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BIO153 Ch 33 SN.pdf

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Christoph Richter

Freeman, Biological Science, 4e, Chapter 33 Chapter 33 - Protostome Animals Learning Objectives: Students should be able to ... • Describe the major characteristics that differentiate the Ecdysozoa and the Lophotrochozoa. • List and describe the basic traits of four major phyla of lophotrochozoans (including subgroups of mollusks) and two major phyla of ecdysozoans (including subgroups of arthropods). • Describe the major evolutionary innovations that triggered the diversification of the protostomes, especially relating to the water-to- land transition, appendages and mouthparts, and metamorphosis. Lecture Outline I. An Overview of Protostome Evolution A. Protostomes are the most abundant animals on Earth. (Fig. 33.1) B. Protostomes can be divided into Lophotrochozoa and Ecdysozoa. (Fig. 33.2) C. What is a lophotrochozoan? 1. The Lophotrochozoa comprise 13 phyla and include the mollusks, annelids, and flatworms. 2. The Lophotrochozoa lack a distinct synapomorphy but many have: a. A feeding structure called a lophophore (in three phyla) (Fig. 33.3a) b. A type of ciliated larva called a trochophore (in several marine phyla) (Fig. 33.3b) c. Growth by incremental additions to the body rather than by molting (Fig. 33.4a) D. What is an ecdysozoan? 1. The Ecdysozoa comprise seven phyla and include nematodes and arthropods. 2. All grow by molting—shedding an exoskeleton or covering. (Fig. 33.4b) II. Themes in the Diversification of Protostomes A. How do body plans vary among phyla? 1. All 22 protostome phyla are triploblastic and bilaterally symmetric and undergo embryonic development in a similar way. 2. The coelom was reduced or eliminated in some groups. a. Wormlike protostomes have a well-developed coelom that serves as a hydrostatic skeleton or for circulation. b. In some other phyla, including flatworms, arthropods, and mollusks, the coelom was no longer needed for these functions and was reduced or eliminated. 3. The arthropod body plan (Fig. 33.5a) a. Arthropods have segmented bodies divided into sections called tagmata (singular: tagma). © 2011 Pearson Education, Inc. Freeman, Biological Science, 4e, Chapter 33 b. The exoskeleton is made of chitin and hardened by calcium carbonate. c. Movement occurs by contraction of muscles that pull against the exoskeleton to move jointed limbs. d. A fluid-filled hemocoel holds internal organs and circulates body fluid, and functions as a hydrostatic skeleton in some larvae. e. Students should be able to describe the major features of the arthropod body. 4. The molluscan body plan (Fig. 33.5b) a. A muscular foot is used for movement. b. A visceral mass contains the internal organs. c. A mantle covers the visceral mass and may secrete a shell. 5. Variation among body plans of the wormlike phyla a. These phyla all have similar body plans but are distinguished by specialized mouthparts or feeding structures. b. Echiurans (a group of annelids) have a proboscis with which they capture pieces of detritis in marine mud. (Fig. 33.6a) c. Priapulids have a tooth-lined throat that they can turn inside out to grab their prey. (Fig. 33.6b) d. Nemerteans are active predators that move around the ocean floor and spear their prey with a proboscis. (Fig. 33.6c) B. The water-to-land transition 1. Evidence for multiple transitions a. Phylogenetic analysis indicates that the ability to live in terrestrial environments evolved independently in arthropods (at least twice), mollusks, roundworms, and annelids. (Fig. 33.7) 2. Adaptations to terrestrial environments a. The protostomes that made the water-to-land transition already had adaptations for support and locomotion (hydroskeletons, limbs, etc.). b. New adaptations were required to accomplish gas exchange without drying out. (1) Roundworms and earthworms exchange gases across their entire body surface, but they must stay in moist environments. (2) Arthropods and mollusks have gills that are located inside the body where they are protected from desiccation. (3) Insects evolved a waxy layer to minimize water loss, with respiratory passages that can be closed when necessary. 