Chapter 32
Given that I do not usually give the taxonomy lectures
(the material is addressed via projects and assignments) these notes will not
have any words removed.-Dr. G.
Animal Origins and the Evolution of Body Plans
Animal Origins and the Evolution of Body Plans
Animals: Descendants of a Common Ancestor
Body Plans: Basic Structural Designs
Sponges: Loosely Organized Animals
Cnidarians: Two Cell Layers and Blind Guts
Ctenophores: Complete Guts and Tentacles
The Evolution of Bilaterally Symmetrical Animals
Simple Lophotrochozoans
Lophophorates: An Ancient Body Plan
Spiralians: Spiral Cleavage and Wormlike Body Plans
Animals: Descendants of a Common Ancestor
Evidence indicates that all animals are descendants of a single ancestral lineage.
All animals share a set of derived traits:
Similarities in their small-subunit ribosomal RNAs
Similarities in their Hox genes
Special types of cellcell junctions: tight junctions, desmosomes, and gap junctions
A common set of extracellular matrix molecules, including collagen
Animals: Descendants of a Common Ancestor
Animals evolved from ancestral colonial flagellated protists.
Within these ancestral colonies, a division of labor arose.
Cells became specialized for different functions, such as movement, nutrition, and reproduction.
The specialized units continued to differentiate while improving their coordination with other working groups of cells.
These coordinated groups of cells evolved into animals.
Animals: Descendants of a Common Ancestor
Generalized traits characterize animals:
They are multicellular organisms that must take in pre-formed organic molecules.
They acquire these organic molecules by ingesting other organisms, living or dead, and digesting them within their bodies.
Animals must expend energy to acquire these organic molecules.
Most have circulatory systems that carry O2, CO2, and nutrients.
Animals: Descendants of a Common Ancestor
Much of the diversity in the animal kingdom evolved as animals acquired the ability to capture and eat many different types of food and to avoid becoming food for other animals.
The need to move in search of food has favored sensory structures that provide animals with detailed information about their environment.
Animals expend a considerable amount of energy to maintain relatively constant internal conditions while taking in foods that vary chemically.
Animals: Descendants of a Common Ancestor
Clues to the evolutionary relationships among animals are found in the fossil record, patterns of embryonic development, comparative physiology and morphology, and the structure of molecules such as the small-subunit RNAs and mitochondrial genes.
The sponges, cnidarians, and ctenophores separated from the other animal lineages early in evolutionary history.
The remaining animals have been divided into two major lineages: the protostomes and the deuterostomes.
Animals: Descendants of a Common Ancestor
Animals form layers of cells during their development from a single-celled zygote to a multicellular adult.
The embryos of diploblastic animals have two cell layers: an outer ectoderm and an inner endoderm.
The embryos of triploblastic animals have a third layer, the mesoderm.
The existence of three cell layers distinguishes the protostomes and deuterostomes from simple animals that diverged earlier.
Animals: Descendants of a Common Ancestor
Protostomes and deuterostomes differ in the fate of the blastopore , the opening of the cavity that forms in the spherical embryo.
In the protostomes, the mouth arises from the blastopore.
In the deuterostomes, the blastopore gives rise to the anus.
Animals: Descendants of a Common Ancestor
Most protostomes and the deuterostomes exhibit a pattern of early cell division in the fertilized egg called radial cleavage.
In radial cleavage, cells divide along a plane either parallel to or at right angles to the long axis of the fertilized egg.
One major protostome lineage evolved a pattern of early cell division called spiral cleavage.
Body Plans: Basic Structural Designs
The entire structure of an animal, its organ systems, and the integrated functioning of its parts are known as its body plan.
Body Plans: Basic Structural Designs
Overall shape is referred to as symmetry. A symmetrical animal can be divided into similar halves along at least one plane.
Animals that have no plane of symmetry are said to be asymmetrical.
In spherical symmetry body parts radiate out from a central point. Spherical symmetry is widespread among the protists.
An organism with radial symmetry has one main axis around which its body parts are arranged.
