Physical Description
A sea urchin's spherical body is completely covered by sharp spines that can grow up to 8 cm (3.1 in). These spines grow on a hard shell called the "test", which encloses the animal. The oldest ones have been measured to be around 19 cm (7.5 in) in diameter. It can vary in color from red to dark burgundy. Rarely, albino specimens are found. A sea urchin has no visible eyes or legs. It has a mouth located on its underside, which is surrounded by five teeth. During larval development, the body of a sea urchin transitions from radial to bilateral symmetry.
This bilaterally symmetrical larva, called an echinopluteus, subsequently develops a type of pentaradiate symmetry that characterises echinoderms. It crawls very slowly over the sea bottom using its spines as stilts, with the help of its tube feet. Scattered among its spines are rows of tiny tube feet with suckers that help it to move and stick to the sea floor.
Read more about this topic: Red Sea Urchin
Famous quotes containing the words physical and/or description:
“The preservation of health is a duty. Few seem conscious that there is such a thing as physical morality.”
—Herbert Spencer (1820–1903)
“Whose are the truly labored sentences? From the weak and flimsy periods of the politician and literary man, we are glad to turn even to the description of work, the simple record of the month’s labor in the farmer’s almanac, to restore our tone and spirits.”
—Henry David Thoreau (1817–1862)