Key to Class Echinoidea  (Urchins and Sand Dollars)

Phylum Echinodermata

Taken primarily from  Kozloff, 1987, 1996  (Copyright 1987, 1996, University of Washington Press.  Used in this web page by permission of University of Washington Press)
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1a  A sand dollar--body disk-shaped, its diameter several times greater than the thickness
Dendraster excentricus
1b Body not disk-shaped, its diameter not more than twice the thickness 2
2a A heart urchin--the body nearly heart-shaped when viewed from the oral or aboral side; ambulacra in distinct furrows, one of which is much deeper than the others; symmetry essentially bilateral, the mouth at one end of the oral surface (close to the deepest ambulacral furrow) and the anus at the margin of the test on the opposite side Brisaster latifrons
2b Sea urchins--body tomato-shaped, circular in outline when viewed from the oral or aboral side; ambulacra not in distinct furrows; symmetry almost perfectly radial, the mouth at the center of the oral surface and the anus at the center of the aboral surface 3
3a Prevailing general color greenish or whitish, definitely not red, purple, or orange-pink 4
3b Prevailing color red, purple, or orange-pink 5
4a Prevailing color of spines whitish, although some, especially near the center of the aboral surface, may be greenish or reddish; ridges on the spines with nearly flat surfaces and without obvious sculpturing; tube feet usually either lighter than the spines or about the same color, rarely much darker; diameter of test not often greater than 6 cm (but sometimes nearly 8 cm); cleaned test mostly white, but with light green to reddish tints in the region of the periproct; primary spines with 17-26 lengthwise wedges; usually with 6 or 7 pore pairs in each arc; oral spines white, with reddish brown or brown tips, the color persisting for several days after specimens have been preserved in formalin; strictly subtidal and usually at depths greater than  30 m Strongylocentrotus pallidus
4b Prevailing color of spines pale green; ridges on spines with rounded surfaces and with periodic sculpturings (these somewhat fan-shaped); tube feet darker than the spines, and usually purple; diameter of test frequently more than 6 cm (maximum about 8 cm); cleaned test decidedly greenish; primary spines with 26-36 lengthwise wedges; with 5 or 6 pore pairs in each arc; oral spines slightly purplish, sometimes with white tips, becoming distinctly purple after specimens have been preserved in formalin; intertidal and subtidal
Strongylocentrotus droebachiensis
5a Prevailing color of living animal orange-pink; cleaned test pale orange-pink, extremely fragile; subtidal and mostly at depths greater than 100 m Allocentrotus fragilis
5b Prevailing color of  living animal red or purple; cleaned test usually gray or pale purple not extremely fragile; intertidal and subtidal 6
6a Prevailing color usually bright red, reddish purple, or maroon, although the larger spines of lighter individuals may be rose and the smaller spines may be almost white; diameter of test up to about 15 cm; spines up to 7 cm long; intertidal and subtidal (Strongylocentrotus franciscanus in original key) Mesocentrotus franciscanus
6b Prevailing coloration purple; diameter of test not exceeding 9 cm; spines rarely more than 2.5 cm long; largely intertidal on rocky shores that have considerable wave action, but subtidal to some extent
Strongylocentrotus purpuratus

Taxonomic Levels Represented in This Key:

  Dendraster excentricus
  Mesocentrotus franciscanus
  Strongylocentrotus droebachiensis
  Strongylocentrotus pallidus
  Strongylocentrotus purpuratus

Page created by Heidee Leno, 7-26-2002
Edited by:  Dave Cowles 8-2002, 2005, 2006-