Evolution of the Animal Phyla The Living World Fifth Edition George B. Johnson Jonathan B. Losos Chapter 25 Evolution of the Animal Phyla Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

25.1 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 unique tissues

25.2 The Animal Family Tree the multicellular animals 35 very different phyla to judge which phyla are more closely related, taxonomists compare anatomical features and aspects of embryological development the end result are phlyogenies, which are basically like family trees the main branches of the phylogenies make possible the evolutionary history of animals

25.2 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

25.2 The Animal Family Tree although very different, the Parazoa and Eumetazoa are thought to have evolved from a common ancestor the shared ancestor was probably a choanoflagellate the choanoflagellate lived over 700 million years ago and was a colonial, flagellated protist

25.2 The Animal Family Tree within the Eumetazoan phylogeny, the family tree branches on the basis of the type of embryological layering Radiata have two embryological layers, an outer ectoderm and an inner endoderm this body plan is called diploblastic Bilateria have a third embryological layer, the mesoderm, that occurs between the ectoderm and the endoderm this body plan is called triploblastic

25.2 The Animal Family Tree additional branches to the phylogenetic tree were assigned by identifying traits that were important to the evolutionary history of phyla for example, the presence or absence of a body cavity the traditional phylogeny of taxonomists relies on the either-or-nature of categories

Figure 25.1 The animal family tree: the traditional viewpoint.

25.2 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 clusters of genes as means to detect relatedness this new approach has resulted into significant refinements of the traditional phylogeny for example, the protostomes have a more complex evolutionary history

Figure 25.2 The animal family tree: A new look.

25.3 Five Key Transitions in Body Plan the evolution of animals is marked by five key transitions in body plan evolution of tissues bilateral symmetry body cavity deuterostome development segmentation

25.3 Five 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 besides members of the Parazoa possess tissues they belong to the Eumetazoa

25.3 Five 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 along a central axis if a plane passing through the central axis divides the organism in halves, the halves will be mirror images bilateral symmetry is body plan with distinct right and left halves that are mirror images the plan allows for specialization among body regions

25.3 Five 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

25.3 Five 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

25.3 Five Key Transitions in Body Plan the subdivision of the body into segments is a key transition to the animal body plan that occurs early on during development 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

Figure 25.3 Evolutionary trends among the animals.

25.4 Sponges: Animals Without Tissues sponges, 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 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

Figure 25.4 Diversity in sponges. (a) (b)

Figure 25.5 Phylum Porifera: sponges.

25.5 Cnidarians: Tissues Lead to Greater Specialization the Radiata 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 a major evolutionary advance in the Radiata is extracellular digestion of food digestion begins outside the body in a gut cavity called, the gastrovascular cavity this form of digestion allows animals to digest an animal larger than itself

Figure 25.6 Representative cnidarians.

25.5 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 harpoon-like barb, called a nematocyst, which cnidarians use to spear their prey and they retract towards the tentacle the nematocyst can discharge so explosively that it is capable of piercing the hard shell of a crab

Figure 25.7 Phylum Cnidaria: cnidarians.

25.5 Cnidarians: Tissues Lead to Greater Specialization cnidarians have two basic body forms medusae are the floating form polyps are the sessile form Figure 25.8 The two basic body forms of cnidarians.

25.5 Cnidarians: Tissues Lead to Greater Specialization medusae are often called “jellyfish,” because of their gelatinous interior, or “stinging nettles,” because of their nematocyts polyps are pipe-shaped animals that usually attach to rock in corals, the polyps secrete a deposit of calcium carbonate in which they live

Figure 25.9 The life cycle of Obelia, a marine colonial hydroid.

25.6 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

Figure 25.10 How radial and bilateral symmetry differ.

25.6 Solid Worms: Bilateral Symmetry most bilaterally symmetrical animals have evolved a definitive head end this process is termed cephalization 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

25.6 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 they have separate organs, including a uterus and testes

Figure 25.11 Body plan of a solid worm.

