Prominent among the family Ursidae is a polar bear that is adapted to a highly specialized semi-aquatic lifestyle. Although it is a bear yet it is listed as a marine mammal precisely due to the fact that it spends 50% of its life on ice floes and is able to swim as far as 90 km (60 miles).
urchin. 1  a mischievous boy. 2  any small boy or youngster. 3  sea urchin. 4  either of two small rollers covered with card clothing used in conjunction with the cylinder in carding. 5  Chiefly British Dialect. a hedgehog. 6  Obsolete. an elf or mischievous sprite.
Cidaroid urchins are unusual in that their spines LACK skin on the surface. Remember that ALL echinoderms display their ENDOSKELETONS. What you are seeing (spines, armor, etc.) are NOT a shell or exoskeleton. Because of this, cidaroid urchin spines are affected by the environment.