Overview of Animal Phylogeny and Key Groups

Sponges

Sessile filter feeders with tube-shaped bodies.

Benthic

Organisms that live on the ocean floor.

Spongin

Protein matrix in sponge cells.

Spicules

Needle-like structures made of silica.

Choanocytes

Collar cells that create water currents.

Amoebocytes

Cells that distribute food within sponges.

Choanoflagellates

Sister group to animals; filter feeders.

Cnidaria

Radially symmetric, active predators or filter feeders.

Cnidocytes

Stinging cells used to subdue prey.

Nerve net

Decentralized nervous system without a brain.

Bilateria

Animals with bilateral symmetry and complex structures.

Cephalization

Concentration of sensory organs at the front.

Protostomes

Mouth develops from the blastopore opening.

Deuterostomes

Anus develops from the blastopore opening.

Turbellarians

Free-living flatworms in the phylum Platyhelminthes.

Cestodes

Tapeworms that parasitize hosts.

Annelids

Segmented worms with 15K species.

Molluscs

Diverse group with calcium carbonate shells.

Nematodes

Most abundant animals, many are microscopic.

Arthropods

Jointed appendages and chitinous exoskeleton.

Echinoderms

Radial symmetry; includes sea stars and cucumbers.

Chordates

Animals with notochord and dorsal nerve cord.

Notochord

Flexible rod for muscle attachment in chordates.

Dorsal hollow nerve cord

Develops into the spinal cord in vertebrates.

Pharyngeal gill slits

Present in embryos; forms gills in fish.