These small, hermaphroditic gastropods have a reduced shell and a relatively large foot. The adults burrow into the mud, about half an inch below the surface (although the egg-capsules are deposited at the surface), and can be collected from the mud flats of Hadley Harbor, close to the Middle Gutter, at Woods Hole, Mass. Although the adults are quite abundant during the breeding season, they seem to disappear during the rest of the year.

June to September, with the peak between July 15 and August 15.

A. Care of Adults: If the adults are placed in large fingerbowls covered with cheesecloth, and supplied with a gentle stream of sea water, they will live and breed in the laboratory without any other special care.

B. Procuring Fertilized Ova: There is apparently no relation between capsule production and time of day. Animals have been known to produce, on the average, three to four capsules daily over a period of three weeks. During the last week, however, both eggs and capsules may be abnormal. The number of eggs in each capsule varies, but the average content is about two thousand (Smallwood, 1904b).

C. Preparation of Cultures: Eggs will develop normally in open fingerbowls of sea water if they are kept cool on a water table and have a daily change of sea water. This treatment allows normal development, even of eggs in ruptured capsules.

When the free-swimming larvae appear, they should be decanted daily to fingerbowls of fresh sea water. They will live for four to five days under these conditions. After this time, many die from starvation or because they become trapped and held in the surface film.

NORMAL DEVELOPMENT

A. The Ovum: The fertilized, uncleaved egg is small, having a diameter of about 80 microns. It is enclosed in a thin, structureless membrane, and is very opaque. The ova are laid in spherical, soft gelatinous capsules with no apparent orientation. Capsules produced by laboratory animals average about half an inch in diameter, and, as noted above, contain about two thousand eggs, although this number varies considerably.

B. Cleavage: Shortly after fertilization two polar bodies are extruded. Cleavage is total and spiral, and often unequal. Smallwood (1904a) observed that in about 30 per cent of the eggs, one blastomere was markedly larger than the other.

The micromeres are strikingly large and clear. Gastrulation is apparently by epiboly.

C. Time Table of Development: The following schedule is given for temperatures of 24 to 26û C. Times are calculated from the appearance of the first polar body, which is reported by Smallwood (1904a) to appear about 15 minutes after the egg is deposited.

Stage First cleavage Second cleavage Third cleavage Gastrulation Young veliger Well-formed, rotating veliger Hatching

Time 85 minutes 120 minutes 200 minutes 24 hours 48 hours 72 hours 96 hours

D. Later Stages of Development: A typical gastropod veliger is formed from the gastrula. In young forms, an opaque digestive tract can be seen, and beside it another dark mass which is probably unabsorbed yolk. A clear vesicle, which may represent a head kidney, is sometimes visible. In older veligers the alimentary tract becomes more complex; a triangular mouth, ciliated oesophagus, large ciliated stomach and a narrow intestine with one coil may be seen. A dark lobe, the liver, lies to the left of the stomach. An eyespot and a pair of statocysts develop. At hatching, the veliger is enclosed in a curved shell; the large, two lobed velum, which protrudes from it, propels the free-swimming larva through the water.

BERRILL, N. J., 1931. The natural history of Bulla hydatis Linn. J. Mar. Biol. Assoc., 17: 567-571.

SMALLWOOD, M., 1901. The centrosome in the maturation and fertilization of Bulla solitaria. Biol. Bull., 2: 145-154.

SMALLWOOD, W. M., 1904a. Natural history of Haminea solitaria Say. Amer. Nat., 38: 207-225.

SMALLWOOD, W. M., 1904b. The maturation, fertilization, and early cleavage of Haminea solitaria (Say). Bull. Mus. Comp. Zool., Harvard, 45: 259-318.