This common inhabitant of the tidepools and rocky shore wharf pilings often grows to a diameter of ten inches. Sea anemones expand during the day and close at night. Their tentacles are armed with stinging cells which are used to para1yze small animals such as crabs that come in contact with the anemone. An animal that is very sensitive to stagnant or polluted water, it is usually found near an influx of clean ocean water. The green color of the body is due to a green alga which lives in the animal's tissues, apparently with a mutually beneficial (symbiotic) relationship.