Colonies of this form are fairly common on the Littorina shells inhabited by the small hermit crab, Pagurus. There are three types of individuals in the fully developed colony: (1) ordinary polyps (feeders), with a single whorl of tentacles; (2) thread-like coiling forms with no mouth and an apical knob of nematocysts (stingers, commonest around the lip of the shell); and (3) gonosomes. All three types arise singly from a hydrorhiza network covered by a rust-red spine-studded crust (Nutting, 1901).

The snails on which the colonies grow are common in the littoral near Woods Hole, Mass., and can be gathered in considerable numbers at Sheep Pen Harbor and Tarpaulin Cove.

A. Care of adults Colonies may be kept in large beakers or other deep vessels, supplied with running sea water.

B. Methods of Observation: If a number of snail-shells bearing ''male'' and "female" colonies are placed in a large uncovered dish of sea water and left overnight, eggs will usually be shed and fertilized between 7 and 9 A.M. on the following morning. Colonies kept in running sea water have been known to shed daily for a week before becoming exhausted. The shedding can be controlled by light, however, if eggs are desired at some other time of day. Colonies should be kept in running sea water, under a glowing 100-watt bulb, from the time of collection until gametes are needed. They should then be placed in the dark for one or more hours and subsequently re-exposed to light. By the use of a hand lens or dissecting microscope, the sexes can be segregated to separate fingerbowls of fresh sea water. The males will shed 50 minutes after re-illumination, the females five minutes later. The eggs should be transferred to fingerbowls of fresh sea water and inseminated with one or two drops of water taken from a dish of shedding males. Ballard (1942) gives further details of this method for controlling shedding.

A. Asexual Reproduction: The gonosomes, or reproductive individuals, are usually without tentacles and have a large knob of nematocysts on the proboscis; each bears a number of gonophores, which are medusa-buds reduced to the status of sporosacs. Ripe "male" and "female" colonies can be distinguished from one another with the unaided eye, since the eggs within the sporosacs are dull green against the red hydrorhiza, and the sperm, when mature, are a white mass. For details of gonophore development, see the papers by Goette (1907, 1916).

B. Sexual Reproduction: The maturation of the eggs within the gonophores occurs as a direct response to light and can be seen in eggs dissected from colonies placed in the light after several hours of darkness. In such eggs, the large germinal vesicle begins to break down soon after the exposure to light. The first polar body is given off 45 minutes after exposure to light, the second polar body ten minutes later. Occasionally the first polar body may divide. The eggs are shed immediately after the second maturation division (Ballard, 1942).

The eggs are yolky and usually green; occasionally grey, orange or pink ova are shed. Teissier and Teissier (1927) give the average egg-diameter as between 160 and 170 microns. When shed, the eggs are covered by a highly transparent, radially striated jelly, which swells on exposure to sea water. The swelling of this layer causes the polar bodies to be lifted from the egg surface and they are soon lost. Cleavage may be irregular, but usually the somewhat amoeboid egg undergoes three equal, total cleavages, each of which is at right angles to the preceding one. The separating pairs of blastomeres tend to retain broad protoplasmic connections with one another on the side opposite the cleavage furrow, until just before the succeeding cleavages begin. There is much variation in the time and degree of shifting of positions of the blastomeres, but the bizarre cleavage patterns often seen in the laboratory are commonly the result of evaporation of the sea water, or other unfavorable factors.

Mitotic synchronism quickly disappears. Gastrulation is said to start as early as the 16-cell stage, by mixed delamination and multipolar proliferation. The gastrula loses its spherical form and remains for a few hours an irregular mass; then it returns to the spherical form and gradually lengthens into the planula form. For illustrations of the cleavage pattern, see the papers by Beckwith (1914) and Bunting (1894).

C. Later Stages of Development and Metamorphosis: At the end of 24 hours the embryo is a "preplanula" (Teissier and Teissier, 1927) with an elongated oval form, recognizable polarity and ciliation. During the course of a few days, it lengthens, one end becoming progressively slimmer, while it rolls and crawls along the bottom like a planarian. The large end (which goes first in this movement) is the end which later produces the adhesive disc by which the larva attaches for metamorphosis; it becomes the aboral end of the polyp.

Following attachment of the attenuated planula, there is a delay of a few hours to several days, and then the tapering free end shrinks down almost to the substrate, where it produces a mouth and a succession of tentacles. The new polyp elongates, its attached end meanwhile actively sending out a number of anastomosing and encrusting hydrorhiza processes from which branch new polyps. For further details of planula development and metamorphosis, see the paper by Teissier and Teissier (1927) .

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

BECKWITH, C. J., 1914. The genesis of the plasma-structure in the egg of Hydractinia echinata. J. Morph., 25: 189-251.

BERRILL, N. J., 1953. Growth and form in gymnoblastic hydroids. VI.. Polymorphism within the Hydractiniidae. J. Morph., 92: 241-272.

BUNTING, M., 1894. The origin of sex cells in Hydractinia and Podocoryne, and the development of Hydractinia. J. Morph., 9: 203-236.

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

GOETTE, A., 1916. Die Gattungen Podocoryne, Stylactis und Hydractinia. Zool. Jahrb. abt. Syst., Geog., Okol. der Tiere, 39: 443-510.

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

TEISSIER, L., AND G. TEISSIER, 1927. Les principales étapes du développement d Hydractinia echinata (Flem.). Bull. Soc. Zool. France, 52: 537-547.