Phylum Annelida Essays

Phylum Annelida Essays Phylum Annelida Essay Phylum Annelida Essay Phylum Annelida L. annelleus = little ring Annelida have bodies consisting of many essentially similar ringlike segments (somites or metameres). This segmentation usually shows in both external and internal features, including muscles, nerves, and circulatory, excretory and reproductive organs. Phylum Annelida This phylum is divided into three classes: Oligochaeta: Earthworms Most earthworms and their equals are inhabitants of damp soil and fresh waters. Hirudinea: Leeches The leeches are found mainly in fresh water or on moist ground. Polychaeta: Bristle-worms The marine worms are found chiefly in close vicinity of the shore. OLIGOCHAETA = Earthworms HIRUDINEA = Leeches POLYCHAETA = Bristle-worms Some annelids are free-living, many inhabit burrows or dwell in tubes, some are commensals on other aquatic animals (few are ecto- or endoparasites), and many of the leeches attach to vertebrates. Symmetry bilateral. Triploblastic. The body is elongate and usually conspicuously segmented both internally and externally. Appendages are minute rodlike chitinous setae, little to many per somite. Polychaeta have fleshy tentacles on its head and has the setae situated on the lateral fleshy parapodia. Most species of the class Hirudinea lack setae. The Body is covered by a thin moist cuticle over columnar epithelium containing unicellular gland cells and sensory cells. Both the body wall and the digestive canal has layers of circular and longitudinal muscles. The body cavity (coelom) is well developed (except in the leeches) an d is divided by septa in the Oligochaeta and Polychaeta. A complete digestive canal is present in a tubular shape, extending the whole length of the body. A closed circulatory system of longitudinal blood vessels with branches extending laterally in each segment. Dissolved hemoglobin and free amoebocytes are usually present in the blood plasma. Respiration occurs via the epidermis or through gills in some tube dweller species. Excretory system consists of one pair of nephridia per segment(somite). Each of these nephridia removes waste from the coelom and bloodstream directly to the outside world. A nervous system is present with a pair of cerebral ganglia ( a brain) and connectives to a solid (double) midventral nerve cord extending the length of the body. The midventral nerve cord is connected to pairs of lateral nerves in each segment. Sensory cells and organs for touch, taste and the perception of light is also present. The sexes are united and the development in the case of Oligochaeta and Hirudinea are direct or the sexes are separate and the development includes a trocophore larval stage as with the Polychaeta. Some species of Oligochaeta and Polychaeta reproduce asexually by budding. Earthworms have long, cylindrical body that is divided into similar segments. The grooves that extend around the body of the worm show the arrangement of the segments. Earthworms have bilateral symmetry The first body segment is called the peristomium. The peristomium contains the mouth. Instead, they depend on their prostomium and sensory receptors in their skin to feel their way through the soil. The periproct is the last segment of an earthworm. Reference: Hickman Jr. C. P. and et al. , 2007. Animal Diversity 4th edition. Boston: McGrawHill naturewatch. ca/english/wormwatch/resources/anatomy. html