Metridium is a large yellow-brown anemone, which is common along the Atlantic coast. It has a broad pedal disc and a lobed oral disc bearing many short tentacles.

A. Care of Adults: These anemones are easily kept in the laboratory for considerable periods of time, if they are maintained in aquaria with adequate supplies of running sea water.

B. Methods of Observation: The sex of the mature animals cannot be ascertained macroscopically. If a number of ripe individuals are placed together in large dishes containing sea water, natural spawning and fertilization will occur. The same individuals have been shown to spawn several times, at intervals of two to ten days, for a month. Since the eggs are heavy, they will sink to the bottoms of the containers; they can be picked up and transferred to fingerbowls with a pipette. The jelly, which may surround the eggs when they leave the stomodeum, soon dissolves. The larvae can be maintained in the laboratory if they are changed to dishes of fresh, aerated sea water at intervals of a few days.

A. The Unfertilized Ovum: The eggs are mature and surrounded by a delicate membrane at the time of shedding; they are spherical, opaque, and usually pink in color. McMurrich (1891) reports that they measure between 100 and 160 microns in diameter.

B. Fertilization and Cleavage: Fertilization occurs soon after the eggs are shed, as the eggs are sinking to the bottom. Cleavage is total and slightly irregular, being either equal or sub-equal. The hollow, single-layered blastula is converted into a gastrula by invagination.

C. Time Table of Development: No details of the exact developmental rate are available, although the first cleavage is said to occur 45 minutes after insemination. Fixation occurs in about a month.

D. Later Stages of Development: The young gastrula is top-shaped, the oral surface being somewhat flattened. In the older larva the body is lengthened, and a tuft of long, stiff cilia appears on the anterior (aboral) pole. The stomodeum is well formed, and the two lateral mesenteries appear as conspicuous folds in the gastric cavity. Nematocysts are present at both the oral and the aboral poles of the developing larva. Although temporary attachments (probably for feeding purposes) are made by the oral surface, the permanent attachment occurs at the aboral pole.

GEMMILL, J. F., 1920. The development of the sea-anemones Metridium dianthus (Ellis) and Adamsia palliata (Bohad). Phil. Trans. Roy. Soc., London, ser. B, 209: 351-375.

MCMURRICH, J. P., 1891. Contributions on the morphology of the Actinozoa. II. On the development of the Hexactiniae. J.. Morph., 4: 303-330.