Introduction to Echinoderms
Bivalves have bilateral (mirror-image) symmetry, while adult starfish possess radial symmetry. Various species of starfish, along with the sea urchins ( UR -chins), belong to a phylum of invertebrates called the Echinoderms (ih- KY -nuh-derms). The Echinoderms are a group of small sea animals covered by a spiny “skin” (derm) that forms a hard protective shell. This phylum gets its name from the numerous spines on the surface of the animals, which give them the appearance of a sea-dwelling “hedgehog” (Echino-). The sea urchins, in particular, look much like real “hedgehogs” (urchins)! The sea urchins (Figure 10.6, A) are a group of small, round Echinoderms with hard shells consisting of moveable, calcium-hardened spines. Like a hedgehog, the animal curls up and projects its sharp spines as a protective reaction when it is being attacked by a hungry predator.
The starfish (not being fish), are more accurately called sea stars. Like the sea urchins, the sea stars possess radial symmetry in their adult form. Interestingly enough, however, all the Echinoderms grow into radially symmetrical adults from larvae ( LAR -vee) or immature “ghosts” (larva) that are bilateral and free-swimming. The conversion of immature larvae with bilateral symmetry into mature Echinoderm adults with radial symmetry apparently reflects an adaptation to a more-or-less stationary and slow-moving existence on the ocean floor.
Water Vascular System
The sea star consists of a central body, with five arms radiating out like spokes from the hub of a wheel (Figure 10.6, B). Its inferior mouth opens into a digestive tract. Besides its prickly skin, the sea star shows another feature unique to the Echinoderms: a water vascular ( VAS -kyoo-lar) system . This vascular system is contained within the animal’s coelom (main body cavity). A small sieve plate, present on the superior surface of the animal, filters seawater (like a sieve with holes) just before it enters the sea star and flows down a short tube. The tube connects to a central ring canal , and from there into individual radial canals, which circulate the filtered water into each prickly arm.
Besides acting as a circulatory system for nutrients and waste products, the water vascular system serves as a hydrostatic ( HIGH -druh- stat -ik) skeleton . The “water” ( hydro -) within the vascular canals provides a “steady” ( static ) pressure. This hydrostatic pressure allows the sea star to use the suckers on its hundreds of tube feet to push down and slowly drag its prickly body across the ocean floor. The water-driven pressure is even great enough to allow the star to raise its arms and grasp prey, such as sponges, mollusks, and oysters.
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