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EVOLUTION OF THE ANIMAL PHYLA CHAPTER 19 GENERAL FEATURES OF ANIMALS • Animals share many important characteristics, such as they: • • • • • • • are heterotrophs are multicellular and lack cell walls can move from place to place have diverse forms and habitats reproduce, mostly by sexual reproduction have a common pattern of development have unique tissues THE ANIMAL FAMILY TREE • Animals consist of 35 very different phyla. • To judge which phyla are more closely related, taxonomists traditionally have compared anatomical features and aspects of embryological development. • The end result are phylogenies, which are basically like family trees. THE ANIMAL FAMILY TREE • The kingdom Animalia is traditionally divided into two main branches based on tissue presence. • Parazoa possess neither tissues nor organs and have no discernible symmetry. • They are represented mostly by the phylum Porifera, the sponges. • Eumetazoa have a definite shape and symmetry and, in most cases, tissues organized into organs and organ systems. • Although very different, the Parazoa and Eumetazoa are thought to have evolved from a common ancestor. THE ANIMAL FAMILY TREE • Eumetazoans are divided into two groups: • Radiata have radial symmetry and two embryological layers, an outer ectoderm and an inner endoderm. • This body plan is called diploblastic. • Bilateria have bilateral symmetry and a third embryological layer, the mesoderm, that occurs between the ectoderm and the endoderm. • This body plan is called triploblastic. THE ANIMAL FAMILY TREE • Traditionally, the Bilateria have been subdivided based on traits that were important to the evolutionary history of phyla. • For example, the presence or absence of a body cavity. • The traditional animal phylogeny relies on the either-or nature of categories. THE ANIMAL FAMILY TREE: THE TRADITIONAL VIEWPOINT No coelom Pseudocoel No tissues Parazoa Radial symmetry Radiata Acoelomates Pseudocoelomates Protostomes Coelom forms from mesoderm cell mass Mix of characteristics Coelom forms from embryonic gut Coelomates Mix of characteristics Deuterostomes Sponges Cnidarians Ctenophorans Flatworms Nematodes Mollusks Annelids Arthropods Lophophorates Echinoderms Chordates THE ANIMAL FAMILY TREE • The traditional animal phylogeny is being revised because some of the important characters may not be conserved to the extent previously thought. • Molecular systematics offers a means to construct phylogenic trees using unique gene sequences as means to detect relatedness. • New molecular data has resulted in a variety of new phylogenies, most of which divide protostomes into: • Lophotrochozoans • Ecdysozoans THE ANIMAL FAMILY TREE: A NEW LOOK Grow by increasing body mass, locomotion is ciliary, trochophore larvae No tissues Parazoa Radial symmetry Radiata Lophotrochozoa Grow by molting Coelom forms from embryonic gut Ecdysozoa Protostomes Sponges Cnidarians Ctenophorans Lophophorates Flatworms Mollusks Annelids Deuterostomes Nematodes Arthropods Echinoderms Chordates SIX KEY TRANSITIONS IN BODY PLAN • The evolution of animals is marked by six key transitions in body plan: 1. 2. 3. 4. 5. 6. tissues bilateral symmetry body cavity segmentation molting deuterostome development SIX KEY TRANSITIONS IN BODY PLAN • The presence of tissues is the first key transition in the animal body plan. • Only the Parazoa, the sponges, lack defined tissues and organs. • These animals exist as aggregates of cells with minimal intercellular coordination. • All other animals, the Eumetazoa, possess tissues. SIX KEY TRANSITIONS IN BODY PLAN • Virtually all animals other than sponges have a definite shape and symmetry. • Radial symmetry is a body plan in which all parts of the body are arranged around a central axis. • Any plane passing through the central axis divides the organism in halves that are approximate mirror images. • Bilateral symmetry is a body plan with distinct right and left halves that are mirror images. • The plan allows for specialization among body regions and more efficient movement. SIX KEY TRANSITIONS IN BODY PLAN • The evolution of a body cavity was an important step in animal evolution. • This internal space allowed for the support of organs, distribution of materials, and coordination of development. • For example, the digestive tract can be larger and longer. SIX KEY TRANSITIONS IN BODY PLAN • Bilateral animals can be divided into two groups based on differences in the basic pattern of development. • Protostomes include the flatworms, nematodes, mollusks, annelids, and