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~6500 species
 

Characteristics
"All animal phyla are unique, but some are more unique than others"  (Nichols - "The Uniqueness of Echinoderms", 1976)

Echinoderms are fairly easy to describe b/c characteristics are so unique.  Echinoderms have a very different body plan.

• pentamerous radial symmetry.  What would you predict about the lay out and sophistication of the nervous system?

• water vascular system

 

• calcareous ossicles forming endoskeleton of interlocking plates and spines.

• connective tissue mutable (stiffness and fluidity changes rapidly under nervous control)

• respiratory, circulatory, and excretory typically unsophisticated or absent

• dipleurula larvae  ciliated, planktonic, bilaterally symmetrical larva

Generalized dipleurula larva of echinoderm (upper left)
and ophiuronid, asteroid echinoid, holothuroid larvae

Classes

Class Crinoidea
   sea lillies and feather stars

Oral side is upward supported by stalk or claws (cirri).

How do they feed?
 

 
 

Much more diverse in fossil record.

Class Stelleroidea
   seastars and brittle stars.

• Arms arrayed star-like around central disc (not stalked)

Previously consider as seperate classes, but fossil record and unique gene inversion unite these into single class.

Subclass Ophiuroidea the brittle stars
long, slender, fragile arms (with vertebral ossicles for flexibility)
 

 
 

No intestine or anus, and solid waste is extruded through the mouth. Typically deposit feeders.

basket star

SubClass Asteroidea Sea stars,

Arms are not distinct from the body both externally and internally
    

pedicellaria
 

Tend to take larger prey than brittle stars.

• 2 concentric water-vascular rings.
• tube feet around the edge of the disk.
• Only 2 species found on wood in deep waters; discovered in 1986

Class Echinoidea
   sea urchins, heart urchins, and sand dollars

• without arms, often though with spines
  
• ossicles form of tightly fused skeletal plates (ridgid test)

 

 

Aristotle's lantern (from left to right: internal view,  aboral view, and close-up of tooth)
 

Class Holothuroidea
     Sea cucumbers

• Worm-shaped body elongated along aboral-oral axis
• Reduced microscopic ossicles floating in body wall
• Respiratory tree

 

 

Phylogeny
 

Molecular evidence supports the conventional phylogeny of echinoderm classes where crinoids are the oldest living class of echinoderms.
 

Hemichordata, Echinodermata, and Chordata as a monophyletic group of deuterostomes appear to be withstanding molecular sequencing examination.

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