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Christoph Richter

Freeman, Biological Science, 4e, Chapter 34 Chapter 34 - Deuterostome Animals Learning Objectives: Students should be able to ... • List the basic traits of echinoderms and describe their ecological role. • List the basic traits of vertebrates and describe their ecological role. • Describe these innovations that occurred during the evolution of the vertebrates: jaws, limbs, flight, the amniotic egg, the placenta, and parental care. • Briefly describe the known history of the hominid lineages, starting from the common ancestor shared with chimpanzees and continuing to modern humans. Lecture Outline • All deuterostomes undergo development in a similar way. o The gut develops from posterior to anterior; that is, the anus forms first. o The coelom, if present, develops from outpocketings of mesoderm. • There are four phyla of deuterostomes: (Fig. 34.1) o Echinodermata, including sea stars and sea urchins o Hemichordata, or acorn worms o Xenoturbellida, containing a single genus with two wormlike species o Chordata, including the vertebrates I. What Is an Echinoderm? A. The echinoderm body plan 1. Echinoderm larvae are bilateral, but adults have evolved pentaradial symmetry (five-sided symmetry). (Fig. 34.2a) 2. Echinoderms have an endoskeleton made of plates of calcium carbonate secreted inside the skin. (Fig. 34.2b) 3. Echinoderms have a system of branching, fluid-filled tubes, and chambers called the water vascular system. (Fig. 34.3) a. Tube feet are elongated, fluid-filled appendages of this system. b. Sections of tube feet called podia project outside the body wall to make contact with the substrate. c. The water vascular system is a sophisticated hydrostatic skeleton used for movement. 4. Students should be able to indicate the origin of pentaradial symmetry in adults, the water vascular system, and the echinoderm endoskeleton on Figure 34.1. B. How do echinoderms feed? 1. Echinoderms suspension feed, deposit feed, or harvest algae or other animals. 2. Echinoderms use their podia to gather food. a. For example, sea stars eat bivalves by using their podia to pry open the shells. (Fig. 34.4a) © 2011 Pearson Education, Inc. Freeman, Biological Science, 4e, Chapter 34 b. Podia are also used in suspension feeding or deposit feeding, or to move food toward cilia that then sweep it into the mouth. (Fig. 34.4b) C. Key lineages 1. Echinoderms include five major lineages. (Fig. 34.5) 2. Feather stars and sea lilies are sessile suspension feeders. 3. Brittle stars and basket stars have five or more long arms that radiate out from a central disk, with clear, joint-like articulations. 4. Sea cucumbers are sausage-shaped animals that feed with modified tube feet called tentacles. 5. Asteroidea (sea stars)(Fig. 34.6) a. Habitat: They are entirely marine, living on hard or soft substrates along coasts. b. Morphology: Five or more long arms radiate from a central region. Most do not have the long spines characteristic of other echinoderms. c. Feeding: Sea stars are predators or scavengers. Many prey on bivalves, sponges, barnacles, or snails. They may function as keystone predators that increase biodiversity by keeping prey species in check. d. Movement: They crawl with tube feet. e. Reproduction: Reproduction is sexual, with separate sexes. At least one arm is filled with reproductive organs that produce gametes. Most species can regenerate arms, and some can reproduce asexually via fission. 6. Echinoidea (sea urchins and sand dollars) (Fig. 34.7) a. Morphology: They have globe-shaped bodies (sea urchins) or flattened disks (sand dollars). b. Feeding: Most are herbivores, using either a jaw-like structure (sea urchins) or suspension feeding (sand dollars). (1) Sea urchins are ecologically important grazers and can overgraze kelp forests if not kept in check by predators. c. Movement: They crawl with spines, sometimes with podia. (1) Sea urchins crawl via their podia and movement of their spines. d. Reproduction: Reproduction is sexual, with separate sexes and external fertilization. Larvae may undergo asexual reproduction. II. What Is a Chordate? A. Chordates are defined by four morphological features: 1. Pharyngeal gill slits in the throat 2. A bundle of nerves that runs the length of the body, called the dorsal hollow nerve cord 3. A stiff, but flexible, notochord that runs the length of the body 4. A muscular tail that extends past the anus © 2011 Pearson Education, Inc. Freeman, Biological Science, 4e, Chapter 34 5. Together, these traits create a "torpedo" that can swim forward rapidly. 