Phylum Echinodermata

Sea stars, sea urchins, sea cucumbers, sea feathers, etc.

First appearance of an endoskeleton in animals

Underneath epidermis are calcareous ossicles formed by mesoderm tissue

Ossicles are individual and can be attached to each other rigidly or hinged

• Exclusively marine species

• Triploblastic eucoelomates

• Deuterostomes (mouth second)

• Bilateral and radial

• No cephalization

• Radial segments

  
  

Most ancient of all deuterostomes

• Larval body plan: bilateral

• Adult body plan: radial

• Hox genes control the shift in body symmetry from bilateral larvae to radial adult

Echinodermata Water Vascular System:

• Central coelomic cavity transports sea water within the animal

• Ventral surface contains tube feet, which are thin-walled extensions of the epidermis

• Water enters through madreporite (sieve plate), passes through stone canal and into ring canal. Radial canals extend into each arm and connect with the ring canal.

• Ampullae, connected to the radial canals, fill with seawater. Contraction of ampullae extends the tube feet, release of ampullae retracts the tube feet. This allows the animal to coordinate movement.’

Respiratory System:

• Gas exchange occurs through the tube feet

• Oxygen diffuses into tube feet then travels along water vascular system

• CO2 travels along water vascular system then diffuses out of tube feet

Morphology:

• Each arm or section has digestive glands, gonads, nerve cord and tube feet

• Digestive system is complete

• Move on ventral side, anus on dorsal side

• Ingested food moves into cardiac stomach, then into pyloric stomach which moves food into radial canals to each arm.

• Each arm has digestive glands that digest food

• Nervous system is simple nerve ring aligning with ring canal. Radial nerve cords branch from the nerve sing, one cord extends into each arm.

• No organized brain

• Segment nerve pulses into one direction/arm

• Metabolic waste transported via water vascular system, diffuses out of tube feet

• Osmoregulation is not necessary as body fluid is very similar to sea water

• Larvae have metanephridia excretory systems

Reproduction:

• Animals are dioecious

• Either male or female

• Broadcast spawning

• External fertilization

• Reproduce via fragmentation: Crinoidea

5 Echinodermata Classes:

1. Class Asteroidea

   Five thick arms extending from central body

   Sea stars

2. Class Ophiuroidea

   Five long, thin arms extending from central body

   Brittle stars

3. Class Echinoidea

   No arms, spherical shape with tube feet

   Sea urchins and sand dollars

4. Class Crinoidea

   Sea feathers and sea lilies

5. Class Holothuroidea

   Exhibit secondary bilateral symmetry as adults

   Sea cucumber

Class Crinoidea:

• Most ancient

• Arms in multiples of 5

• Mouth on dorsal on sea lilies

• Sessile and motile

• Tube feet collect food and carry it into mouth

• Sea feathers can switch between being sessile and being motile

• Their arms have increased surface area via feather-like modifications

• They swim by waving their arms up and down

Class Asteroidea:

• Sea stars

• Large animal with five thick arms extending from central body

• Predators or scavengers

• Some species “suck” food into their stomach, some species event their stomach and begin digestion externally

• Use tube feet for walking

• Have glands for excreting adhesive carbohydrates and proteins

• Move in a directed way (radial symmetry)

• One arm becomes the leading arm, other arms cooperate in that direction

• Cooperation is coordinated through the nerve ring

• Have light sensing eye spot at end of each arm

• Can regenerate arm or new starfish (not reproduction but defense)

Class Ophiuroidea

• Brittle stars

• Small animal with five thin arms extending from central body

• Much harder endoskeleton than sea stars

• But ossicles in arms are hinged, giving greater flexibility of arms

• Predators

• Food is grabbed by arms, helped by mucus excreted from tube feet which do not have ampullae. Arms move food to mouth.

• To walk or swim they have one leading atm, two arms on each side sweep back and forth. Creates jerky walking/swimming motion

• Arms have spine projections made of ossicles, creates traction

• Arms have nerve ganglia along each arm, allows better coordination between arms

• Most species have light intensity sensing cells

• Allows animal to sense light/shadows from a specific direction

• They move fast so need more complex sensory system

Class Echinoidea

• Sea urchins (regular echinoids) and sand dollars (irregular echinoids)

• Flattened spherically shaped

• Have fixed lattice of ossicles which form a fused body called a “test”

• Test is covered in spines

• Sea urchins are browsing herbivores

• They move slowly

• Scrape seaweed from rocks using movable ossicles called Aristotles Lantern

• 5 jaws and 1 tooth in each jaw, connected by ligament

• Sand dollars are burrowing deposit feeders

• Have tube feet modified for burrowing

• In place of arms there are five ambulacral grooves where tube feet are located

• Use tube feet for walking but water vascular system is not as important due to the rigid test, water always retained in animal

• Tube feet have ampullae

• Hard bodied urchins can not regenerate

• More difficult to regenerate

Class Holothuroidea

• Sea cucumbers

• Secondarily bilaterally symmetrical (radial)

• They essentially lay on one side and have elongated body axis extending from mouth to anus

• Endoskeleton is very reduced with only a few internal ossicles, no external spines

• Have tube feet in grooves that run along the main body axis

• Have specialized tube feet anterior end that form long tentacles

• They are deposit feeders and use tentacles to gather food

• Respiration takes place in respiratory tree which branches from the intestines

• They “breathe” through their anus by taking water into their anus to fill respiratory tree

• Can not fragment/regenerate