3. Students should be able to name three problems that arise during a water-to-land transition in animals. © 2011 Pearson Education, Inc. Freeman, Biological Science, 4e, Chapter 33 C. Adaptations for feeding 1. Protostomes have extremely diverse mouthparts, which allow them to eat a wide diversity of feeds with a wide variety of methods. (Fig. 33.8) 2. Larvae and adults often exploit different food sources. D. Adaptations for moving 1. Protostomes that lack limbs often move via a hydrostatic skeleton. 2. Arthropods evolved jointed limbs that permit rapid, precise movement. (Fig. 33.9a) 3. The evolution of the insect wing allowed flight. (Fig. 33.9b) a. About two-thirds of the multicellular species living today are winged insects. 4. Mollusks can glide with waves of muscle contractions of the foot. (Fig. 33.9c) 5. Cephalopods can move rapidly using jet propulsion. (Fig. 33.10) 6. Students should be able to explain the advantage of a jointed limb versus an unjointed limb. E. Adaptations in reproduction 1. Many wormlike phyla can perform asexual reproduction via splitting or fragmenting of the body. 2. Some crustaceans and insects reproduce asexually via parthenogenesis, when an unfertilized egg develops into an offspring. 3. Sexual reproduction with external fertilization occurs in clams, bryozoans, brachiopods, and other groups. 4. Sexual reproduction occurs with internal fertilization in crustaceans, snails, and insects⎯probably because individuals in these groups can move (i.e., can meet each other). 5. A few groups of snails and insects are ovoviviparous, retaining eggs until they hatch. 6. Metamorphosis occurs in many protostomes. a. In marine species, this allows larvae to disperse to new habitats. b. In insects, it reduces competition between larvae and adults. 7. Terrestrial lineages evolved desiccation-resistant eggs several times. III. Key Lineages: Lophotrochozoans A. Rotifera (rotifers) (Fig. 33.11) 1. Habitat: Habitats are damp or aquatic. Rotifers are important components of plankton. 2. Morphology: Most are less than 1 mm long. They have a coelom and a distinctive cluster of cilia at the anterior end called a corona. 3. Feeding: Many rotifers suspension feed using the cilia of the corona. 4. Movement: Most swim via the beating of cilia in the corona. © 2011 Pearson Education, Inc. Freeman, Biological Science, 4e, Chapter 33 5. Reproduction: Some are parthenogenetic, with direct development (no metamorphosis). B. Platyhelm inthes (flatworms) (Fig. 33.12) 1. Habitat: This large and diverse phylum includes free-living species (Turbellaria), endoparasitic tapeworms (Cestoda), and endoparasitic or ectoparasitic flukes (Trematoda). 2. Morphology: The body is broad and flat, unsegmented, and has no coelom. Flatworms have no gas exchange structures; instead, they perform gas exchange across the body wall (and are thus restricted to moist environments). 3. Feeding: There is no lophophore. The digestive tract is blind, with only one opening. 4. Movement: Most do not move much. 5. Reproduction: a. All three groups can reproduce sexually by cross-fertilization and self-fertilization. b. Turbellarians reproduce asexually by splitting in half or by fragmentation. c. Flukes and tapeworms have complex life cycles. (1) Sexual reproduction occurs in a definitive host. (2) Asexual reproduction occurs in one or more intermediate hosts. C. Annelida (segmented worms) (Fig. 33.13) 1. Habitat: There is wide variety of habitats. 2. Morphology: The body is usually segmented, with a coelom that functions as a hydroskeleton. 3. Findings of recent phylogenetic analyses a. The common ancestor of annelids had bristle-like extensions called chaetae, extending from appendages called parapodia. b. Polychaeta is a paraphyletic group consisting of several separate lineages of marine annelids that retained the ancestral trait of chaetae. c. Several other lineages independently lost the chaetae, including Clitellata (earthworms, leeches) and two other groups now known to be annelids: Sipunculida (peanut worms) and Echiura (spoon worms). (1) Sipunculida and Echiura also lost segmentation. 4. Feeding: Feeding methods include suspension feeding, active predation, deposit feeding (earthworms), and blood-feeding ectoparasitism (about half of leeches). 5. Movement: Annelids crawl, burrow, or swim. 6. Reproduction a. Many can reproduce asexually via transverse fission or fragmentation. b. Sexual reproduction in polychaetes involves separate sexes with eggs released into the water. © 2011 Pearson Education, Inc. Freeman, Biological Science, 4e, Chapter 33 c. Sexual reproduction in oligochaetes and lee
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