Bilaterally symmetric animals can be divided into mirror images by a single plane.
Body Plans: Basic Structural Designs
Bilateral symmetry is a common characteristic of animals that move freely through their environments.
Bilateral symmetry is often associated with cephalization: the presence of a head bearing sensory organs and central nervous tissues at the anterior end of the animal.
Body Plans: Basic Structural Designs
Body cavities are fluid-filled spaces that lie between the cell layers of the bodies of many kinds of animals.
The type of body cavity an animal has influences how it can move.
Animals can be grouped into three major categories based on the type of body cavity they have: the acoelomates, the pseudocoelomates, and the coelomates.
Body Plans: Basic Structural Designs
Acoelomates lack an enclosed body cavity. The space between the gut and body wall is filled with cells called mesenchyme.
Pseudocoelomates have a pseudocoel, a liquid filled space in which organs are suspended.
Coelomates have a coelom that develops within the mesoderm. It is lined with the peritoneum and enclosed on the inside and outside by muscles.
Body Plans: Basic Structural Designs
The fluid-filled body cavities of simple animals function as hydrostatic skeletons.
When the muscles surrounding fluids contract, the fluids can be moved to other parts of the body, causing these body regions to expand.
Other forms of skeletons developed in many lineages, including internal skeletons (vertebrate bones), and external skeletons (crab shells, clam shells).
The form of an animals skeleton and body cavities strongly influences the degree to which it can control and change its shape and thus the complexity of the movements it can perform.
Sponges: Loosely Organized Animals
The lineage leading to modern sponges (phylum Porifera) separated from the lineage leading to other animals very early during animal evolution.
Sponges are sessilethey live attached to the substratum.
The body plan of sponges is an aggregation of cells built around a water canal system.
Specialized feeding cells called choanocytes line the inside of the water canal. Beating of the choanocyte flagella moves water through the animal. Water exits through a larger opening called the osculum.
Sponges: Loosely Organized Animals
Sponges have a supporting skeleton, either in the form of branching spines called spicules or as an elastic network of fibers.
Sponges are loosely organized; if a sponge is completely disassociated, its cells can reassemble into a new sponge.
Sponges depend on water movement through their bodies to obtain food and are often oriented at right angles to current flow so that they may intercept water as it flows past.
Sponges reproduce both sexually and asexually. In most species, a single individual produces both eggs and sperm. Asexual reproduction is by budding and fragmentation.
Cnidarians: Two Cell Layers and Blind Guts
The cnidarians (phylum Cnidaria) were the next lineage to split off from the main line of animal evolution after the sponges.
They are diploblastic and have a blind gut with only one entrance.
Despite their relatively simple structures, the Cnidarians have structural molecules, such as actin and collagen, and homeobox genes.
Cnidarians: Two Cell Layers and Blind Guts
Cnidarians appeared early in evolutionary history and radiated in the late Precambrian.
There are about 11,000 species living today.
The cnidarian body plan combines a low metabolic rate with the ability to capture large prey, allowing cnidarians to survive in environments where prey is scarce.
Cnidarians: Two Cell Layers and Blind Guts
Cnidarians have tentacles with specialized cells called cnidocytes. These cells contain structures called nematocysts that can discharge toxins into their prey.
The mouth of a cnidarian is connected to a blind sac called the gastrovascular cavity. It functions in digestion, circulation, and gas exchange.
Cnidarians have epithelial cells with muscle fibers whose contractions allow them to move, as well as nerve nets that integrate body activities.
Cnidarians: Two Cell Layers and Blind Guts
The generalized cnidarian life cycle has two stages:
The polyp is typically asexual; individual polyps may reproduce by budding to form colonies.
The medusae produce eggs and sperm and release them into the water.
A fertilized egg becomes a free-swimming, ciliated larva called a planula that eventually settles to the bottom and transforms into a polyp.
The polyp and the medusa have the same body plan; a medusa is essentially a polyp without a stalk.
The mesoglea is a jellylike material that lies between the two cell layers of the cnidarian.
Cnidarians: Two Cell Layers and Blind Guts
The species in the class Anthozoa are all marine and include the sea anemones and corals.
