Phylum Cnidaria

    Phylum Cnidaria includes all stinging marine organisms that exhibit radial symmetry, such as jellyfish, hydras, sea anemones, and coral. Cnidarians have a true digestive gut like other animals, but one opening serves as both the mouth and anus. Additionally, their body walls are made up of only two layers of cells: endoderm and ectoderm.

 

    Phylum Platyhelminthes (Flatworms)

    Flatworms are bilaterally symmetric and are the most primitive animals to possess all three embryonic tissue layers. Like cnidarians, most flatworms have a digestive gut with only a single opening. Flatworms are also the most primitive animals to exhibit discernable organs, internal structures with at least two tissue layers and a specialized function. There are three main kinds of flatworms: free-living carnivorous planarians, parasitic flukes that feed off the blood of other animals, and parasitic tapeworms that live inside the digestive tracts of other animals.

 

    Phylum Nematoda (Roundworms)

    Most nematodes, also called roundworms, are free-living; however, some live as parasites in the digestive tracts of humans and other animals. Soil-dwelling roundworms play an important ecological role by helping to decompose and recycle organic debris. Roundworms are bilaterally symmetric, have a complete gut tube with two openings, and possess all three embryonic tissue layers with a cavity in between the mesodermal and endodermal tissues. The roundworm species Caenorhabditis elegans was the first animal to have its entire genome sequence determined.

 

    Phylum Mollusca

    Phylum Mollusca includes many familiar animals such as snails, slugs, squid, octopuses, and shellfish such as clams and oysters. Mollusks are bilaterally symmetric and have a complete digestive tract and a circulatory system with a simple heart. They move by means of a muscular structure called a foot, and they have a rasping tongue called a radula and a mantle that secretes a hard shell. Mollusks generally live in aquatic regions.

 

    Phylum Annelida (Segmented Worms)

    Annelida means “ringed” and refers to the repeated ringlike segments that make up the bodies of annelids such as earthworms and leeches. Annelids exhibit bilateral symmetry have a complete digestive tract with two excretory organs called nephridia in each segment and a closed circulatory system. Their nervous system consists of a simple brain in front and a ventral (near the belly) nerve cord connecting smaller clusters of nerve cells, or ganglia, within each segment. Earthworms live freely within the soil, while most leeches, on the other hand, are bloodsucking parasites. All annelids must live in moist environments. Having not yet developed more sophisticated respiratory systems, they exchange gases directly with their surroundings.