Calcitic Endoskeleton

Although Echinoderms lack backbones, they have a protective calcitic endoskeleton, such as the sand dollars shown above. This calcium carbonate-rich structure is made up of skeletal units called ossicles. The layering of ossicles give Echinoderms their spiny appearance through the thin skin that covers its surface. Endoskeletons from dead Echinoderms are the main ingredient in limestone, an important sedimentary rock used in building materials, livestock feed, and toothpaste.

How are dead Echinoderm endoskeletons naturally "recycled"?

Dead Echinoderm endoskeletons are the main ingredient in forming the sedimentary rock, limestone.

Bilaterally Symmetric Larvae

When Echinoderm larvae hatch from eggs, they have a much different body shape than adult Echinoderms. As shown in the picture above, Echinoderm larvae can be divided down the center, showing a mirror image of two body halves. This is called bilateral symmetry.

Radially Symmetric Adults

As an Echinoderm matures from larva to adult, they change from bilateral to radial symmetry. Notice how the adult brittle star shown above has a circular body with five arms branching out. The brittle star can be divided into five "pie slices" that look the same. As a matter of fact, most Echinoderms have a multiple of five branching body parts. This is called pentaradial symmetry.

Using context clues, which number does the prefix "penta-" mean?

The prefix "penta-" means five.

Water Vascular System

Sea Urchin Cross Section

Echinoderms control their movement with the use of a water vascular system, which is a network of water vessels. Ocean water enters the Echinoderm through the sieve plate, which is a calcium filter that prevents large particles from entering. The ocean water moves to the radial canal, which is a pathway that circulates water throughout the Echinoderm's body. From the radial canal, smaller water vessels branch throughout the animal's body, leading to the tube feet. Tube feet move when the Echinoderm alternates its internal water pressure, controlled by ampullae (singular: ampulla). When ampullae are closed, the water pressure increases and the tube feet extend. When ampullae are open, the water pressure decreases and the tube feet withdraw. The Echinoderm water vascular system is a naturally occurring hydraulics system!

What do echinoderms use their hydraulics system for?

Echinoderms use their hydraulics system for movement.

Mutable Collagenous Tissue

A sea star eating a mussel

Echinoderms have mutable collagenous tissue, which can shape the animal into any position and lock. This locking ability means that an Echinoderm saves its energy, especially when eating a shellfish, like the mussel shown above. The Echinoderm just locks onto a shellfish until the shellfish's muscle becomes tired. The Echinoderm then pulls apart the two shells and eats the meat inside. The locking mechanism is controlled by connective tissue called collagen.

What is the advantage of an Echinoderm locking into position?

An Echinoderm locked into position does not use a lot of energy.

Regenerative Body

Most Echinoderms can regenerate lost or damaged body parts as long as part of their nervous tissue is present. In the image above, a sea star is regenerating one of its arms. This regenerative ability is why fishermen who want to lessen the competition from sea stars should not cut them in half.

What happens when a sea star is cut in half?

Both halves of the sea star will grow into a new sea star.