The anthozoans entirely lack the medusa stage of the life cycle.
Polyps produce eggs and sperm, and the fertilized egg develops into a planula that develops directly into more polyps.
Sea anemones are solitary. Some species are able to swim, others can crawl slowly, and some can burrow.
The sea pens are generally sessile and colonial.
Cnidarians: Two Cell Layers and Blind Guts
Corals are also usually sessile and colonial.
The polyps of corals secrete a matrix of organic molecules upon which calcium carbonate is deposited.
This matrix forms the eventual skeleton of the coral colony.
As coral colonies grow, old polyps die and leave their calcareous skeletons behind.
Living members of the colony form a layer on top of a growing reef of skeletal remains.
Cnidarians: Two Cell Layers and Blind Guts
The Great
Barrier Reef along the northeastern coast of
Corals flourish in nutrient-poor, clear, tropical waters.
Symbiotic photosynthetic dinoflagellates help corals obtain enough nutrients to grow rapidly.
The corals provide protection and nutrients for the dinoflagellates, which in turn provide the products of photosynthesis to the corals.
Global warming and nutrient runoff are threatening coral reefs throughout the world.
Cnidarians: Two Cell Layers and Blind Guts
Cnidarians in the class Hydrozoa have diverse life cycles.
The polyp commonly dominates the life cycle; however, some species have only medusae while others have only polyps.
Most hydrozoans are colonial. The polyps are interconnected and share a continuous gastrovascular cavity.
The polyps may be specialized for different tasks such as food capture, defense, and reproduction, as in the man-of-war.
Cnidarians: Two Cell Layers and Blind Guts
Members of the class Scyphozoa are commonly known as jellyfish and are all marine.
The medusa typically has the form of an inverted cup, and the tentacles with nematocysts extend downward from the margin of the cup.
The medusa dominates the life cycle of the scyphozoans.
Ctenophores: Complete Guts and Tentacles
The phylum Ctenophora (comb jellies) was the next lineage to separate from the lineage leading to all other animals.
They have body plans that are superficially similar to those of cnidarians.
Both groups have two cell layers separated by thick mesoglea, and both have radial symmetry and feeding tentacles.
The ctenophores also have low metabolic rates.
Ctenophores: Complete Guts and Tentacles
Ctenophores have a complete gut with a separate mouth and anus.
Ctenophores have eight comblike rows of fused plates of cilia, called ctenes, that they use for movement.
The tentacles do not have nematocysts but are covered with sticky filaments to which prey adhere.
Ctenophores: Complete Guts and Tentacles
Ctenophores have simple life cycles. Gametes from gonads located on the walls of the gastrovascular cavity are released into the cavity and then discharged through the mouth or pores.
Fertilization takes place in the open seawater, and the fertilized egg develops into a miniature ctenophore that grows into an adult.
The Evolution of Bilaterally Symmetrical Animals
A common ancestor of all bilaterally symmetrical animals is postulated.
Zoologists use evidence from genes, development, and the structure of existing animals to infer the form of ancient bilaterians.
The development of all bilaterally symmetrical animals is controlled by homologous Hox and homeobox genes. It is unlikely that these genes evolved separately in several animal lineages.
Fossilized tracks from the late Precambrian suggest that early bilaterians had circulatory systems, antagonistic muscles, and a tissue- or fluid-filled body cavity.
The Evolution of Bilaterally Symmetrical Animals
The protostomes and the deuterostomes that dominate todays fauna have been evolving separately since the Cambrian period.
Members of both lineages are bilaterally symmetrical and have cephalization.
