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

2. 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

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

4. 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)

5. Animal Phyla

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

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

8. Trichoplax is the sole member of the placozoan phylum

9. Placozoan digestion

11. 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

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

13. 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

14. 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

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

16. 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 cells arranged in two layers
Two classes:
1. Rhombozoa
2. Orthonectida

17. 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

18. 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

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

20. 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