These animals form flat, encrusting colonies on sea-weeds and stones. They can be recognized by the ovate zooecia which are punctured by minute oval pores. The mouth of each zooecium is rimmed by a circle of from four to twelve short spines; a single, longer spine lies directly below the opening. The zoids are hermaphroditic and possibly protandrous. Breeding colonies can be recognized by the presence of eggs within the body cavity of the zoids.

This has not been determined for the Woods Hole, Mass., region. In European waters, the season lasts from June until December, with the peak in November.

A. Care of Adults: Colonies can be maintained easily in the laboratory, if an adequate supply of running sea water is provided.

B. Methods of Observation: Breeding colonies should be placed in fingerbowls of fresh sea water and examined at periodic intervals under the dissecting microscope, to ascertain whether shedding of eggs has begun. Since these eggs are small, they may not be visible to the unaided eye. Just (1934) warns that they are few in number and rather sensitive to environmental changes.

A. The Unfertilized Ovum: Although the eggs are irregular in outline while they are in the body cavity, they soon round up after they are shed. When freshly shed, the eggs are greyish in color, and measure 79 microns in diameter.

B. Fertilization and Cleavage: Fertilization presumably occurs within the body cavity, before the eggs are shed. Although it is absent in body-cavity eggs, a membrane rises from the surface of shed eggs; when this membrane is fully formed, it is connected to the egg surface by fine radiating lines. The polar bodies are given off within a few minutes of shedding. The cleavages are regular and equal as far as the 16-cell stage. At this time, a bilateral symmetry becomes apparent, and by the 32-cell stage, the future ectoderm and endoderm cells are distinguishable and the cells are arranged in a flat plate. Gastrulation is by epiboly.

C. Later Stages of Development: The larva is a typical "cyphonautes," with a complete digestive tract. It is conical in shape, the broad end being termed the oral surface. At the aboral pole, a tuft of long cilia springs from the apical organ. The margin of the oral surface is rimmed with a circle of powerful cilia, forming the corona. A bivalve shell and a peculiar pyriform organ are formed soon after the free-swimming stage is reached; the internal sac seems to be associated solely with metamorphosis and appears late in development. Details and figures of the development of the eggs and larvae are given by Prouho (1892), Kupelweiser (1905) and MacBride (1914).

BONNEVIE, K., 1907. Untersuchungen über Keimzellen. II. Physiologische Polyspermie bei Bryozoen. Jenaische Zeitschr. f. Naturw., 35: 567-598.

JUST, E. E., 1934. Fertilization in Membranipora pilosa. Carnegie Inst. Year Book, 33: 268-270.

KUPELWEISER, H., 1905. Untersuchungen über den feineren Bau und die Metamorphose des Cyphonautes. Zoologica, 19: Hf. 47, pp. 1-50.

MACBRIDE, E. W., 1914. Text-Book of Embryology. Vol. I. Invertebrata. Macmillan and Co., Ltd., London.

PROUHO, H., 1892. Contribution a l'histoire des Bryozoaires. Arch. de Zool. Exp., 20: 557656.

Silén, L., 1944. The main features of the development of the ovum, embryo and ooecium in the ooeciferous Bryozoa Gymnolaemata. Arkiv f. Zool., K. Svenska Vetenskap., 35A: no. 17, pp. 1-34.