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:
“I have often been reproached with the aridity of my genius; a deficiency of imagination has been imputed to me as a crime; and the Pyrrhonism of my opinions has at all times rendered me notorious. Indeed, a strong relish for physical philosophy has, I fear, tinctured my mind with a very common error of this age—I mean the habit of referring occurrences, even the least susceptible of such reference, to the principles of that science.”
—Edgar Allan Poe (1809–1849)
“Once a child has demonstrated his capacity for independent functioning in any area, his lapses into dependent behavior, even though temporary, make the mother feel that she is being taken advantage of....What only yesterday was a description of the child’s stage in life has become an indictment, a judgment.”
—Elaine Heffner (20th century)