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.