Exercise 2a - Kingdom Protista and Animal Classification

• Kingdom Protista is a paraphyletic group of eukaryotic, mostly unicellular organisms.
• One proposed mechanism called the syncytial ciliate hypothesis proposes that animals arose from a syncytial (multinucleate) ciliated protest which later evolved cell boundaries to form a multicellular animal.
• Another mechanism, which is more accepted by zoologists, is called the colonial flagellate hypothesis which states that animals descended from a colony of flagellated protists whose individual cells became differentiated for specialized roles.
• Among the protists, it is believed that the closest unicellular relative (or sister group) of the animals are the choanoflagellates.
• Animals in the Parazoa lack true tissues and organs. Symmetry is radial or lacking. The Parazoa contains a single phylum, Porifera.
• Phylum Porifera (sponges): These animals are predominantly marine organisms that are attached to a substrate.
• Some sponges are also found in freshwater environments. They feed by filtering water through choanocytes (flagellated internal collar cells).
• Water enters sponges through microscopic incurrent openings (ostia, sing = ostium).
• The simpler species may be base-shaped, with a single excurrent opening (osculum, pl = oscula), but the larger and more complex sponges are asymmetric and have numerous excurrent oscula.
•  Many sponges secrete glassy or calcerous skeletal structures (spicules) some (such as the bath sponges) lack these skeletal elements and instead have skeletons composed of flexible proteinaceous material (spongin).
• Animals in the Eumetazoa are composed of definite tissues and organs; symmetry may be radial or bilateral; alimentary tract (if present) may have one opening (mouth) or two openings (mouth and anus); bodies may be comprised of 2 (ectoderm and mesoderm) or 3 (ectoderm, endoderm and mesoderm) basic embryonic tissue types.
• These are two main branches in the Eumetazoa, the Radiata and Bilateria.
• Animals from the Radiata show primary radial symmetry and generally are diploblastic (composed of 2 main embryonic tissue layers, an outer ectoderm.
• the inner and outer cell layers are separated by a thick or thin, secreted, largely or completely noncellular gelatinous matrix called the mesoglea.
• The two phyla of the Radiata branch are the Ctenophora and the Cnidaria.
• BILATERIA. Animals show primary bilateral symmetry and are triploblastic.
• There are two main subgroups in the Bilateria, the Protostomia and the Deuterostomia.
• PROTOSTOMIA: The animals in this branch display a characteristic embryology with spiral, determinate cleavage, mesoderm arising from cells near the lip of the blastopore, coelom arising as a split in an originally solid mass of mesoderm, and mouth arising from the embryonic blastopore
• Two branches exist within the Protostomes: The Lophotrochozoa and the Ecdysozoa.
• DEUTEROSTOMIS: Animals in this clade often show radial, indeterminate cleavage, mouth arising away from the blastopore, and coelom arising from outpouching of the gut.