38 Reading: Echinoderms
Phylum: Echinodermata
Reading
Echinoderms are coelomate, and deuterostomes.
Echinoderms include sea stars (starfishes), sea urchins, sand dollars, sea cucumbers, and sea lilies. There are 6,000 species of echinoderms; they are all marine.
Although echinoderm adults have radial symmetry, they evolved from ancestors that were bilaterally symmetrical. They have free-swimming, bilateral larvae that metamorphose (change as they mature) into adults with radial symmetry.
The adult body usually has five-part organization.
They possess an internal skeleton (endoskeleton) composed of calcium carbonate plates just beneath the surface of the skin. The plates often bear spines that protrude through the skin.
Echinoderms have numerous tube feet underneath each arm. The tube feet are connected to a system of pipes referred to as the water vascular system. Water enters the system by a sieve plate on the aboral surface. Each tube foot has a fleshy bulb or ampulla attached so that the entire structure looks like a medicine dropper or pipette. When muscles surrounding the ampulla contract, fluid inside the bulb moves down into the tube foot, extending it.
Large digestive glands produce enzymes necessary for digestion. Sexes are separate and gametes are shed into the water. The gonads are large due to the necessity of releasing large numbers of gametes into the marine environment. Coelomic fluid circulates substances and carries amoeboid cells that clean up particulate wastes. Gas exchange is done with numerous tiny gills that extend from the surface of the skin.
The nervous system consists of a central nerve ring with nerve branches extending into the arms. They do not have a brain.
Identify the blastopore. What structure in the adult does this give rise to? Notice that the embryo shows bilateral symmetry yet the adult is radial. The ancestral echinoderms were bilateral and bilateral symmetry is maintained in the larval stage.
Sea Star (Class Asteroidea)
Sea stars and their relatives are composed of a central disk, usually with five or a multiple of five arms (rays) extending outward.
They feed on bivalves (clams) by pulling apart the shell and lowering their stomach into the mollusk, releasing enzymes and digesting the mollusk, then absorbing the digested material into the body of the starfish, where digestive glands in each arm continue the process of food breakdown.
Obtain a preserved sea star for dissection. Identify the oral and aboral surfaces. On the aboral surface, find the sieve plate (madreporite), a structure that allows water to enter the water vascular system. Notice the spines that protrude from the skin. The word “echinoderm” means “spiny-skinned.”
On the oral surface, identify the tube feet and mouth.
Examine the oral surface of a sea star under a dissecting microscope and observe the spines. Most of the fleshy projections are skin gills and are used to absorb oxygen. Claw-like projections called pedicellariae function to cap