arthropods. • Deuterostomes include the echinoderms and the chordates. • Deuterostomes evolved from protostomes more than 630 million years ago. SIX KEY TRANSITIONS IN BODY PLAN • The subdivision of the body into segments is another key transition in the animal body plan. • In highly segmented animals, each segment can develop a more or less complete set of adult organ systems. • Each segment can function as a separate locomotory unit. Parazoa Eumetazoa Radiata Bilateria Coelomates Pseudocoelomates Deuterostomes Protostomes Echinodermata Jointed appendages exoskeleton, molting Lophophorates Segmented Arthropoda Annelida Mollusca Rotifera (rotifers) Nematoda (roundworms) Platyhelminthes (flatworms) Porifera (sponges) EVOLUTIONARY TRENDS AMONG THE ANIMALS Radiata (Cnidaria and Ctenophora) Segmented Chordata Acoelomates Notochord, segmentation, jointed appendages Segmentation Molting ? Deuterostome development, endoskeleton Pseudocoel Coelom Body cavity Radial symmetry Bilateral symmetry No true tissues Tissues Multicellularity Ancestral protist TABLE 19.2 THE MAJOR ANIMAL PHYLA Approximate Number of Named Species Phylum Typical Examples Key Characteristics Arthropoda (arthropods) Insects, crabs, spiders, millipedes Most successful of all animal phyla; chitinous exoskeleton covering segmented bodies with paired, jointed appendages; most insect groups have wings; nearly all are freshwater or terrestrial Mollusca (mollusks) Snails, clams, octopuses, nudibranchs Soft-bodied coelomates whose bodies are divided into three parts: head-foot, visceral mass, and mantle; many have shells; almost all possess a unique rasping tongue called a radula; most are marine or freshwater but 35,000 species are terrestrial 110,000 Chordata (chordates) Mammals, fish, reptiles, birds, amphibians Segmented coelomates with a notochord; possess a dorsal nerve cord, pharyngeal pouches, and a tail at some stage of life; in vertebrates, the notochord is replaced during development by the spinal column; most are marine, many are freshwater, and 20,000 species are terrestrial 56,000 Platyhelminthes (flatworms) Planaria, tapeworms, flukes Segmented coelomates with a notochord; possess a dorsal nerve cord, pharyngeal pouches, and a tail at some stage of life; in vertebrates, the notochord is replaced during development by the spinal column; most are marine, many are freshwater, and 20,000 species are terrestrial 20,000 Nematoda (roundworms) Ascaris, pinworms, hookworms, Filaria Pseudocoelomate, unsegmented, bilaterally symmetrical worms; tubular digestive tract passing from mouth to anus; tiny; without cilia; live in great numbers in soil and aquatic sediments; some are important animal parasites 20,000 Annelida (segmented worms) Earthworms, marine worms, leeches Coelomate, serially segmented, bilaterally symmetrical worms; complete digestive tract; most have bristles called setae on each segment that anchor them during crawling; marine, freshwater, and terrestrial 12,000 1,000,000 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. TABLE 19.2 (CONTINUED) Approximate Numberof NamedSpecies Phylum Typical Examples Key Characteristics Cnidaria (cnidarians) Jellyfish, hydra, corals, sea anemones Soft, gelatinous, radially symmetrical bodies whose digestive cavity has a single opening; possess tentacles armed with stinging cells called cnidocytes that shoot sharp harpoons called nematocysts; almost entirely marine 10,000 Echinodermata (echinoderms) Sea stars, sea urchins, sand dollars, sea cucumbers Soft, gelatinous, radially symmetrical bodies whose digestive cavity has a single opening; possess tentacles armed with stinging cells called cnidocytes that shoot sharp harpoons called nematocysts; almost entirely marine 6,000 Porifera (sponges) Barrel sponges, boring sponges, basket sponges, vase sponges Asymmetrical bodies without distinct tissues or organs; saclike body consists of two layers breached by many pores; internal cavity lined with food-filtering cells called choanocytes; most marine (150 species live in freshwater) 5,150 Lophophorates (Bryozoa, also called moss animalsor Ectoprocta) Bower bankia, Plumatella, sea mats, sea moss Microscopic, aquatic deuterostomes that form branching colonies, possess circular or U-shaped row of ciliated tentacles for feeding called a lophophore that usually protrudes through pores in a hard exoskeleton; Bryozoa are also called Ectoprocta because the anus, or proct, is external to the lophophore; marine or freshwater 4,000 Rotifera (wheel animals) Rotifers Small, aquatic pseudocoelomates with a crown of cilia around the mouth resembling a