GENERAL HISTOLOGY

Detailed explanation-1: -Sea urchins and sand dollars are examples of Echinoidea. These echinoderms do not have arms, but are hemispherical or flattened with five rows of tube feet that help them in slow movement; tube feet are extruded through pores of a continuous internal shell called a test.

Detailed explanation-2: -Even sea cucumbers, which show a more worm-like appearance, usually exhibit this body plan, known as pentameral symmetry. Most species have an arm number that is an increment of five-there are even echinoderms that display great numbers of arms, with some sea stars having up to 50.

Detailed explanation-3: -Sea stars may be red, blue, or many other colors. Most sea stars have a central disk with five radial arms; some species have 15 to 40 arms. A few species have arms so short that they barely protrude.

Detailed explanation-4: -So, echinoderms have spiny skin. All echinoderms have some form of radial symmetry as adults, meaning they have body parts that branch out from a central point-think of the five arms of a starfish. That’s fivefold radial symmetry.