The Evolution of Bilaterally Symmetrical Animals
Shared, derived traits that unite the protostomes include:
A central nervous system consisting of an anterior brain that surrounds the entrance to the digestive tract
A ventral nervous system consisting of paired or fused longitudinal nerve cords
Free-floating larvae with a food-collecting system consisting of compound cilia on multiciliate cells
A blastopore that becomes the mouth
Spiral cleavage (in some species)
The Evolution of Bilaterally Symmetrical Animals
The major shared, derived traits that unite the deuterostomes inlcude:
A dorsal nervous system
Larvae, if present, that have a food-collecting system consisting of cells with a single cilium
A blastopore that becomes the anus
Radial cleavage
The Evolution of Bilaterally Symmetrical Animals
Evidence suggests that the protostomes split into two major lineages that have been evolving separately since ancient times:
Lophotrochozoans grow by adding to the size of their skeletal elements and use cilia for locomotion. Many have a free-living larva called a trochophore.
Ecdysozoans increase in size by molting their external skeletons, move by mechanisms other than ciliary action, and share a common set of homeobox genes.
Simple Lophotrochozoans
The flatworms (phylum Platyhelminthes) are the simplest of the lophotrochozoans.
The flatworms are bilaterally symmetrical, unsegmented, acoelomate animals.
They lack organs for transporting oxygen to internal tissues.
They have simple organs for excreting metabolic wastes.
Their flattened form allows each body cell to be near a body surface, a requirement of their body plan.
Simple Lophotrochozoans
The flatworm digestive tract is a mouth opening into a blind sac.
The sac is often highly branched, increasing the surface area available for the absorption of nutrients.
Flatworms feed on living or dead animal tissue.
The motile flatworms move by beating broad bands of cilia.
Simple Lophotrochozoans
The flatworms of the class Turbellaria are probably most similar to ancestral flatworm forms.
Turbellarians are small, free-living, marine and freshwater animals.
The head has chemoreceptor organs, simple eyes, and a small brain.
Simple Lophotrochozoans
Most living flatworms are parasitic, such as the tapeworms (class Cestoda) and the flukes (class Trematoda).
Parasitic flatworms lack digestive tracts; they absorb digested food from their hosts.
Some species cause serious diseases, such as schistosomiasis.
Most parasitic species have complex life cycles involving one or more intermediate hosts and several larval stages.
Simple Lophotrochozoans
The phylum Rotifera is comprised of bilaterally symmetrical, pseudocoelomate, unsegmented animals that have three cell layers.
Most rotifers are tiny but they have highly developed internal organs.
The rotifers have a complete gut passing from an anterior mouth to a posterior anus.
Their pseudocoelom functions as a hydrostatic skeleton.
They move by means of rapidly beating cilia.
Simple Lophotrochozoans
A ciliated organ called the corona surmounts the head of many species. Beating of its cilia sweeps particles of organic matter from the water into the mouth.
Once food enters the mouth, it travels down to a complex structure called the mastax, where it is ground.
Males and females are found in most species, but some have only females that produce diploid eggs.
Most of the 1,800 known species live in fresh water.
Lophophorates: An Ancient Body Plan
The lophotrochozoan lineage divided into two branches after the platyhelminthes and rotifers diverged from it.
The descendants of these branches became the lophophorates and the spiralians.
There are three phyla of lophophorate animals surviving today:
Phoronida
Brachiopoda
Ectoprocta
Lophophorates: An Ancient Body Plan
Members of these phyla are primarily marine.
Lophophorate animals obtain food by filtering it from ocean waters.
These animals have a unique feature, the lophophore, a circular or U-shaped ridge around the mouth that bears two rows of ciliated, hollow tentacles.
The lophophore is used for both food collection and gas exchange.
Nearly all lophophorate animals are sessile.
Lophophorates: An Ancient Body Plan
There are 20 known species of phoronids in the phylum Phoronida.
They are sedentary worms that live in muddy or sandy sediments or attached to a rocky substrate.
They range in size from 5 to 25 cm in length and secrete chitinous tubes in which they live.
In most species eggs are fertilized in the water, but some produce large eggs that are fertilized internally.
Lophophorates: An Ancient Body Plan
Ectoprocts (phylum Ectoprocta) are colonial lophophorates. They live in a house secreted by the body wall.
Strands of tissue connect the individuals within the colony.
Ectoprocts have greater control over the lophophore than members of other lophophorate phyla.
Colonies of ectoprocts are formed by asexual reproduction and can have as many as 2 million members.
