The colonies of Obelia commissuralis are tree-like in form, and reach a height of 6 to 8 inches. The annulated, rather sparse side branches are given off at right angles from a long central trunk. Colonies of O. geniculata are not more than 30 mm. high and usually consist of a single stem bearing alternate hydranths on broad processes. Hydroids of both species have cup-like hydrothecae and a single row of filiform tentacles surrounding the hypostome.

The hydroid colonies are commonly found attached to docks, sea-weed or floating timbers at Woods Hole, Mass. The medusae are often caught in tow nets during the summer.

A. Care of Adults: Colonies are easily maintained in the laboratory, if they are provided with an adequate supply of fresh sea water.

B. Methods of Observation: To obtain medusae, ripe hydroid colonies should be placed, without crowding, in large fingerbowls of sea water. Within an hour or two, swimming medusae are usually released; these can be removed to depression slides for study. Young stages of medusa formation must be teased from the gonosomes with needles; older gonophores will usually be released if a well-matured gonosome is mounted on a slide under a coverslip and pressure applied to the coverslip with needles.

To obtain fertilized eggs, several mature medusae should be placed together in a jar of sea water. Although it has not been demonstrated for the Woods Hole form, Merejkowsky (1883) states that the Mediterranean species of Obelia always sheds in the early morning hours.

NORMAL DEVELOPMENT

A. Asexual Reproduction: The gonosomes of both species are borne in the axils of the branches, and are several times as large as the hydranths. Each consists of a vase-like gonotheca surrounding a central blastostyle whose expanded tip forms a loose plug for the mature gonotheca. The gonophores which mature as medusae develop as buds along the blastostyle, the most mature buds being found toward the neck of the gonotheca. When fully mature, they break loose and escape past the blastostyle plug to the outside. Details and diagrams of the development of the medusae can be found in the paper of Goette (1907).

B. Sexual Reproduction: When first shed, the medusae of O. geniculata have 24 tentacles, while those of O. commissuralis have only 16. Both species have four radial canals and a rather short manubrium. The velum is reduced to a narrow and somewhat lobed membrane near the bases of the tentacles. This reduction of the velum makes possible an eversion of the bell when the medusa comes to rest, so that the manubrium protrudes from the center of the convex surface. The gonads are not visible when the medusae are shed, but slowly mature during the two months of free-swimming existence. When mature, they appear as oval structures hanging from the mid-part of the radial canals. Diagrams of the medusae of the two species are presented by Nutting (1901).

Merejkowsky (1883) gives the details of sexual reproduction in an undesignated species of the genus Obelia. The eggs (about 130 microns in diameter, according to Hargitt, 1919) are fertilized externally after the gametes have been shed. No fertilization membrane is produced. A single polar body is given off after the eggs are shed. Regular and total cleavage leads to the formation of a one-layered coeloblastula, which soon becomes ciliated and motile. A few hours after the blastula is formed, the endoderm develops by an inward migration of cells from the posterior pole of the larva, which is now elongated. Continued multiplication of the endoderm cells leads to the formation of a typical solid, ciliated planula, with a broad anterior end and a narrow, pointed posterior end. This planula remains as a surface-swimmer for about a day, gradually developing nematocysts and epithelio-muscular cells. It then drops to the bottom, loses its cilia, and becomes affixed by the broader anterior end. Tentacles and mouth develop at the free end of the attached larva, and eventually asexual budding leads to the formation of a new hydroid colony.

FAULKNER, G. H., 1929. The early prophases of the first oocyte division as seen in life, in Obelia geniculata. Quart. J. Micr. Sci., 73: 225-242

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

HARGITT, G. T., 1919. Germ cells of coelenterates. VI. General considerations, discussion, conclusions. J. Morph., 33: 1-58.

MEREJKOWSKY C., 1883. Histoire du developpement de la meduse Obelia. Bull. Soc. Zool. France, 8: 98-129.

NUTTING, C. C., 1901. The hydroids of the Woods Hole region. Bull. U. S. Fish Comm., 19: 325-386.