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.

Response and Movement

Nervous and sensory systems are generally poorly developed in echinoderms. There is a simple nervous system sensitive to temperature, light, and vibrations. They lack specialized sense organs nose, eyes, and ears. No brain, or head but they do have a nerve net and a nerve ring. Their nerve cells can detect light and touch. Starfish have eyespots (which are a cluster of nerve cells) at the tips of each arm that sense each ray for light.
The water-vascular system supplies water through canals of small muscular tubes to the tube feet (ambulacral feet). As the tube feet press against a moving object, water is withdrawn from them, resulting in a suction effect. When water returns to the canals, suction is released. Sea stars, sea cucumbers, and sea urchins move by alternately extending and retracting groups of tube feet, gripping with the suction cups and pulling themselves along. The resulting locomotion is generally very slow, but effective.

Respiration

Echinoderms lack respiratory systems. Tube feet and skin gills are used for gas exchange.
Tube feet immensely contribute to the respiratory function in most echinoderms; the tissue’s outer thin wall forms the main respiratory surface. The paper-like wall is suitable for the diffusion of oxygen into the body cavity and the diffusion outward of carbon dioxide and wastes. However, many groups have developed skin gills (auxiliary respiratory structures).

Reproduction!

Echinoderms have separate sexes, male or female, although some are hermaphroditic. The males and females discharge their eggs and sperm through broadcast spawning into the water where they are fertilized. Fertilization is external; the gametes are simply shed into the water. A female can release one hundred million eggs at once. The floating embryo develops into a ciliated, free-swimming, bilaterally symmetrical larva, which undergoes radical metamorphosis into the radial symmetrical adult. In some species, the whole digestive system is changed during metamorphosis, and the organism acquires a new mouth and anus. Regeneration is achieved by means of asexual binary fission. If a piece of certain echinoderms is cut off or injured, their great ability to regenerate quickly creates a new body part or even a new echinoderm can grow.

Feeding

This phylum is known for its diversity in its feeding. Echinoderms are carnivores, herbivores, filter feeders, and detritus feeders. All echinoderms are heterotrophs (including active, omnivorous, scavenging, selective predators and mud swallowing). Many carnivorous echinoderms, like the sea star, use their tube feet to force open shells, particularly mussels, clams and scallops. Once the shell has been pried open, the echinoderm expels its stomach out of the mouth, releasing enzymes into the preys open cavity. The enzymes then liquefy the meat, and the prey is digested in its own shell. The herbivorous sea urchin scrapes algae from rocks with their five-part jaw arranged in a structure known as "Aristotle's lantern." Many echinoderm filter feeders, such as sea lilies and basket stars feed on small suspended organisms that are floating in the water or the bottom of the ocean floor. The sea cucumber, a detritus feeder slowly moves across the ocean floor ingesting a combination of detritus and sand, and digest the organic material leaving the sand grains behind.

Habitat and Adaptations

Echinoderms are marine, although few can survive being washed on the shore by a tide for several hours. They are the only phylum with no freshwater or land animals. There is no fossil evidence that shows any exception to this. Within the marine waters echinoderms can survive in a variety of habitats. The habitat use of echinoderms also depends on the area where their communities are developed. One study counted twenty-eight species on the rocky bottom while thirty-eight species were found on the sandy bottom. Many echinoderm species such as sea stars or sea cucumbers cling to rocks and therefore prefer rocky areas, while other echinoderm species prefer it to be sandy where they can burry themselves in order to remain concealed. There are also many echinoderms that enjoy living in mud. In addition, echinoderms are known to commonly use other animals as homes such as sponges or the skin of other fishes. Echinoderms are found in all marine waters except the Arctic. Within the marine waters, echinoderms are located in different areas depending on their preferred temperatures. Sea cucumbers have adapted to face predators without a calcium carbonate endoskeleton. When threatened, they are able to release all internal organs into their surroundings. This confuses the predator and gives the sea cucumber an escape route. Many echinoderms are sand, coral or reef coloured to camoflauge themselves in their habitat.

General Anatomy

Echinoderms are characterized by radial symmetry, radiating from a central body (pentamerous). It can be expressed in different body forms such as five arms, a five-part jaw, and five radial canals. The body consists of five equal segments, each containing a duplicate set of various internal organs. They have an endoskelteon that is made from hard calcium carbonate (CaCO3). They have no heart, brain, or eyes, but some brittle stars have light sensitive areas on their arms. Their mouth is situated on the underside and their anus on upper body (except feather stars, sea cucumbers and some urchins).Echinoderms have tentacle-like structures called tube feet with suction pads situated on their extremities. They are many small, flexible, fluid filled tubes that project from the body and are used in locomotion, feeding, gas exchange, and nitrogen excretion. These tube feet are hydraulically controlled by a remarkable water-vascular system. The madreporite is a sieve-like opening on the aboral surface of a sea star through which water enters the water vascular system and connects to a circular ring canal. Five radial canal branch from the ring canal and a portion of the sea star’s water vascular system that runs along inside the ambulacral ridge in each arm. The body is covered by many ossicles, which are small calcium carbonate plates that forms the endoskeleton.