Ectoprocts also reproduce sexually. Fertilization is internal and developing embryos are brooded before they exit as larvae.
Lophophorates: An Ancient Body Plan
The brachiopods (phylum Brachiopoda) are solitary, marine lophophorate animals that superficially resemble bivalve mollusks.
The shell differs from that of mollusks in that its two halves are dorsal and ventral rather than lateral.
Brachiopods are either attached to a solid substrate by a short, flexible stalk or embedded in soft sediment.
Most species release gametes into the water, where they are fertilized.
More than 26,000 fossil species have been described, but only 350 species survive today.
Spiralians: Spiral Cleavage and
Wormlike Body Plans
The spiralian lineage gave rise to many phyla, with members of more than a dozen of these phyla being wormlike.
Wormlike characteristics include bilateral symmetry, absence of legs, and soft bodies that are many times longer than they are wide.
This body form enables animals to move efficiently through muddy and sandy marine sediments.
Spiralians: Spiral Cleavage and
Wormlike Body Plans
Ribbon worms (phylum Nemertea) are carnivorous spiralians.
They are similar in structure to the flatworms, but they have a complete digestive tract.
Small ribbon worms move by beating their cilia; larger ones move by waves of contraction of body muscles.
Spiralians: Spiral Cleavage and
Wormlike Body Plans
Within the body of most ribbon worms is a fluid-filled cavity called the rhynchocoel within which is the hollow, muscular proboscis.
When the muscles surrounding the rhynchocoel contract, the proboscis is everted explosively through the anterior end.
The proboscis is armed with a sharp stylet that pierces the prey and through which paralysis-inducing toxins are discharged.
Spiralians: Spiral Cleavage and
Wormlike Body Plans
A body cavity that is segmented allows an animal to alter the shape of its body in complex ways and to control its movements precisely.
Segmentation evolved several times among spiralians.
The annelids (phylum Annelida) are a diverse group of segmented worms.
Annelid species can be found in marine, freshwater, and terrestrial environments.
Spiralians: Spiral Cleavage and
Wormlike Body Plans
Each segment in an annelid is controlled by a separate nerve center called a ganglion. All the ganglia are connected by nerve cords that coordinate their function.
The coelom in each segment is isolated from those in other segments.
Most species lack a rigid, external protective surface.
The thin body wall serves as a surface for gas exchange and also limits annelids to moist environments, as they lose body water rapidly in dry air.
Spiralians: Spiral Cleavage and
Wormlike Body Plans
More than half of all annelid species are members of the class Polychaeta, which are mostly marine.
Thin outgrowths called parapodia extend laterally from the body wall in most segments. They function in gas exchange and sometimes movement.
Stiff bristles called setae protrude from each parapodium and form temporary attachments to the substrate that prevent the animal from slipping backward when its muscles contract.
Fertilization takes place in the water, and fertilized eggs develop into trochophore larvae.
Spiralians: Spiral Cleavage and
Wormlike Body Plans
The trochophore is a distinctive larva type in polychaetes, mollusks, and several other lineages with spiral cleaveage.
The trochophore is believed to represent an evolutionary link between the annelids and the mollusks.
Spiralians: Spiral Cleavage and
Wormlike Body Plans
More than 90 percent of oligochaete (class Oligochaeta) species live in freshwater or terrestrial habitats.
They have no parapodia, eyes, or anterior tentacles.
All oligochaetes are hermaphroditiceach individual is both male and female.
Eggs are laid in a cocoon outside the adults body.
Spiralians: Spiral Cleavage and
Wormlike Body Plans
The leeches (class Hirudinea) probably evolved from oligochaete ancestors.
Leeches live in freshwater or terrestrial habitats. They lack parapodia and tentacles and are hermaphroditic.
Groups of segments at the end of a leech are modified to form suckers that serve as temporary anchors to aid in movement.
Many leeches are external parasites of other animals.
An anticoagulant secreted by the leech keeps the hosts blood flowing.
