Definition of "sea urchin" [sea• ur•chin]

• Any of various echinoderms of the class Echinoidea, having a soft body enclosed in a round, symmetrical, calcareous shell covered with long spines. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "FIGURE 9.3 Schematic drawing of a developmental gene regulatory network for sea urchin endomesoderm."
• "Kernels in general can be expected to have a degree of complexity similar to that for sea urchin endomesoderm, so we can infer that other kernels also were designed."