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.