Caducous

from WordNet (r) 3.0 (2006)

caducous
    adj 1: shed at an early stage of development; "most amphibians
           have caducous gills"; "the caducous calyx of a poppy"
           [syn: {caducous}, {shed}] [ant: {lasting}, {persistent}]

from The Collaborative International Dictionary of English v.0.48

Caducous \Ca*du"cous\, [L. caducus falling, inclined to fall,
   fr. cadere to fall. See {Cadence}.] (Bot. & Zool.)
   Dropping off or disappearing early, as the calyx of a poppy,
   or the gills of a tadpole.
   [1913 Webster]