6. These traits may be present only briefly, but they are present in all chordates at some point in the life cycle. B. Three "subphyla" 1. Cephalochordates (lancelets or amphioxus) are small, mobile, suspension feeders. Adults burrow into sand. (Fig. 34.8a) 2. Urochordates (tunicates and sea squirts) are small suspension feeders that live attached to hard substrates in the ocean. (Fig. 34.8b) a. The mobile larvae have the diagnostic chordate traits, but the sessile adults may lose them. 3. Vertebrates include sharks, bony fishes, reptiles, and mammals. (Fig. 34.8c) a. The human spinal cord is the dorsal hollow nerve cord. b. The pharyngeal slits give rise to gill slits in fishes and are present during development in the embryos of terrestrial vertebrates. c. The notochord is present only in development, when it helps organize the body plan; for example, it helps induce the formation of vertebrae and muscles. C. Key lineages: the invertebrate chordates 1. Two phyla of the Chordata lack vertebrae: 2. Cephalochordata (lancelets) (Fig. 34.9) a. Habitat: A few dozen species are all found in marine sands. b. Morphology: The notochord is retained in adults, where it functions as an endoskeleton. c. Feeding: Lancelets are suspension feeders using the pharyngeal slits. d. Movement: Efficient swimmers, they use large blocks of muscle arranged in series along the length of the notochord. e. Reproduction: Reproduction is sexual, with separate sexes and external fertilization. 3. Urochordata (tunicates) (Fig. 34.10) a. Habitat: All are marine. Sea squirts live on the ocean floor, and salps live in open water. b. Morphology: Tunicates have an exoskeleton-like coat of polysaccharide called a tunic, a U-shaped gut, and two openings called siphons. c. Feeding: They are suspension feeders with the pharyngeal slits. d. Movement: Larvae swim; adults are sessile or float. e. Reproduction: Reproduction is sexual, with hermaphroditism. They may have internal or external fertilization. Some use asexual reproduction. © 2011 Pearson Education, Inc. Freeman, Biological Science, 4e, Chapter 34 III. What Is a Vertebrate? A. Vertebrates are a monophyletic group distinguished by two traits: 1. A vertebral column 2. A cranium that encloses the brain B. The large vertebrate brain is divided into three regions: 1. The forebrain houses the sense of smell and may be elaborated into a large cerebrum. 2. The midbrain is associated with vision. 3. The hindbrain is associated with balance and vision and may include a cerebellum and medulla oblongata. C. An overview of vertebrate evolution (Fig. 34.11) 1. The first vertebrates lived about 540 mya. a. They had streamlined bodies and a skull made of cartilage. b. A cartilaginous endoskeleton was a basic vertebrate feature. 2. Bone appeared in the Ordovician, about 480 mya. a. Bone consists of cells and blood vessels in a matrix of hydroxyapatite and protein fibers. b. At first, bone was not part of the endoskeleton; rather, bony plates formed an exoskeleton. 3. Jawed vertebrates appeared in the Silurian, about 430 mya. a. Jaws gave vertebrates the ability to bite, allowing them to diversify their diet to herbivory or carnivory. b. Teeth appeared soon after jaws. 4. Bony endoskeletons appeared in the early Devonian. a. The cartilaginous endoskeleton began to be stiffened by bone. 5. Tetrapods moved to land about 365 mya. 6. The amniotic egg evolved about 345 mya in a lineage called the amniotes (all tetrapods except amphibians). 