Introduction

The phylum Echinodermata consists of five main classes: Asteroidea (sea stars), Ophiuroidea (brittle stars), Echinoidea (sea urchins and sand dollars), Holothuroidea (sea cucumbers) and Crinoidea (feather stars). The word chinoderm is derived from the Greek origin meaning “spiny skin,” in reference to the plate-like calcareous endoskeleton of these animals. Penta-radial symmetry is an eminent characteristic in echinoderms. The water-vascular system aids in many life functions of the Echinoderms. Echinodermata is a very diverse phylum of marine species.

Interesting Facts!

• Echinoderms are intricate parts of their ecosystems, many are keystone predators and without them a population boom/decline can occur in species lower in the food chain and emensly change their habitat.



• Sea urchin eggs are also edible and often served in sushi bar



• 500 mollion years ago when echiniderms first appeared they were sessile



• 6000 species of echinoderms

Excretion

Echinoderms do not have a specialized excretory system. Excretion is accomplished by simple diffusion of metabolic wastes (ammonia) across thin permeable regions of the body wall. The excretion of solid wastes is processed through the anus. Brittle stars are a exception because they have a blind gut, therefore excrete solid wastes through the mouth.

Connections to other Phylums

Echinoderms are the most closely related to the major phyla Chordates. It may not seem so at first, but looking closely at embryological evidence, it is believed that chordates and echinoderms shared a common relative. This relative is believed to have been something along the lines of sea urchin - an organism a bit larger than a golf ball with spines extending radially. In addition, Echinoderms and Chordates are both deuterostomes.

Digestion

Echinoderms have a complete gut, this includes a mouth and an anus. First, the food source is ingested from the mouth, lead through the esophagus and broken down in two different stomachs. Cardiac stomach is portion of an echinoderm’s stomach that is extruded through the mouth during feeding. The pyloric stomach is connected to the digestive glands and short intestine. The cavity contains large phagocytic cells that function in the transport of food and the storage of insoluble wastes. After digestion, the feces leave through the anus.

Vocabulary

Water Vascular System: the internal network of fluid-filled canals involved in feeding, respiration, internal transport, excretion and movement

Monoecious: A term to distinguish sexes are separate

Ossicle: One of the small calcium carbonate plates that forms the endoskeleton of echinoderms

Pedicellaria: Small pincher-like structure on the surface of certain echinoderm which clean the body’s outer surface

Tube Feet: many small, flexible, fluid filled tubes that project from the body of an echinoderm and are used in locomotion, feeding, gas exchange, and nitrogen excretion

Madreporite: Sieve-like opening on the aboral surface of a starfish through which water enters the water vascular system

Ganglion: A mass of nerve cells

Stone Canal: Part of the water vascular system that connects the madreporite and ring canal

Ampulla: Bulb-like sac at the base of the tube foot which controls water entering and leaving the foot

Cardiac stomach: Portion of an echinoderm’s stomach that is extruded through the mouth during feeding

Hemolymph: The fluid that is circulated through the body of an animal with an open circulatory system

Hemocoel: The hemolymph-filled space or body cavity of some invertebrates

Bipinnaria: Winged larva seen in echinoderms

Pyloric stomach: In a sea star, the stomach that is connected to the digestive glands

Skin gills: One of the many hollow tubes that project from the surface of a sea star through which gas exchange and nitrogen excretion takes place

Nerve ring: Nerve that encircles the mouth in a sea star

Deuterostome: Organism whose embryonic blastopore becomes the anus

Ambulacral groove: Groove on the oral surface of a sea star that holds the tube feet

Radial nerve: Nerve that runs along inside the ambulacral ridge in each arm of a starfish

Radial (ambulacral )Canal: Portion of the sea star’s water vascular system that runs along inside the ambulacral ridge in each arm

Internal TRansport and Circulation

Internally, echinoderms have a thickened vessel which acts as the heart, pumping blood through a closed circulatory tract. In the body cavity, circulation occurs in an open system of channels and sinuses, which is lined with flagellated cells that create an internal current. The water-vascular system consists of a circular passageway, the ring canal, which surrounds the digestive tract and five radial canals that radiate from the ring canal like spokes of a wheel. Each radial canal underlies an ambulacral area. The ring canal is usually connected to a porous plate in the body wall, the madreporite (by a lime-walled tube called the stone canal). The position of the madreporite varies in the different groups. Seawater enters the system through the madreporite, which is regulated by the animal. Short lateral canals equipped with valves lead from the radial canals into the tube feet. Generally a muscular, water-filled bulb, the ampulla, is connected to each tube foot. When the valve closes and the ampulla contracts, water is squeezed into the tube feet, causing the feet to extend. Feet are retracted by the contraction of the attached muscles, thereby forcing the water back into the ampulla.