The Collaborative International Dictionary of English v.0.48:

Pluteus \Plu"te*us\, n.; pl. L. Plutei, E. Pluteuses. [L., a
   shed.] (Zool.)
   The free-swimming larva of sea urchins and ophiurans, having
   several long stiff processes inclosing calcareous rods.
   [1913 Webster]