An Online Introduction to the Biology of Animals and Plants





Key Concepts




Section 3

Chapter 8










With the Arthropods, the progression of protostome animal groups hits its end.  Now we go back to the deuterostome  line of animals, which only has two major groups on it:  our group, the chordates, which well get to soon, and the echinoderms, the starfish, sea urchins, and their relatives, a rather odd group to be our nearest relatives.

The echinoderms have a number of odd features:  

They show a five-fold circular pattern called pentaradial symmetry:  in theory, any one-fifth "pie slice" of an echinoderm should have all of the same structures as any other.  It is likely that echinoderms originated as a bilaterally symmetrical group, since their early embryo development has that layout, but they apparently became sessile, which seems to lead to radial symmetry, and retained that pattern even in the descendants who again became crawling browsers, although its odd that crawling animals didnt tend to also evolve head ends and bilateral symmetry (only one echinoderm subgroup has).  Since they have no head end, echinoderms lack central processors or "brains," although processing gets done on a more spread-out system.

They have a unique powering system that depends upon pumping of water into specialized structures.  This water vascular system exerts a lot of what force an echinoderm can exert.  They do have muscles, which are important for control, but they do a lot with tube feet, extendable tubes with suction-cup ends that are used for crawling and holding onto things.

They usually have an endoskeleton that winds up acting more like a shell.  Technically, the skeleton, of calcium salts and proteins (like ours), is inside, but it is usually covered only by a very thin skin.  Even the spines that give the group its name (echinoderm means "spiny skin") are really projections of the skeleton.

Echinoderms are an entirely marine group, with no evidence of there ever being any fresh water or land forms even though many species live in the variable environments of tidal pools.    









The echinoderm phylum is divided into several subgroups:

The sea lilies and feather stars may represent the sessile ancestors of this radial group.  Many have their feathery arms and mouth on top, over a stalk-like body, with a shape somewhat like a hydra.  Many of the feather stars are sort of like sea lilies that have bent over and gone crawling off.

The starfish  are probably the most familiar echinoderms.  These slow-moving animals are often, surprisingly, predators, eating things even slower than they are.  One odd predatory behavior can be seen in starfish that prey upon clams:  the starfish wraps itself around a clam, uses the power of its many tube feet to pry the clam open a tiny bit, and then shoves its stomach into the clam.  Eventually, the digestive juices kill and break down the clam, which then opens easily.  This process used to be interpreted as a "tug of war" that the starfish eventually won.

The brittle stars are similar to starfish but have a more obvious middle disc and much narrower arms attached.

The sea urchins and sand dollars are ball-shaped and disc-shaped, sometimes covered with long movable spines, that generally browse across the bottom.

The sea cucumbers are fat and wormlike creatures that usually eat their way through loose sediments, kind of the same way that earthworms function.  These animals are bilaterally symmetrical, making them the only creatures that begin as bilateral embryos, develop into pentaradial fetuses, and then change into bilateral animals again.






Informational Links



An introductory site with basic info and some nice pictures.






Click on term to go to it in the text.
Terms are in the order they appear.



Deuterostome Groups  
Echinoderm Features  
Pentaradial Symmetry  
Water Vascular System 
Echinoderm Subgroups  
Sea Lilies & Feather Stars  
Brittle Stars  
Sea Urchins  
Sand Dollars  
Sea Cucumbers  





Go On to Next Chapter - Introduction to Chordates






Online Introduction to the Biology of Animals and Plants.

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