Colonial Theory - Metazoa
This is the most popular theory on the origin of Metazoa. The idea was conceived by Haeckel (1874) modified by Metschnikoff (1886) and revived by Hyman (1940). This theory considers the flagellates to be the most probable ancestral group of metazoans. For support of this theory the following evidences are cited:
1) Flagellated spermatazoa occur throughout the metazoan series.
2) Monoflagellated cells (with a single flagellum) are also common in lower metazoans (especially among sponges and coelenterates).
3) True sperms and eggs (feature of metazoans) are present in phytoflagellates.
4) Phytoflagellates also display a type of colonial organization that could have led to a multicellular construction. In fact, differentiation between somatic and reproductive cells has been attained in Volvox. Although Volvox is often used as a model for the flagellated colonial ancestor, it is not the likely ancestor of metazoans. Ultrastructural evidence points to the choanoflagellates (a small group of animal like flagellates) as the probable ancestor. Choanoflagellates have mitochondria and flagellar structures very similar to those in metazoan cells. Also choanocytes i.e. cells with a collar of microvilli are found in a number of groups of metazoans notably the sponges.