How did multicellular animals arise?
Three major hypotheses are usually presented for the presence of metazoans: 1. Syncytical ciliate hypothesis — metazoans arose from an ancestor in common with the ciliated protozoans. (see text) • Colonial flagellate hypothesis — metazoans arose from a colonial, spherical ancestor that possessed flagellated cells. • Polyphyletic hypothesis — metazoans arose from several ancestral groups, resulting in the divesity seen today. Current biochemical evidence suggests that the colonial flagellate hypothesis should receive the most support for the origin of metazoan animals. PHYLUM MESOZOA Small, ciliated, worm-like animals that live as parasites in marine invertebrates. Each animal consists of 20-30 cells. Since they are parasitic, it is difficult to determine their phylogenetic heritage. PHYLUM PORIFERA The sponges are multicellular organisms that lack true tissue organization (no true tissues, no true organs). They are sessile animals that utilize the water currents to provide them with
