SR

Chapter 32: The Deuterostomes

What Are Deuterostomes?

  • The deuterostomes include echinoderms, hemichordates, and chordates.
    • Shared derived characters include: radial, indeterminate cleavage; the blastopore becomes (or is near the future site of) the anus; and pharyngeal slits at some time in the life cycle.
    • Basal deuterostomes have a larva with a loop-shaped ciliated band used for locomotion.
  • Hemichordates (acorn worms) are marine deuterostomes with a three-part body, including proboscis, collar, and trunk.

Echinoderms

  • Echinoderms (phylum Echinodermata) are marine animals with a spiny "skin," water vascular system, tube feet, and endoskeleton.
    • The larvae exhibit bilateral symmetry; most of the adults exhibit pentaradial symmetry.
  • Class Crinoidea includes sea lilies and feather stars.
    • The oral surface of crinoids is turned upward; some crinoids are sessile.
  • Class Asteroidea consists of the sea stars.
    • They have a central disc with five or more arms, and they use tube feet for locomotion.
  • Class Ophiuroidea includes the brittle stars, which resemble sea stars but have longer, more slender arms that are set off more distinctly from the central disc.
    • They use their arms for locomotion.
    • Their tube feet lack suckers and are not used in locomotion.
  • Class Echinoidea includes the sea urchins and sand dollars.
    • Echinoids lack arms; they have a solid shell and are covered with spines.
  • Class Holothuroidea consists of sea cucumbers, animals with elongated flexible bodies.
    • The mouth is surrounded by a circle of modified tube feet that serve as tentacles.

The Chordates: Major Characteristics

  • The chordates (Phylum Chordata) include three subphyla: Urochordata, Cephalochordata, and Vertebrata.
    • At some time in its life cycle, a chordate has a flexible, supporting notochord; a dorsal, tubular nerve cord; a muscular postanal tail; and an endostyle, or thyroid gland; they are also characterized by pharyngeal slits, but that is a derived character of deuterostomes.

Invertebrate Chordates

  • The tunicates, which are urochordates, are suspension-feeding marine animals with tunics.
    • Larvae have typical chordate characteristics and are free-swimming.
    • Adults of most groups are sessile suspension feeders.
  • The lancelets are cephalochordates, small, segmented, fishlike animals; their chordate characteristics are highly developed.
    • The available evidence suggests that urochordates are the sister group of the vertebrates.

Introducing the Vertebrates

  • The vertebrates have a vertebral column composed of vertebrae that forms the chief skeletal axis of the body and a braincase, or cranium.
    • Neural crest cells are embryonic cells important in the development of many structures, including the cranium and jaws.
    • Vertebrates have pronounced cephalization, a complex brain, a muscular pharynx, and muscles attached to the endoskeleton.
  • Vertebrates can be assigned to nine classes.
    • The hagfishes, which make up the Myxini, and the lampreys, which make up
    • The Petromyzontida, have neither jaws nor paired fins.
    • The Chondrichthyes comprise the sharks, rays, and skates; they are jawed fishes with skeletons of cartilage.
    • The extant (living) bony fishes can be assigned to three classes: Actinopterygii, ray finned fishes; Actinistia, coelacanths; and Dipnoi, lungfishes.
  • The tetrapods (Tetrapoda) include the amphibians (class Amphibia; salamanders, frogs, and caecilians), many of which have aquatic larvae that undergo metamorphosis, and the amniotes (Amniota), which include reptiles and mammals.
    • Reptiles (class Reptilia) include turtles, lizards, snakes, alligators, and birds.
    • Reptiles are amniotes with keratin scales or feathers and reproduction adapted for terrestrial life; mammals (class Mammalia) include monotremes, marsupials, and placental mammals.
    • Mammals are amniotes with hair and mammary glands.

Jawless Fishes

  • The extant jawless fishes are the hagfishes (Myxini) and the lampreys (Petromyzontida).
    • Jaws and paired fins are absent in both hagfishes and lampreys.
    • Hagfishes are marine scavengers that secrete slime as a defense mechanism.
    • Many lampreys are parasites on other fishes.

