely branched, and between 10 and 15 cm. in height. The hydranths have a trumpet-shaped hypostome surrounded by a single row of about 20 filiform tentacles. They are very common on piles or docks in shallow water.

A. Care of Adults: Colonies should be provided with adequate supplies of fresh sea water.

B. Methods of Observation: The release of planulae from ripe colonies occurs about mid-day, if the colonies are collected in the morning. Few larvae can be obtained from colonies collected late in the afternoon.

Eggs dissected from the gonophores will not cleave normally; if a study of normal development is desired, it is necessary to clip off not only the intact gonophore but also a considerable portion of the hydroid colony.

NORMAL DEVELOPMENT

A. Asexual Reproduction: The gonosomes are rudimentary, sessile medusaforms or gonophores, borne at the bases of special hydranths which lose their tentacles and degenerate during the period in which the gonophores are ripening. The gonophores are strikingly different in the two sexes. The "female" colonies bear loose irregular tufts of sporosacs attached to the stems, each ripe sporosac being bright orange in color. "Male" colonies bear strings of light pink sporosacs (two to four or more per string), radiating like the spokes of a wheel from a common point on the base of the degenerate hydranth. Hyman (1940) shows illustrations of the gonophores; the paper of Goette (1907) may be consulted for the details of gonophore development.

B. Sexual Reproduction: The eggs are fertilized within the gonophores; they are large (230 microns in diameter) and very opaque, due to the presence of a large supply of orange-colored yolk. The details of cleavage can be ascertained only by histological study, but it has been shown that cleavage is very similar to that of insects and crustaceans, involving the formation of yolk-pyramids and an early period during which nuclear, but not cytoplasmic, divisions occur. Following a migration of nuclei to the surface of the syncytial mass, an outer layer of ectoderm is cut off. The inner mass remains syncytial for a considerable period of time and the endoderm is not fully differentiated until the time of metamorphosis.

C. Later Stages of Development and Metamorphosis: When first liberated, the ciliated planulae are elongated and pyriform; however, they become slender after a few hours of free-swimming existence. They show a marked positive phototropism when they are first released, but this later declines. Fixation normally occurs after two or three days; the broader aboral pole is attached to the surface and fixed there by a slimy secretion. Metamorphosis is completed in 12 to 24 hours.

For details of cleavage and planula formation, see the paper by Hargitt (1904); metamorphosis is described by Hargitt (1904), and by Allman (1871).

ALLMAN, J. G., 1871. A monograph of the gymnoblastic or tubularian hydroids. London, The Ray Society.

BERRILL, N. J., 1952. Growth and form in gymnoblastic hydroids. II. Sexual and asexual reproduction in Rathkea. III. Hydranth and gonophore development in: Pennaria and Acaulis. IV. Relative growth in Eudendrium. J. Morph., 90: 1-32.

GOETTE, A., 1907. Vergleichende Entwicklungsgeschichte der Geschlechtsindividuen der Hydropolypen. Zeitschr. f. wiss. Zool., 87: 1-336.

HARGITT, C. W., 1889. Origin of sex-cells in Eudendrium. Proc. Amer. Assoc. Adv. Sci., 1889.

HARGITT, C. W., 1904. The early development of Eudendrium. Zool. Jahrb. abt. Anat. u. Ontog., 20: 257-276.

HYMAN, L. H., 1940. The Invertebrates: Protozoa through Ctenophora. McGraw-Hill Book Co., New York.