Echinoderm

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