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 physical world is meaningless tonight
And there is no other.”
—Wallace Stevens (1879–1955)
“Everything to which we concede existence is a posit from the standpoint of a description of the theory-building process, and simultaneously real from the standpoint of the theory that is being built. Nor let us look down on the standpoint of the theory as make-believe; for we can never do better than occupy the standpoint of some theory or other, the best we can muster at the time.”
—Willard Van Orman Quine (b. 1908)