Professor, PhD, DSc
Department of Invertebrate Zoology, Saint Petersburg University
& Department of  Paleontology, University of Vienna

Abstract

Placentation has evolved multiple times among both chordates and
invertebrates. Although they are structurally less complex, invertebrate
placentae are much more diverse in their origin, development and
position. Aquatic colonial suspension-feeders from the phylum Bryozoa
acquired placental analogues multiple times, representing an outstanding
example of their structural diversity and evolution. Among them, the
clade Cyclostomata is the only one in which placentation is associated
with viviparity and polyembryony—a unique combination not present in
any other invertebrate group. Yet, cyclostome nutritive tissue is
currently the only known example of a coenocytic placental analogue that
was previously not described among animals. Finally, evolution of the
cyclostome placenta, involving transformation of the hydrostatic
apparatus (membranous sac) and change of its function to embryonic
nourishment, is an example of exaptation that is rather widespread among
matrotrophic bryozoans.