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Hickman−Roberts−Larson: Animal Diversity, Third Edition

3. Animal Architecture Text © The McGraw−Hill Companies, 2002

New Designs for Living

Zoologists today recognize 32 phyla of multicellular animals, eachphylum characterized by a distinctive body plan and array of biologi-cal properties that set it apart from all other phyla. Nearly all are thesurvivors of perhaps 100 phyla that were generated 600 million yearsago during the Cambrian explosion, the most important evolutionaryevent in the history of animal life.Within the space of a few millionyears virtually all of the major body plans that we see today, togetherwith many other novel plans that we know only from the fossilrecord, were established. Entering a world sparse in species andmostly free of competition, these new life forms began widespreadexperimentation, producing new themes in animal architecture.Nothing since then has equaled the Cambrian explosion. Later burstsof speciation that followed major extinction events produced onlyvariations on established themes.

Once forged, a major body plan becomes a limiting determi-nant of body form for descendants of that ancestral line. Molluscsbeget only molluscs and birds beget only birds, nothing else. Despitethe appearance of structural and functional adaptations for distinc-tive ways of life, the evolution of new forms always develops withinthe architectural constraints of the phylum’s ancestral pattern. Thisis why we shall never see molluscs that fly or birds confined withina protective shell.

c h a pt e rt h r e e

Animal Architecture

Cnidarian polyps haveradial symmetry and cell-tissue grade of organiza-tion, (Dendronephthya sp.).

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51



52 c h ap t e r t h re e

he English satirist Samuel Butler proclaimed that thehuman body was merely “a pair of pincers set over abellows and a stewpan and the whole thing fixed upon

stilts.”While human attitudes toward the human body are dis-tinctly ambivalent, most people less cynical than Butler wouldagree that the body is a triumph of intricate, living architec-ture. Less obvious, perhaps, is that humans and most other ani-mals share an intrinsic material design and fundamentalfunctional plan despite vast differences in structural complex-ity. This essential uniformity of biological organization derivesfrom the common ancestry of animals and from their basic cel-lular construction. In this chapter, we will consider the limitednumber of body plans that underlie the diversity of animalform and examine some of the common architectural themesthat animals share.

The Hierarchical Organization of Animal ComplexityAmong the different metazoan groups, we can recognize fivemajor grades of organization (table 3.1). Each grade is morecomplex than the preceding one and builds upon it in a hier-archical manner.

The unicellular protozoan groups are the simplest animal-like organisms. They are nonetheless complete organisms thatperform all basic functions of life as seen in more complexanimals. Within confines of their cell, they show remarkableorganization and division of labor, possessing distinct sup-portive structures, locomotor devices, fibrils, and simple sen-sory structures. The diversity observed among unicellularorganisms is achieved by varying the architectural patterns ofsubcellular structures, organelles, and the cell as a whole (seeChapter 5).

Metazoa, or multicellular animals, evolved greater struc-tural complexity by combining cells into larger units. A meta-zoan cell is a specialized part of the whole organism and,unlikea protozoan cell, it is not capable of independent existence.Cells of a multicellular organism are specialized for performingthe various tasks accomplished by subcellular elements in uni-cellular forms. The simplest metazoans show the cellulargrade of organization in which cells demonstrate division oflabor but are not strongly associated to perform a specific col-lective function (table 3.1). In the more complex tissue grade,similar cells are grouped together and perform their commonfunctions as a highly coordinated unit. In animals of the tissue-organ grade of organization, tissues are assembled into stilllarger functional units called organs. Usually one type of tis-sue carries the burden of an organ’s chief function, as muscletissue does in the heart; other tissues—epithelial, connective,and nervous—perform supportive roles. The chief functionalcells of an organ are called its parenchyma (pa-ren´ka-ma;Gr.para, beside, + enchyma, infusion). The supportive tissues areits stroma (Gr. bedding). For instance, in the vertebrate pan-creas the secreting cells are the parenchyma; the capsule andconnective tissue framework represent the stroma.