Spiralians: Spiral Cleavage and
Wormlike Body Plans
The vestimentiferans (class Pogonophora) are burrowing forms with a crown of tentacles through which gases are exchanged.
They have entirely lost their digestive systems.
The coelom consists of an anterior compartment and a long, subdivided cavity that extends much of the length of its body.
The most remarkable species live near deep-ocean hydrothermal vents. These species harbor endosymbiotic prokaryotes that fix carbon using energy obtained from the oxidation of H2S.
Spiralians: Spiral Cleavage and
Wormlike Body Plans
The mollusks (phylum Mollusca) range in size from small snails to giant squids that can be more than 18 meters long.
Mollusks have a unique body plan with three major structural components: foot, mantle, and visceral mass.
The molluscan foot is a large, muscular structure that originally was both an organ of locomotion and support for the internal organs.
Spiralians: Spiral Cleavage and
Wormlike Body Plans
The mantle is a fold of tissue that covers the visceral mass of internal organs.
In many mollusks, the mantle extends beyond the visceral mass to form the mantle cavity.
The gills lie within the mantle cavity; the beating of their cilia creates a flow of oxygenated water over them.
Mollusks have a much-reduced coelom.
The radula is a rasping feeding structure.
The three body parts are the unique shared derived characteristics of all mollusks.
Spiralians: Spiral Cleavage and
Wormlike Body Plans
The monoplacophorans (class Monoplacophora) were the most abundant mollusks during the Cambrian period.
Today there are only a few surviving species.
They have multiple gills, muscles, and excretory structures that are repeated over the length of their body.
Spiralians: Spiral Cleavage and
Wormlike Body Plans
The chitons (class Polyplacophora) have multiple gills and segmented shells, but their other body parts are not segmented.
They have simple internal organs.
Most are marine herbivores that scrape algae from rocks with their radulae.
Adult chitons spend most of their lives glued tightly to rock surfaces by their large foot.
Spiralians: Spiral Cleavage and
Wormlike Body Plans
The bivalves (class Bivalvia) have a hinged, two-part shell that extends over the sides and top of their body.
Bivalves are largely sedentary.
They have greatly reduced heads.
Feeding is accomplished by bringing water in through an opening called an incurrent siphon and extracting food from the water using their gills.
Water and gametes exit through another opening, the excurrent siphon.
Spiralians: Spiral Cleavage and
Wormlike Body Plans
The gastropods (class Gastropoda) are mostly motile, using their large foot to move across a substrate or to burrow through it.
The gastropods are the most species-rich and widely distributed of the molluscan classes.
Some gastropods can crawl, whereas others have a modified foot that functions as a swimming organ.
Gastropods are the only terrestrial mollusks. They have a mantle cavity that is modified into a highly vascularized lung.
Spiralians: Spiral Cleavage and
Wormlike Body Plans
The cephalopods (class Cephalopoda) have a modified excurrent siphon.
This modification allowed early cephalopods to control the water content of the mantle cavity.
The modification of the mantle into a device for forcibly ejecting water from the cavity enabled cephalopods to move rapidly through the water.
It also allows the animals to control their buoyancy.
Their greatly enhanced mobility allowed some cephalopods, such as squids and octopuses, to become the major predators in open ocean waters.
Spiralians: Spiral Cleavage and
Wormlike Body Plans
Cephalopods include the squids, octopuses, and nautiluses.
They appeared near the beginning of the Cambrian period about 600 million years ago.
They were the first large, shelled animals able to move vertically in the ocean.
Nautiloids are the only cephalopods with external chambered shells that survive today.
Animation 32.1 Life Cycle of a Cnidarian
Video 32.1 Portrait
of a marine ecosystem: A coral reef in the
Video 32.2 Deep-sea bioluminescence in ctenophores, jellyfish, and squid
Video 32.3 The ctenophore Beroe sp.
Video 32.4 The flatworm Mesostoma, catching and eating Daphnia
Video 32.5 The rotifers Floscularia and Colletheca feeding
Video 32.6 Rotifers feeding via flagella-induced vortices
Video 32.7 The bryozoan Plumatella sp. feeding