The colonies vary from two to six inches in height, and grow in a branched, fan-like pattern. The terminal hydranths are large, with vermilion bodies and white tentacles. The hypostome is long and is covered with stubby, knobbed tentacles. In addition, there is a basal ring of about 12 long, slender tentacles. Diagrams are given by Hyman (1940) and Nutting (1901). The adult colonies are very common at Woods Hole, Mass., and can be collected from Fucus or other sea-weeds, or from pilings three feet below sea level. They are abundant on the vegetation of the Spindle.

A. Care of Adults: The animals are extremely sensitive and should never be crowded. Place a few colonies bearing the best embryological material (large eggs and full spermaries) in large fingerbowls on a water table, allowing a gentle stream of sea water to flow through the dishes.

B. Methods of Observation: About 3 P.M. on the second afternoon after collection, cut a few small stems from ripe "male" and "female" colonies, carefully rinse them in sea water, and place them together in a fingerbowl containing filtered sea water. Cover the dish and return it to the water table. Examine the dish during the evening for evidence of shedding, and pipette the fertilized eggs to a fingerbowl of fresh sea water as soon as possible after they are shed. If unfertilized eggs are desired, the male and female colonies can be isolated in separate fingerbowls. Eggs procured in this manner can be artificially inseminated with a few drops of sperm solution taken from a dish containing a shedding male colony.

It is possible to modify the time of shedding and to procure gametes at any time of day which is convenient (Baker, 1936; Ballard, 1942). If colonies are placed in the dark for 24 hours, as soon as they are brought into the laboratory, and then exposed to continuous light (either artificial or natural), shedding can be expected 10-14 hours after the return to light. Since the animals do not live long in the laboratory, this procedure can be used only once.

The free-swimming larvae should be transferred to Syracuse dishes of sea water and kept in a moist-chamber on a sea water table until attachment. After attachment has occurred, the dishes can be stacked in an inverted position, in wooden racks placed in aquaria of running sea water.

A. Asexual Reproduction: The gonophores bud off singly from the hydranth body just above the proximal tentacles; there may be one to three per gonosome. A single colony bears gonophores of one sex only, but in living individuals sex is difficult to ascertain until the gonophores are mature; the eggs will be pink and the sperm white (Smallwood, 1899). "Male" and "female" colonies are actually asexual, bearing male and female gonophores, respectively. For illustrations and details of gonophore development, see the paper of Goette (1907).

The mature medusae are similar in the two sexes; they have an elongate bell, a velum, four radial canals and four rudimentary tentacles. The vermilion manubrium (spadix), to which the gametes are attached, is short and there is no mouth. In southern waters, Pennaria medusae generally break away from the colony and swim about during the discharge of the sex products; in Woods Hole, however, they usually remain attached, and the eggs may not be ejected until long after fertilization.

At the onset of spawning, the ripe medusae gradually begin a rhythmic twitching. The males emit puffs of white sperm, the females eject three to six eggs. The spent medusae finally drop off, swim feebly if at all, and shrivel- and die in a few hours.

B. Sexual Reproduction The newly shed eggs are opaque and usually pale pink in color, although this can vary from a cream-white to orange. The irregular shape makes accurate measurements difficult; C. W. Hargitt (1900) gives 400-500 microns as the diameter, while G. T. Hargitt (1919) states that the average diameter is only 237 microns. The large size of at least some of the ova is probably associated with the fact that certain of the maturing oocytes absorb other oocytes during development. The polar bodies are given off and lost before the eggs are shed. Fertilization is external and is accompanied by amoeboid movements of the egg; no fertilization membrane is formed.

The cleavages, which begin about 30 minutes after insemination, are rapid. Although at times they may be quite regular until the 8-cell stage, they become chaotic and without pattern after this stage. Nuclear division with delayed cytoplasmic division is quite common. The embryo has a flat, disc-like appearance during the later cleavages, but rounds up to form a solid spherical morula. This differentiates an ectodermal layer, which becomes ciliated. In 12-24 hours, the pyriform embryo becomes a young free-swimming planula. See the papers of C. W. Hargitt (1900) and G. T. Hargitt (1909) for details of early development.

C. Later Stages of Development and Metamorphosis: The early, free-living planula is a solid, ciliated organism, but eventually the endoderm differentiates from the central mass and the beginnings of a coelenteron become visible. In about five days the larva attaches and begins to secrete a delicate perisarc. Two days later, the rudiments of the proximal tentacles of the first hydranth are visible about the mouth, which develops at the free end of the metamorphosing larvae. Hargitt (1900) presents diagrams and describes metamorphosis in detail.

BARER, E. G. S., 1936. Photoperiodicity in the spawning reaction of Pennaria tiarella McCr. Proc. Indiana Acad. Sci., 45: 251-252.

BALLARD, W. W., 1942. The mechanism for synchronous spawning in Hydractinia and Pennaria. Biol. Bull., 82: 329-339.

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., 1900. A contribution to the natural history and development of Pennaria tiarella MeCr. Amer. Nat., 34: 387-415.

HARGITT, G. T., 1909. Maturation, fertilization, and segmentation of Pennaria tiarella (Ayres) and of Tubularia crocea (Ag.). Bull. Mus. Comp. Zool., Harvard, 53: 159-212.

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

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

NUTTING, C. C., 1901. The hydroids of the Woods Hole region. Bull. U. 5. Fish Comm., 19: 325-386.

SMALLWOOD, M., 1899. A contribution to the morphology of Pennaria tiarella McCrady. Amer. Nat., 33: 861-870.

SMALLWOOD, W. M., 1909. A reexamination of the cytology of Hydraetinia and Pennaria. Biol. Bull., 17: 209-240.