7. Evaluating molecular phylogenies a. The phylogenetic tree built from molecular data concurs with the order of appearance observed in the fossil record. (Fig. 34.12) 8. Several different vertebrate lineages are called fishes. a. "Fish" is a grade, not a monophyletic taxon. D. Key innovations 1. The most species-rich and diverse vertebrate lineages are the ray- finned fishes and the tetrapods. (Fig. 34.13) 2. The vertebrate jaw a. Ancient vertebrates like hagfish and lampreys do not have jaws and cannot bite. b. Jaws probably evolved from gill arches. (Fig. 34.14) (1) Jaws and gill arches both consist of flattened bars of bone or cartilage that are hinged. (2) Muscles that move gill arches and jaws are derived from the same population of cells. © 2011 Pearson Education, Inc. Freeman, Biological Science, 4e, Chapter 34 (3) Jaws and gill arches are both derived from neural crest cells. c. Modifications in the jaw structure make ray-finned fishes particularly diverse in their eating habits. (1) In most species, the jaw protrudes to be able to nip out or bite food. (2) Most species also have a pharyngeal jaw, consisting of modified gill arches that act as a second set of jaws at the back of the mouth. This makes food processing more efficient. d. Students should be able to explain the adaptive significance of the jaw. 3. The tetrapod limb a. Lungfish morphology providesclues for how vertebrates may have made the water-to-land transition. (Fig. 34.15) b. The fossil record includes a series of fossil species between lungfishes and tetrapods, documenting a gradual transition from a fin to a limb that could support walking. (Fig. 34.16) c. Genetic data also show that the regulatory proteins involved in fish fins and the upper parts of mammal limbs are homologous. 4. Feathers and flight a. Wings and flight evolved in three lineages of tetrapods: pterosaurs, bats, and birds. b. A series of fossils discovered in the early 2000s indicate that at least one lineage of dinosaurs had feathers. (Fig. 34.17) (1) Feathers probably evolved in a series of steps, beginning with simple projections from the skin. (2) It is not yet clear whether flight evolved from the ground up or from the trees down. (3) Once flight evolved, further adaptations made powered, flapping flight increasingly efficient. Examples: keeled sternum, reduction in weight of skeleton, endothermy. (Fig. 34.18) 5. The amniotic egg a. Amniotic eggs allow reptiles, birds, and some mammals to lay their eggs on land. b. These eggs have membranes surrounding a food supply, water supply, and waste repository. (Fig. 34.19) c. The membranes provide support and extra surface area for gas exchange, allowing the production of larger, better- developed young. d. Amniotic eggs usually have leathery shells that are not waterproof (most reptiles), but in birds the shell is hard and waterproof. e. Students should be able to explain why the additional membranes enabled amniotic eggs to become larger. © 2011 Pearson Education, Inc. Freeman, Biological Science, 4e, Chapter 34 6. The placenta a. Some species are viviparous, meaning that embryos are retained in the mother's body and are nourished through a placenta rather than from a yolk. b. The placenta is an organ that forms from both fetal and maternal tissue; it is rich in blood vessels and facilitates the flow of oxygen and nutrients from the mother to the embryo. (Fig. 34.20) c. Why did viviparity and the placenta evolve? (1) Offspring develop at a more constant, favorable body temperature. (2) Offspring are protected. (3) Offspring are portable (the mother is not tied to a nest). 7. Parental care a. Parental care represents a fitness trade-off: Parents produce fewer offspring, but each offspring has a better chance of survival. b. Parental care involves supplying food, warmth, and protection. c. Many taxa exhibit some parental care (ray-finned fishes, frogs, insects), but the most extensive parental care is found in mammals and birds. (Fig. 34.21) (1) Mammals and birds often feed their young after birth (with lactation in mammals). (2) The evolutionary success of mammals and birds has been attributed to the time and energy they invest in parental care. E. Key lineages 1. Myxinoidea (hagfish) and Petromyzontoidea (lampreys) (Fig. 34.23) a. It is not yet clear whether this group comprises two independent lineages or one monophyletic lineage (Cyclostomata). b. Morphology: They have long, slender bodies with a cranium and no paired appendages. Lampreys have a primitive vertebral column, but hagfish do not. c. Feeding: They are scavengers (hagfish) or ectoparasites (lampreys). d. Movement: They swim using the well-developed notochord as a brace. e. Reproduction: Lampreys are anadromous (swim up streams to breed), wi
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