Figure 3.

The gastraea, the last common ancestor of living phyla. (a) The gastraea. This cup-shaped animal resulted from simple infolding of the initial blastaea and first cell-type diversification. Colours relate to similar cell types in (b) and (c). (b) Infolding of the amphiblastula larva of calcareous sponges into a cup-shaped transitory stage that resembles a gastrula. The inner cells express endomesodermal markers, after [41,42] and various sources. (c) The initial diversification of metazoan cell types into sensory-contractile cell types and cell types related to feeding. The colour scheme matches the cells of the gastraea in (a). A multifunctional choanocyte-like cell type gives rise to ectochoanocytes, endochoanocytes and kopeocytes (black arrows). These ancestral cells had an apical, undulating cilium or flagellum, propelling water away from the cell, surrounded by a circle of long, contractile, actin-containing microvilli. A well-developed Golgi complex produces lysosomes for intracellular digestion, and a prominent endocytotic machinery mediates phagocytosis (Pc). Mm, mucoid mesh; Fp, filopodia.