The ancestral
opisthokont cell is assumed to have possessed slender filose (thread-like) projections or 'tentacles'. In some opisthokonts (Mesomycetozoa and
Corallochytrium) these were lost. They are retained in Filozoa, where they are simple and non-tapering, with a rigid core of
actin bundles (contrasting with the flexible, tapering and branched
filopodia of nucleariids and the branched
rhizoids and
hyphae of fungi). In choanoflagellates and in the most primitive animals, namely
sponges, they aggregate into a filter-feeding collar (made from
microvilli, that are also made from actin) around the
cilium or
flagellum; this is thought to be an inheritance from their most recent common filozoan ancestor. ==References==