Figure 25.12 Flatworms.

25.6 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 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, the parasites lack cilia in the adult stage and do not need eye spots this loss of features that lack adaptive purpose for parasitism is sometimes called degenerative evolution

Figure 25.13 Life cycle of the human liver fluke, Clonorchis sinensis. Flukes often require two or more hosts to complete their life cycles.

25.6 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

Figure 25.14 Phylum Platyhelminthes: solid worms.

25.6 Solid Worms: Bilateral Symmetry if flatworms have a digestive cavity, then it is incomplete the gut branches throughout the body and is involved in both digestion and excretion they are capable of performing some extracellular digestion the parasitic flatworms lack a gut entirely and absorb food directly through their body walls

25.6 Solid Worms: Bilateral Symmetry flatworms have an excretory system the system consists of a network of fine tubules that run through the body enlarged flame cells (cilia-lined bulbs) are located on the side branches of the tubules the cilia move water and excretory substances into the tubules and then into exit pores the primary function of the flame cells is in the regulation of water balance most of the metabolic wastes are excreted through the gut

Figure 25.15 Diagram of flatworm anatomy.

25.6 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 they use sensory pits or tentacles along the sides of the head to detect food, chemical, and movement free-living forms have eyespots to distinguish light from dark

25.6 Solid Worms: Bilateral Symmetry reproduction in flatworms is complex most flatworms are hermaphroditic, meaning that each individual contains both male and female reproductive structures some flatworms have a complex succession of distinct larval stages some flatworms are capable of asexual regeneration

25.7 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

25.7 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 located between the mesoderm and the endoderm coelomates have a body cavity (called a coelom) that develops entirely within the mesoderm

Figure 25.16 Three body plans for bilaterally symmetrical animals.

25.7 Roundworms: The Evolution of a Body Cavity seven phyla of bilaterally symmetrical animals have a pseudocoelom the pseudocoelom serves as a hydrostatic skeleton, a skeleton that gains its rigidity from fluids kept under pressure all pseudocoelomates lack a circulatory system most pseudocoelomates have a complete digestive tract

25.7 Roundworms: The Evolution of a Body Cavity the phylum Nematoda contains the greatest number of species among the phyla that are pseudocoelomates the members of this phylum include nematodes, eelworms, and other roundworms they are unsegmented, cylindrical worms covered by a flexible cuticle that is molted as they grow nematodes move in a whip-like fashion

Figure 25.17 Pseudocoelomates. (a) Nematodes (phylum Nematoda)

25.7 Roundworms: The Evolution of a Body Cavity the mouth of a nematode is often equipped with piercing organs called stylets food passes through a muscular chamber called the pharynx reproduction in nematodes is usually sexual with, usually, separate sexes their development is simple and adults have very few cells Caenorhabditis elegans is a roundworm important to genetic and developmental studies

Figure 25.18 Phylum Nematoda: roundworms.

25.7 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 transmitted to humans who eat undercooked pork intestinal roundworms, Ascaris lumbricoides, live in human intestines

25.7 Roundworms: The Evolution of a Body Cavity another phylum consisting of animals with a pseudocoelomate body plan is the phylum Rotifera rotifers are small, aquatic organisms that have a crown of cilia at their heads the cilia help in both locomotion and feeding

Figure 25.17 Pseudocoelomates. (b) Rotifers (phylum Rotifera)

25.8 Mollusks: Coelomates coelomate animals are more successful than pseudocoelomates because of the nature of embryonic development primary induction is a process in animal development in which one of the three primary embryonic tissues interacts with another the interaction requires physical contact in coelomates, contact is made possible between mesoderm and endoderm this interaction permits localized portions of the digestive tract to become highly specialized

25.8 Mollusks: Coelomates the mollusks, members of the phylum Mollusca, are the only coelomates without segmented bodies the basic body of a mollusk 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

25.8 Mollusks: Coelomates there three major groups of mollusks gastropods—include the snails and slugs bivalves—include clams, oysters, and scallops cephalopods—include the octopi and squids

Figure 25.19 Three major groups of mollusks.

