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28.5C: Phylum Chordata

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    13732
  • The phylum Chordata contains all animals that have a dorsal notochord at some stage of development; in most cases, this is the backbone.

    LEARNING OBJECTIVES

    Name the features that distinguish the members of the phylum chordata

    KEY TAKEAWAYS

    Key Points

    • The phylum chordata is named for the notochord, a longitudinal, flexible rod between the digestive tube and the nerve cord; in vertebrates, this is the spinal column.
    • The chordates are also characterized by a dorsal nerve cord, which splits into the brain and spinal cord.
    • Chordata contains two clades of invertebrates: Urochordata (tunicates) and Cephalochordata (lancelets), both of which are suspension feeders.
    • The phylum chordata includes all animals that share four characteristics, although they might each possess some of them at different stages of their development: a notochord, a dorsal nerve cord, pharyngeal slits, and a postanal tail.
    • Chordata contains five classes of animals: fish, amphibians, reptiles, birds, and mammals; these classes are separated by whether or not they can regulate their body temperature, the manner by which they consume oxygen, and their method of reproduction.

    Key Terms

    • dorsal nerve cord: a hollow cord dorsal to the notochord, formed from a part of the ectoderm that rolls, forming a hollow tube.
    • notochord: a flexible rodlike structure that forms the main support of the body in the lowest chordates; a primitive spine
    • pharyngeal slit: filter-feeding organs found in non-vertebrate chordates (lancelets and tunicates) and hemichordates living in aquatic environments

    Phylum Chordata

    Animals in the phylum Chordata share four key features that appear at some stage of their development:

    • A notochord, or a longitudinal, flexible rod between the digestive tube and the nerve cord. In most vertebrates, it is replaced developmentally by the vertebral column. This is the structure for which the phylum is named.
    • A dorsal nerve cord which develops from a plate of ectoderm that rolls into a tube located dorsal to the notochord. Other animal phyla have solid nerve cords ventrally located. A chordate nerve cord splits into the central nervous system: the brain and spinal cord.
    • Pharyngeal slits, which allow water that enters through the mouth to exit without continuing through the entire digestive tract. In many of the invertebrate chordates, these function as suspension feeding devices; in vertebrates, they have been modified for gas exchange, jaw support, hearing, and other functions.
    • A muscular, postanal tail which extends posterior to the anus. The digestive tract of most nonchordates extends the length of the body. In chordates, the tail has skeletal elements and musculature, and can provide most of the propulsion in aquatic species.

    In some groups, some of these traits are present only during embryonic development. In addition to containing vertebrate classes, the phylum Chordata contains two clades of invertebrates: Urochordata (tunicates) and Cephalochordata (lancelets). However, even though they are invertebrates, they share characteristics with other chordates that places them in this phylum. For example, tunicate larvae have both a notochord and a nerve cord which are lost in adulthood. Most tunicates live on the ocean floor and are suspension feeders. Cephalochordates, or lancelets, have a notochord and a nerve cord (but no brain or specialist sensory organs) and a very simple circulatory system. Lancelets are suspension feeders that feed on phytoplankton and other microorganisms.

    image

    Structures present in a tunicate larva: While tunicates are invertebrates and may seem very different from the more familiar members of Chordata, the tunicate larva possesses both a notochord and a dorsal nerve cord, although both are lost in adulthood.

    The phylum Chordata contains all of the animals that have a rod-like structure used to give them support. In most cases this is the spine or backbone. Within Chordata there are five classes of animals: fish, amphibians, reptiles, birds, and mammals. Three dividing factors separate these classes:

    • Regulation of body temperature: animals are either homeothermic (can regulate their internal temperature so that it is kept at an optimum level) or poikilothermic (cannot regulate their internal temperature, the environment affects how hot or cold they are)
    • Oxygen Absorption: the way in which oxygen is taken in from the air, which can be through gills, the skin (amphibians), or lungs
    • Reproduction: this factor is particularly varied. Animals can be oviparous (lay eggs) or viviparous (birth live young). Fertilization can occur externally or internally. In mammals, the mother produces milk for the young.

    image

    Notochord: All chordates possess a notochord, or a type of flexible support rod, at some point in their development. In this dissected lungfish, which is a member of the chordates, tissues have been dissected away to display the notochord.

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