The opposite idea that all larvae are secondary
has been championed by many authors (e.g.,
Garstang 1922 ; de Beer 1954 ; Hadži 1955 ;
Steinböck 1963 ; Conway Morris 1998 ; Valentine
and Collins 2000 ; Collins and Valentine 2001 ;
Hadfi eld et al. 2001 ), although often without a
precise reference to a specifi c node in the metazoan
tree.