wheel; almost all live in freshwater 2,000 SPONGES: ANIMALS WITHOUT TISSUES • Sponges are members of the Phylum Porifera. • Their bodies a little more than masses of specialized cells embedded in a gel-like matrix. • Clumps of cells disassociated from a sponge can give rise to new sponges. SPONGES: ANIMALS WITHOUT TISSUES • The body of a sponge is perforated by many pores. • Choanocytes are flagellated cells that line the body cavity of the sponge and draw in water through the pores. • The sponge is a filter feeder which traps any food particles. CNIDARIANS: TISSUES LEAD TO GREATER SPECIALIZATION • Radiata are radially symmetrical and include two phyla. • Cnidaria comprises the hydra, jellyfish, corals, and anemones. • Ctenophora comprises the comb jellies. • The members of the Radiata have a body plan that allows them to interact with their environment on all sides. CNIDARIANS: TISSUES LEAD TO GREATER SPECIALIZATION • A major evolutionary advance in the Radiata is extracellular digestion of food. • Digestion begins outside of cells in a gut cavity called the gastrovascular cavity. • This form of digestion allows animals to digest an animal larger than itself. CNIDARIANS: TISSUES LEAD TO GREATER SPECIALIZATION • Cnidarians (phylum Cnidaria) are carnivores that capture prey with tentacles that ring their mouths. • These tentacles and, sometimes, the body surface bear stinging cells called cnidocytes. • Within each cnidocyte is a harpoonlike barb, called a nematocyst, which cnidarians use to spear prey and retract it towards the tentacle • The nematocyst can discharge so explosively that it is capable of piercing the hard shell of a crab CNIDARIANS: TISSUES LEAD TO GREATER SPECIALIZATION • Cnidarians have two basic body forms: • Medusae are a floating form. • Polyps are a sessile form. Gastrovascular cavity Epidermis Mesoglea Medusa Gastrodermis Tentacles Mouth Polyp Gastrovascular cavity CNIDARIANS: TISSUES LEAD TO GREATER SPECIALIZATION • Medusae are often called “jellyfish,” because of their gelatinous interior, or “stinging nettles,” because of their nematocysts. • Polyps are pipe-shaped animals that usually attach to rock. • In corals, the polyps secrete a deposit of calcium carbonate in which they live. THE LIFE CYCLE OF OBELIA, A MARINE COLONIAL HYDROID Medusae Testis Ovary Feeding polyp Sexual reproduction Medusa bud Eggs Sperm Reproductive polyp Zygote Blastula Free-swimming planula larva Mature colony Young colony and asexual budding Settles down to start new colony SOLID WORMS: BILATERAL SYMMETRY • Body symmetry differs among the Eumetazoa. • Radial symmetry means that multiple planes cutting the organism in half will produce mirror images. • Bilateral symmetry means that only one plane can cut the organism in half to produce mirror images. SOLID WORMS: BILATERAL SYMMETRY • Most bilaterally symmetrical animals have evolved a definitive head end. • This process is termed cephalization. SOLID WORMS: BILATERAL SYMMETRY • The bilaterally symmetrical eumetazoans produce three embryonic layers. • Ectoderm will develop into the outer coverings of the body and the nervous system. • Mesoderm will develop into the skeleton and muscles. • Endoderm will develop into the digestive organs and intestine. SOLID WORMS: BILATERAL SYMMETRY • The solid worms are the simplest of all bilaterally symmetrical animals. • The largest phylum of these worms is the Phylum Platyhelminthes, which includes the flatworms. • Flatworms lack any internal cavity other than the digestive tract. • This solid condition is called acoelomate. • Flatworms are the simplest animals in which organs occur. SOLID WORMS: BILATERAL SYMMETRY • Most flatworms are parasitic but some are free-living. • Flatworms range in size from less than a millimeter to many meters long. SOLID WORMS: BILATERAL SYMMETRY • There are two classes of parasitic flatworms: • Flukes • Tapeworms • The parasitic lifestyle has resulted in the eventual loss of features not used or needed by the parasite. • For example, parasitic flatworms do not need eyespots. • This loss of features that lack adaptive purpose for parasitism is sometimes called degenerative evolution. LIFE CYCLE OF THE HUMAN LIVER FLUKE, CLONORCHIS SINENSIS Raw, infected fish is consumed by humans or other mammals 3 Metacercarial cysts in fish muscle Flukes often require two or more hosts to complete their life cycles Liver Adult fluke Bile duct 2 1 Egg containing miracidium Miracidium hatches after being eaten by snail Cercaria Redia Sporocyst SOLID WORMS: BILATERAL SYMMETRY • Tapeworms are a classic example of degenerative evolution. • The