Evolution of Jaws and Limbs: Jawed Fishes and Tetrapods

  • Chondrichthyes, the cartilaginous fishes (sharks, rays, and skates), evolved during the Devonian period.
    • They have jaws, two pairs of fins, and placoid scales
  • During the Devonian period, bony fishes gave rise to two evolutionary lines: th Actinopterygii, or ray-finned fishes, and the Sarcopterygii, or lobe-finned fishes.
    • The ray-finned fishes gave rise to the modern bony fishes.
    • Their lungs have been modified as a swim bladder, an air sac for regulating buoyancy.
  • The Sarcopterygii includes the Tetrapodomorpha, the lung-fishes (Dipnoi), and the coelacanths (Actinistia).
    • Evidence suggests that the Tetropodomorpha gave rise to the tetrapods, the land vertebrates.
    • Tiktaalik was a transitional form between fishes and tetrapods.
  • Early tetrapods were mainly aquatic animals that ventured onto the land to find food or escape predators.
    • These early tetrapods had limbs strong enough to support the weight of their bodies on land.
  • Modern amphibians (Amphibia) include salamanders and newts, frogs and toads, and caecilians.
    • Most amphibians return to the water to reproduce.
    • Frog embryos develop into tadpoles, which undergo metamorphosis to become adults.
    • Amphibians use their moist skin as well as lungs for gas exchange.
    • They have systemic and pulmonary circulations as well as hearts with two atria and one ventricle.

Amniotes: Terrestrial Vertebrates

  • Terrestrial vertebrates, or amniotes, include reptiles (including birds) and mammals.
    • Adaptations for life on land include:
    • (1) the evolution of the amniotic egg with its shell and amnion, a membrane that forms a fluid-filled sac around the embryo:
    • (2) internal fertilization; and
    • (3) a body covering that retards water loss and physiological mechanisms that conserve water.
  • Biologists classify amniotes in two main groups: diapsids and synapsids.
    • Diapsids include the turtles, squamates (snakes and lizards), tuataras, extinct ichthyosaurs, crocodiles, the extinct pterosaurs, the extinct ornithischian dinosaurs, the extinct saurischian dinosaurs, and birds.
    • The synapsids include the therapsids, which gave rise to the mammals.
  • Extant reptiles (including birds) can be classified in five clades:
    • (1) turtles, terrapins, and tortoises;
    • (2) lizards, snakes, and amphisbaenians;
    • (3) tuataras;
    • (4) crocodiles, alligators, caimans, and gavials; and
    • (5) birds (avian reptiles).
  • Most non-avian reptiles have dry skin with horny scales, lungs with many chambers, and a heart with two completely separate atria and two ventricles that are incompletely separated. (In the crocodiles the two ventricles are completely partitioned.)
    • Birds have many adaptations for powered flight, including feathers; wings; and light, hollow bones containing air spaces.
    • They have completely divided ventricles and very efficient lungs, and they excrete solid metabolic wastes (uric acid).
    • They are endotherms; that is, they maintain a constant body temperature.
    • Birds have a well-developed nervous system with excellent vision and hearing.
  • Based on fossil evidence and molecular data, feather evolution took place in terrestrial, bipedal dinosaurs before the evolution of birds or flight.
    • Birds are considered feathered reptiles that evolved from the lineage of saurischian dinosaurs, specifically from the theropods, a group of bipedal, saurischian dinosaurs
    • Like theropods, modern birds have feet with three digits; thin-walled, hollow bones; and a furcula, or wishbone.
  • Mammals have hair, mammary glands, differentiated teeth, lungs with alveoli, completely divided ventricles, and three middle-ear bones.
    • They are endotherms and have a highly developed nervous system and a muscular diaphragm.
  • Protherians, the monotremes, include the duck-billed platypus and spiny anteaters.
    • Monotremes lay eggs.
  • Metatherians, the marsupials, include pouched mammals, such as kangaroos and opossums.
    • The young, born at an immature stage, complete their developments in their mother’s marsupium, where they are nourished with milk from mammary glands.
  • Eutherians are more developed at birth than marsupials; they are characterized by a well-developed placenta, an organ of exchange that develops between the embryo and the mother.