starfish summary

Written and fact-checked by The Editors of Encyclopaedia Britannica

Below is the article summary. For the full article, see sea star.

Principal features of a starfish. Water for the water vascular system enters through the madreporite and passes into a ring canal and on to radial and lateral canals, from which it enters the tube feet, which are connected to saclike ampullae on top. Contraction of the tube feet forces water into the ampullae, creating suction that allows suckers at the ends of the feet to hold to a surface. When the ampullae contract, water is forced into the feet, extending them and releasing the suction. These coordinated actions permit movement, attachment, and capture of prey. In many starfish, the cardiac stomach is everted through the mouth on the body's undersurface to envelop prey, and digestion may begin outside the body before the stomach is drawn back in.

starfish, or sea star, Any of 1,800 echinoderm species (class Asteroidea) that have regenerable arms surrounding an indistinct disk and that inhabit all oceans. Species range from 0.4 to 25 in. (1–65 cm) across, but most are 8–12 in. (20–30 cm) across. Their arms, usually five, are hollow and, like the disk, covered with short spines and pincerlike organs; on the lower side are tube feet, sometimes sucker-tipped, used for creeping or clinging to steep surfaces. Some species sweep organic particles into the mouth on the underside of the disk. Others either evert the stomach upon their prey for external digestion or swallow the prey whole.