body of a tapeworm has been reduced to two primary functions. • Eating • Reproduction SOLID WORMS: BILATERAL SYMMETRY • Those flatworms that have a digestive cavity, have an incomplete gut, one with only one opening. • The gut branches throughout the body and is involved in both digestion and excretion. • The parasitic flatworms lack a gut entirely and absorb food directly through their body walls. SOLID WORMS: BILATERAL SYMMETRY • Flatworms lack a circulatory system and all cells must be within diffusion distance of oxygen and food. • Flatworms have a simple nervous system, either a nerve net or longitudinal nerve cords with cross connections. • Free-living forms have eyespots to distinguish light from dark. ROUNDWORMS: THE EVOLUTION OF A BODY CAVITY • A key transition in the evolution of the animal body plan was the evolution of the body cavity. • The evolution of an internal body cavity helped improve the animal body design in three areas: • circulation • movement • organ function ROUNDWORMS: THE EVOLUTION OF A BODY CAVITY • There are three basic kinds of body plans found in bilaterally symmetrical animals: • Acoelomates have no body cavity. • Pseudocoelomates have a body cavity (called a pseudocoel) located between the mesoderm and the endoderm. • Coelomates have a body cavity (called a coelom) that develops entirely within the mesoderm. Ectoderm Mesoderm Endoderm Acoelomate Ectoderm Mesoderm Pseudocoelomate Endoderm Pseudocoel Ectoderm Mesoderm Coelomate Coelomic cavity Endoderm ROUNDWORMS: THE EVOLUTION OF A BODY CAVITY • The largest pseudocoelomate phylum is Nematoda, containing about 20,000 species. • The members of this phylum include nematodes, eelworms, and other roundworms. • Nematodes are unsegmented, cylindrical worms covered by a flexible cuticle that is molted as they grow. • Nematodes move in a whiplike fashion. http://youtu.be/tp-O3LME3OU ROUNDWORMS: THE EVOLUTION OF A BODY CAVITY • Some nematodes are parasitic in humans, cats, dogs, and animals of economic importance. • Heartworm in dogs is caused by a nematode • Trichinosis is an infection caused by the nematode Trichinella and is transmitted to humans who eat undercooked pork. • Intestinal roundworms, Ascaris lumbricoides, live in human intestines. ROUNDWORMS: THE EVOLUTION OF A BODY CAVITY • Another pseudocoelomate phylum is Rotifera. • Rotifers are small, aquatic organisms that have a crown of cilia at their heads. • The cilia help in both locomotion and feeding. http://youtu.be/cYNJOVDQexA MOLLUSKS: COELOMATES • The body of a mollusk (Phylum Mollusca) is comprised of three regions: • a head-foot • a visceral mass containing the body’s organs. • a mantle that envelopes the visceral mass and is associated with the gills. MOLLUSKS: COELOMATES • There are three major groups of mollusks: • Gastropods—include the snails and slugs. • Bivalves—include clams, oysters, and scallops. • Cephalopods—include the octopuses and squids. MOLLUSKS: COELOMATES • Mollusks have a unique feeding structure, called a radula. • The radula is a rasping tonguelike organ that bears rows of pointed, backward-curving teeth. MOLLUSKS: COELOMATES • In most mollusks, the outer surface of the mantle secretes a protective shell. • The shell has multiple layers comprised of protein, calcium, and pearl. ANNELIDS: THE RISE OF SEGMENTATION • One of the early innovations in body plan to arise among the coelomates was segmentation. • Segmentation is the building of a body from a series of similar segments. • This body plan offers a lot of flexibility in that small changes to segments can produce a new kind of segment with different functions. • The first segmented animals to evolve were the annelid worms, Phylum Annelida. ANNELIDS: THE RISE OF SEGMENTATION • The basic body plan of an annelid is a tube within a tube. • The digestive tract is suspended within the tube of the coelom. • There are three body plan characteristics: • Repeated segments • Specialized segments • Connections ARTHROPODS: ADVENT OF JOINTED APPENDAGES • The most successful of all animal groups is the Phylum Arthropoda, consisting of the arthropods. • These animals have jointed appendages. • In addition to joints, arthropods have an exoskeleton made of chitin. • The muscles of arthropods attach to the interior of this outer shell. • The shell offers protection against predators and water loss. ARTHROPODS: ADVENT OF JOINTED APPENDAGES • Chitin cannot support much weight. • Arthropod size is limited as a result. • Arthropod bodies are segmented like annelids. • Segments often fuse into functional groups in the adult stage. Antennae Mouth