This small, thin-shelled bivalve formerly was found on certain sandy mud bottoms, usually in the vicinity of eel grass; it was collected by digging at low tide. Formerly abundant at Woods Hole, Mass., it has not been available in this region in appreciable numbers since the severe winter of 1934-35, although the population is now slowly increasing.

The sexes are separate and can be distinguished by the color of the gonads which can be seen through the thin shell; the testes are white, the ovaries salmon pink.

Mid-June to mid-September. There is a critical water temperature (about 20û C.) below which the species does not spawn. At Woods Hole, eggs were usually abundant until about August 20, following which there was a lull lasting until the beginning of September, or until the next full moon. Details of this lunar periodicity can be found in a paper by Grave (1927a).

A. Care of Adults: The animals should be transported to the laboratory as quickly as possible after collection, kept barely damp in moist sand, and protected from high temperatures. While it is desirable to use them on the day they are collected, they will survive for one or two days in cool, moist sand. They must be kept away from sea water, to prevent shedding of gametes.

B. Procuring Gametes: Wash the adults free of all sediment and isolate them in small slender dishes half-filled with sea water. If mature, both sexes will shed 30 minutes to one hour after being placed in sea water. Grave (1927a) believes this spawning reaction in the laboratory is induced by shock. Collect the eggs as they are shed, and transfer them to a fingerbowl containing about 200 cc. of sea water. After shedding, remove the adults from the slender dishes. They should then be returned to their normal habitat.

C. Preparation of Cultures: Allow the eggs to stand for about 15 minutes (so that they may advance to the metaphase of the first maturation division) and then inseminate by adding one or two drops of dilute sperm suspension. Polyspermy occurs if too much sperm is used. Allow to stand for about 30 minutes, and then change the sea water several times to remove excess sperm and body fluids. Cover the culture dish and store on the sea water table.

About 12 hours after insemination, pour the upper layers of water, containing the normal, top-swimming larvae, into a large battery jar, and fill it with sea water. Repeat this procedure in about four hours, and then leave undisturbed for one or two days. At this time, the veligers tend to settle and lie quiescent on the bottom.

They are readily collected with a pipette and should be transferred once a day to fresh dishes of sea water until they metamorphose. During this time they must be fed diatoms, preferably Nitzschia. It is advisable to employ some device for gently agitating the water.

D. Methods of Observation: Early stages may be mounted on vaselined slides or on slides with the coverslips supported in some other manner. The jelly which surrounds the eggs is rendered visible if they are placed in a suspension of Chinese ink. Older moving larvae may be trapped in shredded lens paper or lightly anaesthetized with a dilute solution of chloral hydrate in sea water, and mounted in a Chinese ink suspension.

Normal DEVELOPMENT

A. The Unfertilized Ovum: The egg is small; measurements of the diameter vary from 61 to 65 microns, but the majority measure 62 or 63 microns. It is surrounded by a tough vitelline membrane and a thick jelly-envelope. The egg is transparent and contains yolk granules which give it a faintly pink cast. It is shed in the germinal vesicle stage and, on contact with sea water, proceeds spontaneously to metaphase of the first maturation division.

B. Fertilization and Cleavage: The vitelline membrane is not elevated perceptibly at fertilization. The polar body is relatively enormous and seems to round up before reaching the surface of the egg. Two polar bodies are formed, the first usually remaining undivided. The living egg is sufficiently clear for the approach and association of the pronuclei to be observed. Cleavage is spiral and very closely resembles that of Dreissensia (see the paper by Meisenheimer, 1901). The late blastula becomes ciliated.

C. Time Table of Development: The rate of development varies considerably with the temperature. A table showing developmental rates at different temperatures is presented by Grave (1927b). The following schedule was observed at 20 to 21û C.: times are recorded from fertilization.

Stage First polar body Second polar body First cleavage Second cleavage Blastula Gastrula Trochophore Fully-formed veliger Metamorphosis

Time 12-15 minutes 28-35 minutes 60-70 minutes 90-102 minutes 5-6 hours 7-10 hours 12-18 hours 2-24 days 16-24 days

D. Later Stages of Development and Metamorphosis: The early gastrula develops a prototroch and swims vigorously. The trochophore stage is of short duration (less than 10 hours). Yolk obscures the internal organs of this larval form. When first formed, the typical veliger settles temporarily to the bottom, swimming less and less frequently until metamorphosis. A pair of semi-circular valves develops, and extending from the straight hinge line to the ciliated velum are several strands of muscle fibers. The digestive tract is ciliated and consists of oesophagus, stomach and intestine. The mouth and anal openings have formed.

Late veligers rarely swim, and the velum appears to have become more and more an accessory feeding organ (Grave, 1927b).

COSTELLO, D. P., 1934. The hyaline zone of the centrifuged egg of Cumingia. Biol. Bull., 66: 257-263.

GRAVE B. H., 1927a. An analysis of the spawning habits and spawning stimuli of Cumingia tellinoides. Biol. Bull., 52: 418-435.

GRAVE, B. H., 1927b. The natural history of Cumingia tellinoides. Biol. Bull., 53: 208-219.

HEILBRUNN, L. V., 1920. Studies in artificial parthenogenesis. III. Cortical change and the initiation of maturation in the egg of Cumingia. Biol. Bull., 38: 317-339.

MEISENHEIMER, J., 1901. Entwicklungsgeschichte von Dreissensia polymorpha Pall. Zeitschr. f. wiss. Zool., 69: 1-137.

MORGAN, T. H., 1910. Cytological studies of centrifuged eggs. J. Exp. Zool., 9: 593-655.

WHITAKER, D. M., 1931. On the rate of oxygen consumption by fertilized and unfertilized eggs. II. Cumingia tellinoides. J. Gen. Physiol., 15: 183-190.

WHITAKER, D. M., 1939. The effect of fertilization on the rate of staining of Cumingia eggs. Growth, 3: 153-158.