sea urchin

merisiili Any of many marine echinoderms, of the class Echinoidea, commonly found in shallow water, having a complex chewing structure named Aristotle's lantern.
1986, Patrick S. Leahy, Chapter 1: Laboratory Culture of Strongylocentrotus purpuratus Adults, Embryos, and Larvae, Thomas E. Schroeder (editor), Methods in Cell Biology, Volume 27: Echinoderm Gametes and Embryos, http://books.google.com.au/books?id=z8h3ThPqv6YC&pg=PA2&dq=%22sea+urchins%22+echinoderm&hl=en&sa=X&ei=zoKAUq3aM8apkgWvwoD4Dg&redir_esc=yv=onepage&q=%22sea%20urchins%22%20echinoderm&f=false page 2,

Of the many species of sea urchins utilized in biological research today the most widely used in North America is probably the purple sea urchin, Strongylocentrotus purpuratus.

The pericentriolar radial complex characteristic of the spermatozoa of other echinoderms is lacking in the spermatozoa of all sea urchins.

Approximately 20% of sea urchins are direct-developers, meaning that they bypass the pluteal larval stage and develop directly into an adult sea urchin.