ARTHROPODS: ADVENT OF JOINTED APPENDAGES • Chelicerates are arthropods that lack jaws. • They include spiders, mites, scorpions, and horseshoe crabs. • Mandibulates are arthropods with jaws, called mandibles. • They include the crustaceans, insects, centipedes, and millipedes. Antenna Eye Eyes Pedipalp Chelicera Mandible ARTHROPODS: ADVENT OF JOINTED APPENDAGES • The chelicerate fossil record goes back 630 million years. • A surviving type of chelicerate from this period is the horseshoe crab. ARTHROPODS: ADVENT OF JOINTED APPENDAGES • The class Arachnida has 57,000 named species of arachnids, including spiders, ticks, mites, scorpions, and daddy longlegs. • Arachnids have a pair of chelicerae, a pair of pedipalps, and four pairs of walking legs. ARTHROPODS: ADVENT OF JOINTED APPENDAGES • Crustaceans belong to the subphylum Crustacea and comprise a diverse group of mandibulates. • There a 35,000 species of crustaceans described including species of crabs, shrimps, lobsters, crayfish, water fleas, pillbugs, and sowbugs. ARTHROPODS: ADVENT OF JOINTED APPENDAGES • Most crustaceans have two pairs of antennae, three pairs of chewing appendages, and various numbers of legs. ARTHROPODS: ADVENT OF JOINTED APPENDAGES • Crustaceans pass through a larval stage called the nauplius. ARTHROPODS: ADVENT OF JOINTED APPENDAGES • Millipedes and centipedes have bodies that consist of a head region followed by numerous similar segments. • Centipedes have one pair of legs per segment while millipedes have two. • Centipedes are all carnivorous while millipedes are herbivorous. ARTHROPODS: ADVENT OF JOINTED APPENDAGES • Insects belong to the Class Insecta and are the largest group of arthropods. • They are the most abundant eukaryotes on the earth. • Insects have three body sections: • Head • Thorax • Abdomen PROTOSTOMES AND DEUTEROSTOMES • There are two major kinds of coelomate animals representing two distinct evolutionary lines: • Protostomes • The mouth develops from or near the blastopore. • Deuterostomes • The anus forms from or near the blastopore; the mouth forms on another part of the blastula. Mesoderm Protostomes Archenteron Coelom Anus Coelom Mouth 1 cell 2 cells 4 cells 8 cells 16 cells 32 cells Blastula Deuterostomes Blastopore Mesoderm splits Mouth forms from blastopore Archenteron Coelom Mouth Coelom 1 cell 2 cells 4 cells 8 cells 16 cells 32 cells Blastula Blastopore Archenteron outpockets to form coelom Anus Anus forms from blastopore PROTOSTOMES AND DEUTEROSTOMES • Deuterostomes also differ from protostomes in three other fundamental ways: • The pattern of cleavage • Protostomes have spiral cleavage while deuterostomes have radial cleavage. • Fating of cells • Occurs later in deuterostome cleavage than in protostome cleavage. • Origin of the coelom. ECHINODERMS: THE FIRST DEUTEROSTOMES • Echinoderms are deuterostomes that belong to the phylum Echinodermata. • Echinoderm means “spiny skin” and refers to the endoskeleton of calcium-rich ossicles just beneath the echinoderm’s skin. • Entirely marine animals and include sea stars, sea urchins, sand dollars, and sea cucumbers. • All are bilaterally symmetrical as larvae but become radially symmetrical as adults. ECHINODERMS: THE FIRST DEUTEROSTOMES • A key adaptation of echinoderms is the water vascular system that aids movement. • This system is fluid-filled and composed of a central ring canal from which five radial canals extend out into the http://youtu.be/8JOxiT5_zpc arms. • From each radial canal, tiny vessels extend through short side branches into thousands of tiny, hollow tube feet. • Echinoderms can extend the tube feet, attach them to the ocean floor, and pull against them to move. CHORDATES: IMPROVING THE SKELETON • Chordates are deuterostomes that belong to the phylum Chordata. • They exhibit a truly internal endoskeleton with muscles attached to an internal rod, called a notochord. • This innovation opened the door to large body sizes not possible in earlier animal forms. CHORDATES: IMPROVING THE SKELETON • The approximately 56,000 species of chordates share four principal features: • • • • Notochord Nerve cord Pharyngeal pouches Postanal tail • All chordates have all four of these characteristics at some time in their lives. CHORDATES: IMPROVING THE SKELETON • Not all chordates are vertebrates. • Tunicates and lancelets are chordates. CHORDATES: IMPROVING THE SKELETON • Vertebrate chordates differ from tunicates and lancelets in two important respects. • Vertebrates have a backbone that replaces the role of the notochord. • Vertebrates have a distinct and welldifferentiated head.