echinopædium

from The Century Dictionary.

• noun The early larval stage of an echinoderm: a name given by Huxley to the primitive generalized type-form of the Echinodermata, illustrated by the bilaterally symmetrical embryonic stage of nearly all members of that class. See the extract.
• noun Synonyms Echinopædium, Pluteus. Echinopædium is the more general term, used by its proposer to cover any embryonic or larval stage of any echinoderm from the gastrula stage to the assumption of its specific characters. A pluteus is a special pluteiform larva of some echinoderms, as the holothurians, ophiurians, and echinids proper.