Chordates

• By the end of the Cambrian period, 540 million years ago, an astonishing variety of animals inhabited Earth’s oceans.
• Chordates are bilaterian animals
• Chordates belong to the clade of animals known as Deuterostomia
• Two groups of invertebrate deuterostomes, the urochordates and cephalochordates are more closely related to vertebrates than to invertebrates.
• Chordates have bilateral symmetry
• Chordates have a coelom
• Chordates undergo deuterostome development
• Chordates undergo radial and indeterminate cleavage
• Chordates undergo enterocoelous coelom development
• Chordates have metamerism
• Chordates have cephalization
• The notochord is a flexible, rod-like structure derived from mesoderm.
• Five distinctive characteristics define the chordates: notochord, dorsal tubular nerve cord, pharyngeal pouches (gill slits), endostyle, and postanal tail.
• All five distinctive characteristics are found at least at some embryonic stage in all chordates, although they may later be lost
• The notochord is the first part of the endoskeleton to appear in an embryo.
• The notochord is the place for muscle attachment
• In vertebrates, the notochord is replaced by the vertebrae.
• Remains of the notochord may persist between the vertebrae.
• In chordates, the nerve cord is dorsal to the alimentary canal and is a tube.
• In chordates, the anterior end of the nerve cord becomes enlarged to form the brain.
• The hollow cord of chordates is produced by the infolding of ectodermal cells that are in contact with the mesoderm in the embryo
• The dorsal tubular nerve cord is protected by the vertebral column in vertebrates
• Pharyngeal slits are openings that lead from the pharyngeal cavity to the outside.
• Pharyngeal slits are formed when pharyngeal grooves and pharyngeal pouches meet to form an opening.
• In tetrapods, the pharyngeal pouches give rise to the Eustachian tube, middle ear cavity, tonsils, and parathyroid glands.
• The perforated pharynx evolved as a filter feeding apparatus.
• Later, the filter feeding apparatus were modified into internal gills used for respiration
• The endostyle in the pharyngeal floor, secretes mucus that traps food particles.
• The endostyle is found in protochordates and lamprey larvae
• The endostyle secretes iodinated proteins
• The endostyle is homologous to the iodinated-hormone-secreting thyroid gland in adult lampreys and other vertebrates.
• The postanal tail, along with somatic musculature and the stiffening notochord, provides motility in larval tunicates and amphioxus.
• The postanal tail evolved for propulsion in water
• The postanal tail is reduced to the coccyx (tail bone) in humans
• There are two protochordate subphyla: Urochordata and Cephalochordata
• Tunicates (subphylum Urochordata) are found in all seas.
• Most tunicates are sessile and highly specialized as adults
• In most species of tunicates, only the larvae show all of the chordate hallmarks
• Tunicates filter feed using the pharyngeal slits and a mucous net secreted by the endostyle.
• Some tunicates are colonial.