Phylum Mesozoa Phylum Placozoa Phylum Porifera: Sponges Mesozoa and Parazoa Phylum Mesozoa Phylum Placozoa Phylum Porifera: Sponges

Muticellularity Organisms larger than unicellular organisms arose as aggregates of cells Organisms made of many cells have advantages over a large single cell because surface area decreases as the size of a cell increases Dividing into smaller units increases surface area

Multicellular Having or consisting of many cells or more than one cell to perform all vital functions.

Metazoa Multicellular animals typically divided into three grades: 1. Mesozoa (a single phyla) 2. Parazoa (phylum Porifera, sponges and phylum Placozoa 3. Eumetazoa (all other phyla of animals)

Animal Phyla

Mesozoans Cellular level of organization Composed only of an outer layer of somatic cells and an inner layer of reproductive cells Entirely parasitic

Placozoans Composed of two epithelia with fluid and some fibrous cells in between

Trichoplax is the sole member of the placozoan phylum

Placozoan digestion

Sponges (Poriferans) Relatively more complex, with several types of cells differentiated for various functions Have a unique system of water currents which bring food and oxygen to the interior of the sponge

Mesozoa, Placozoa and Poriferans Develop differently than other phyla of animals They do not have germ layers (endoderm, mesoderm and ectoderm)

Syncytical Ciliate Hypothesis Supporters believe that metazoans evolved from an ancestor shared with the single-celled ciliates which developed multiple nuclei Similar to modern ciliates Had bilateral symmetry, does not explain how some animals developed radial symmetry

Colonial Flagellate Hypothesis First proposed by Haeckel in 1874 This idea says that the ancestor of metazoans were spherical, colonies of flagellated cells Individual cells are believed to have become differentiated for specific functions

Polyphyletic Origin Suggests that the sponges, cnidarians and ctenophores and other eumetazoans evolved independently of each other instead of having a common ancestor

Phylum Mesozoa Tiny, ciliated, wormlike animals with extremely simple level of organization. All are parasites in marine invertebrates Most are only 0.5 to 7 mm in length Have only 20-30 cells arranged in two layers Two classes: 1. Rhombozoa 2. Orthonectida

Rhombozoans Live in kidneys of benthic cephalopods (bottom dwelling octopuses, cuttlefishes and squids Adults are called vermiform, reproduce asexually when cells give rise to vermiform larva When crowded they form sperm and egg which unite into an infusoriform larvae unlike the adult

Orthonectids Parasites on a variety of invertebrates, such as brittle stars, bivalve molluscs, polychaetes and nemerteans Reproduce sexually and asexually The asexual stage is a multinucleated mass called a plasmodium, which divides to form males and females

Phylogeny of Mesozoans Believed to possibly have a common ancestor with flatworms

Phylum Placozoa Contains only one species; Trichoplax adhaerens Tiny (2-3 mm) marine organisms The body is plate like and has no symmetry, organs, muscles or nervous system Composed of two layers of epithelium with fluid between Glide over food releasing digestive enzymes, then absorb the nutrients