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Chapter

BIO2231: Phylum Chordata characteristics

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Department
Biology
Course Code
BIO2231
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Various

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PHYLUM CHORDATA Characteristics of Phylum Chordata o Bilaterial symmetry; segmented body; three germ layers; well developed coelom o Notochord (a skeletal rod) present at some life stage o Single, dorsal, tubular nerve cord; anterior end of cord usually enlarged to form brain o Pharyngeal pouches present at some life stage; in aquatic chordates these develop into pharyngeal slits o Endostyle in floor of pharynx or a thyroid gland derived from the endostyle o Postanal tail projecting beyond the anus at some life stage but may or may not persist o Segmentation, if present, restricts to outer body wall, head, and tail and not extending into coelom The Chordates: The notochord is a rod-like, semi-rigid body of fluid-engorged cells enclosed by a fibrous sheath, which extends, in most cases, the length of the body just ventral to the central nervous system. Thus, the notochord is a hydrostatic organ, similar to the hydrostatic skeletons of nematodes. Its primary purpose is to stiffen the body, providing skeletal scaffolding for the attachment of swimming muscles. The structural plan of chordates shares features of many non-chordate invertebrates, such as bilateral symmetry, anteroposterior axis coelom, tube within a tube arrangement, metamerism, and cephalization. However, the exact phylogenetic position of chordates within the animal kingdom is unclear. Chordates share with other deuterostomes several important chracteristics: radial cleavage, an anus derived from the first embryonic opening (blastospore), a mouth derived from an opening of secondary origin, and a coelom formed by fusion of enterocoelous pouches (although in most vertebrates coelom formation is schizocoelus, but independently derived from that of protostomes, as an accommodation for their large yolks). Five Chordate Hallmarks: - Notochord - Dorsal tubular nerve cord - Pharyngeal pouches and slits - Endostyle or thyroid gland - Postanal tail Five characteristics that, taken together, set chordates apart from all other phyla are notochord, dorsal tubular nerve cord, pharyngeal pouches or slits, endostyle, and postanal tail. These characteristics are always found at some embryonic stage, although they may change or disappear in later stages of the life cycle. All but pharyngeal pouches or slits are unique to chordates; hemichordates also have pharyngeal slits and these are presumed ancestral to deuterostomes. A dorsal nerve cord is present in some hemichordates, but it is probably not homologous to that of chordates Notochord: The notochord is a flexible, rod-like structure, extending the length of the body. It is the first part of the endoskeleton to appear in the embryo. The notochord is a hydrostatic organ, but unlike nematodes, which contain fluid in a single, large cavity, the fluid in a notochord is contained within cells or in tiny compartments between cells. Muscles attach to the notochord, and because it can bend without shortening, it permits undulatory movements of the body. In most protochordates and in jawless vertebrates, the notochord persists throughout life. In all vertebrates except hagfish a series of cartilaginous or bony vertebrates are formed from mesenchymal cells derived from blocks of mesodermal cells (somites) lateral to the notochord. In more vertebrates, the notochord is replaced by vertebrae, although remnants of the notochord may persist between or within the vertebrae. Dorsal tubular nerve cord: In most vertebrate phyla that have a nerve cord, it is ventral to the digestive tract and is solid, but in chordates the single cord is dorsal to the digestive tract and is a tube. The anterior end becomes enlarged to form the brain in vertebrates. The hollow cord is produced in the embryo by infolding of ectodermal cells on the dorsal side of the body above the notochord. Among vertebrates, the ne
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