Harry, I never knew that about Chamidae.  Is this true of all Chamidae, or
just Arcinella?  I have seen some extremely tiny Chama shells (under 1 cm)
already attached to larger shells, and looking like typical Chama.  How small
is the Irus-like stage you mentioned?  And how large are Arcinella when they
start looking like Arcinella?
     Andy offered a very plausible theory for the difference in sculpture
between juvenile and mature burrowing bivalves.  However, the same phenomenon
is also seen in many non-burrowing gastropods (sculpture on the early whorls,
with no sculpture or reduced sculpture on the later whorls).  Then again,
there are gastropods AND bivalves which reverse this pattern, having smooth
early whorls and sculptured later whorls.  Probably the principle purpose of
this arrangement is to confuse and frustrate conchologists.  I can't think of
any other reasonable explanation.
     One of my favorite examples of "dichotomous metamorphoses", as Harry
describes the process, is the west coast "rock scallop" Hinnites, which starts
life as a typical free-swimming Chlamys-like "scallop" shell, but later
attaches itself permanently to rocks, and grows into a heavy, misshapen
oyster-like thing.  But of course the all time champions of metamorphosis are
the bivalves of the family Clavagellidae, the "watering pots", in which the
juvenile looks like a typical smooth little clam, which however subsequently
grows into a long, hollow tubelike structure, often with sand grains and other
miscellaneous objects cemented into it, and a bizarre, frilly "head" at one
end, perforated with tiny holes.  Now that's metamorphosis!
Paul M.