Echinoderms

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Phylum Echinodermata
Characteristics

• All marine

• Known as spiny-skinned animals

• Endoskeleton known as the test is made of calcium plates or ossicles with protruding spines

• Includes sea stars, brittle stars, sand dollars, sea urchins, & sea cucumbers

• Undergo metamorphosis from bilateral, free-swimming larva to sessile or sedentary adult

• Larval stage known as dipleurula or bipinnaria

• Adults have pentaradial ( 5 part) symmetry

• Lack segmentation or metamerism

• Coelomate

• Breathe through skin gills as adults

• Capable of extensive regeneration

Bipinnaria Larva

• Ventral (lower) surface called the oral surface & where mouth is located

• Dorsal (upper) surface known as aboral surface & where anus is located

• Have a nervous system but no head or brain in adults

• No circulatory, respiratory, or excretory systems

• Have a network of water-filled canals called the water vascular system to help move & feed

• Tube feet on the underside of arms help in moving & feeding

• One-way digestive system consists of mouth with oral spines, gut, & anus

• Deuterostomes (blastopore becomes the anus)

• Separate sexes

• Reproduce sexually & asexually

• Includes 5 classes:
            * Crinoidea - sea lilies & feather stars
            * Asteriodea - starfish
            * Ophiuroidea - basket stars & brittle stars
            * Echinoidea - sea urchins & sand dollars
            * Holothuroidea - sea cucumbers

Class Crinoidea
Characteristics

• Sessile

• Sea lilies & feather stars

• Have a long stalk with branching arms that attach them to rocks & the ocean bottom

• Can detach & move around

• Mouth & anus on upper surface

• May have 5 to 200 arms with sticky tube feet to help capture food (filter feeders) & take in oxygen

• Common in areas with strong currents & usually nocturnal feeders

Class Asteroidea
Characteristics

• Usually sedentary along shorelines

• Starfish or sea stars

• Come in a variety of colors

• Prey on bivalve mollusks such as clams & oysters

Starfish Feeding on Clam

• Have 5 arms that can be regenerated

• Arms project from the central disk

• Mouth on oral surface (underside)

STARFISH

Class Ophiuroidea
Characteristics

• Largest class of echinoderms

• Includes basket stars & brittle stars

• Live on the ocean bottom beneath stones, in crevices, or in holes

• Have long, narrow arms resembling a tangle of snakes

• Arms readily break off & regenerate

• Move quicker than starfish

• Feed by raking in food with arms or trapping it with its tube feet

Class Echinoidea
Characteristics

• Includes sea urchins & sand dollars

• Internal organs enclosed by endoskeleton or test made of fused skeletal plates

• Body shaped like a sphere (sea urchin) or a flattened disk (sand dollar)

• Lack arms

• Bodies covered with movable spines

• Have a jawlike, crushing structure called Aristotle's lantern to grind food

• Use tube feet to move

• Sea Urchins:
          * Spherical shape
            * Live on ocean bottom
            * Scrape algae to feed
            * Long, barbed spines make venom for protection

• Sand Dollars:
          * Flattened body
            * Live in sand along coastlines
            * Shallow burrowers
            * Have short spines

Class Holothuroidea
Characteristics

• Includes sea cucumber

SEA CUCUMBER

• Lack arms

• Shaped like a pickle or cucumber

• Live on ocean bottoms hiding in caves during the day 

• Have a soft body with a tough, leathery outer skin

• Five rows of tube feet run lengthwise on the aboral (top) surface of the body

• Have a fringe of tentacles (modified tube feet) surrounding the mouth to sweep in food & water

• Tentacles have sticky ends to collect plankton

• Show bilateral symmetry

• Can eject parts of their internal organs (evisceration) to scare predators; regenerate these structures in days

Structure & Function of Starfish
Body Plan

• Range in size from 1 centimeter to 1 meter

• Mouth located on oral surface (underside)

• Have an endoskeleton made of calcium plates

• Sharp, protective spines made of calcium plates called ossicles found under the skin on the aboral (top) surface

ABORAL SURFACE

• Have pedicellariae or tiny, forcep-like structures surrounding their spines to help clean the body surface

Water Vascular System

• Network of canals creating hydrostatic pressure to help the starfish move

WATER VASCULAR SYSTEM

• Water enters through sieve plate or madreporite on aboral surface into a short, straight stone canal

• Stone canal connects to a circular canal around the mouth called the ring canal

• Five radial canals extend down each arm & are connected to the ring canal

• Radial canals carry water to hundreds of paired tube feet

TUBE FEET

• Bulb-like sacs or ampulla on the upper end of each tube foot contract & create suction to help move, attach, or open bivalves

• Rows of tube feet on oral surface (underside) are found in ambulcaral grooves under each arm

Tube Feet in Ambulcaral Grooves

Feeding & Digestion

• Tube feet attach to bivalve mollusk shells & create suction to pull valves apart slightly

• Starfish everts (turns inside out) its stomach through its mouth & inserts it into prey

• Stomach secretes enzymes to partially digest bivalve then stomach withdrawn & digestion completed inside starfish

Other Body Systems

• No circulatory, excretory, or respiratory systems

• Coelomic fluid bathes organs & distributes food & oxygen

• Gas exchange occurs through skin gills & diffusion into the tube feet

• No head or brain

• Have a nerve ring surrounding the mouth that branch into nerve cords down each arm

• Eyespots on the tips of each arm detect light

• Tube feet respond to touch

Reproduction

• Separate sexes

• Two gonads (ovaries or testes) in each arm produce eggs or sperm

• Have external fertilization

• Females produce up to 200,000,000 eggs per season

• Fertilized eggs hatch into bipinnaria larva which settles to the bottom after 2 years & changes into adult

• Asexually reproduce by regenerating arms