25.8 Mollusks: Coelomates mollusks have a unique feeding structure, called a radula the radula is a rasping tongue-like organ that bears rows of pointed, backward-curving teeth in most mollusks, the outer surface of the mantle secretes a protective shell the shell has multiple layers comprised of protein, calcium, and pearl

Figure 25.20 Phylum Mollusca: mollusks.

25.9 Annelids: The Rise of Segmentation one of the early innovations to 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

25.9 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 the tubes run from mouth to anus derived from this basic organization are three characteristics repeated segments specialized segments connections

Figure 25.22 Phylum Annelida: annelids.

25.10 Arthropods: Advent of Jointed Appendages the most successful of all animal groups is the phylum Arthropoda, comprising 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

25.10 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

Figure 25.24 Segmentation in insects.

Figure 25.25 Phylum Arthropoda: arthropods.

25.10 Arthropods: Advent of Jointed Appendages chelicerates are arthropods that lack jaws they include spiders, mites and scorpions mandibulates are arthropods with jaws, called mandibles they include the crustaceans, insects, centipedes and millipedes

Figure 25.26 Chelicerates and mandibulates.

25.10 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 Figure 25.27 Horseshoe crabs.

25.10 Arthropods: Advent of Jointed Appendages the class Arachnida has 57K named species and includes the spiders, ticks, mites, scorpions, and daddy longlegs arachnids have a pair of chelicerae, a pair of pedipalps, and four pairs of walking legs

Figure 25.28 Arachnids.

25.10 Arthropods: Advent of Jointed Appendages crustaceans belong to the phylum Crustacea and comprise a diverse group of mandibulates there a 35K species of crustacea described including species of crabs, shrimps, lobsters, crayfish, water fleas, pillbugs, and sowbugs most crustaceans have two pairs of antennae, three pairs of chewing appendages, and various numbers of legs all crustaceans pass through a larval stage called the nauplius

Figure 25.29 Crustaceans.

Figure 25.30 Body of a lobster, Homarus americanus.

25.10 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

Figure 25.31 Centipedes and millipedes.

25.10 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

Figure 25.33 Insect diversity.

25.11 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

Figure 25.34 Embryonic development in protostomes and deuterostomes.

25.11 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 it occurs later in deuterostome cleavage than in protostome cleavage origin of the coelom

25.12 Echinoderms: The First Deuterostomes echinoderms belong to the phylum Echinodermata echinoderm means “spiny skin” and refers to the calcium-rich ossicles that protude just beneath the echinoderm’s skin they are 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

Figure 25.35 Diversity in echinoderms.

25.12 Echinoderms: The First Deuterostomes a key adaptation of echinoderms is the water vascular system this system is a fluid-filled and composed of a central ring canal around which five radial canals extend out into the arms from each radial canal short side branches extend to form thousands of tiny, hollow tube feet most echinoderms reproduce sexually but asexual regeneration is also common

Figure 25.36 Phylum Echinodermata: echinoderms.

25.13 Chordates: Improving the Skeleton chordates belong to the phylum Chordata and are deuterostome coelomates 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

25.13 Chordates: Improving the Skeleton the approximately 56K 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.

Figure 25.37 Phylum Chordata: chordates.

25.13 Chordates: Improving the Skeleton not all chordates are vertebrates tunicates and lancelets Figure 25.38 Nonvertebrate chordates.

25.13 Chordates: Improving the Skeleton vertebrate chordates differ from tunicates and lancelets in two important respects vertebrates have a backbone this replaces the role of the notochord vertebrates have a distinct and well-differentiated head

Figure 25.39 A mouse embryo.

Inquiry & Analysis How many of the species exhibit variation in the comprehensive character index? How does the magnitude of this variation within species compare with the variation seen between species? Graph of the history of evolutionary change among bryozoa.