Most metazoa (nemerteans and all more structurallycomplex phyla) have an additional level of complexity inwhich different organs operate together as organ systems.Eleven different kinds of organ systems are observed in meta-zoans: skeletal, muscular, integumentary, digestive, respiratory,circulatory, excretory, nervous, endocrine, immune, and repro-ductive. The great evolutionary diversity of these organ sys-tems is covered in Chapters 5–20.

Complexity and Body SizeThe opening essay (p. 51) suggests that size is a major consid-eration in the design of animals. The most complex grades ofmetazoan organization permit and to some extent even pro-mote evolution of large body size (figure 3.1). Large size con-fers several important physical and ecological consequencesfor an organism. As animals become larger, the body surfaceincreases much more slowly than body volume because sur-face area increases as the square of body length (length2),whereas volume (and therefore mass) increases as the cube ofbody length (length3). In other words, a large animal will haveless surface area relative to its volume than will a small animalof the same shape. The surface area of a large animal may beinadequate for respiration and nutrition by cells located deepwithin the body. There are two possible solutions to this prob-lem. One solution is to fold or invaginate the body surface toincrease surface area or, as exploited by flatworms, flatten thebody into a ribbon or disc so that no internal space is far fromthe surface. This solution allows the body to become largewithout internal complexity. However, most large animalsadopted a second solution; they developed internal transportsystems to shuttle nutrients, gases, and waste productsbetween the cells and the external environment.

Larger size buffers an animal against environmental fluc-tuations; it provides greater protection against predation andenhances offensive tactics, and it permits a more efficient useof metabolic energy. A large mammal uses more oxygen than asmall mammal, but the cost of maintaining its body tempera-ture is less per gram of weight for a large mammal than for asmall one.Large animals also can move at less energy cost thancan small animals. A large mammal uses more oxygen in run-ning than a small mammal,but energy cost of moving 1 g of itsbody over a given distance is much less for a large mammalthan for a small one (figure 3.2). For all of these reasons, eco-logical opportunities of larger animals are very different fromthose of small ones. In subsequent chapters we will describethe extensive adaptive radiations observed in taxa of large ani-mals, covered in Chapters 8–20.

Extracellular Components of the Metazoan BodyIn addition to the hierarchically arranged cellular structuresdiscussed, metazoan animals contain two important noncellu-lar components: body fluids and extracellular structural ele-

T



ments. In all eumetazoans, body fluids are subdivided into twofluid “compartments:” those that occupy intracellular space,within the body’s cells, and those that occupy extracellularspace, outside the cells. In animals with closed vascular sys-tems (such as segmented worms and vertebrates),extracellularfluids are subdivided further into blood plasma (the fluidportion of the blood outside the cells; blood cells are reallypart of the intracellular compartment) and interstitial fluid.Interstitial fluid, also called tissue fluid,occupies the space sur-rounding cells. Many invertebrates have open blood systems,however, with no true separation of blood plasma from inter-stitial fluid.

If we were to remove all specialized cells and body fluidsfrom the interior of the body, we would be left with the third

element of the animal body: extracellular structural elements.This is the supportive material of the organism,including looseconnective tissue (especially well developed in vertebrates butpresent in all metazoa), cartilage (molluscs and chordates),bone (vertebrates), and cuticle (arthropods, nematodes,annelids, and others). These elements provide mechanical sta-bility and protection. In some instances, they act also as a

Animal Architecture 53

table 3.1 Levels of Organization in Organismal Complexity

1. Protoplasmic level of organization. Protoplasmic organization is found in unicellu-lar organisms. All life functions are confined within the boundaries of a singlecell, the fundamental unit of life. Within a cell, living substance is differentiatedinto organelles capable of carrying on specialized functions.

2. Cellular level of organization. Cellular organization is an aggregation of cellsthat are functionally differentiated. A division of labor is evident, so that somecells are concerned with, for example, reproduction, others with nutrition.Such cells have little tendency to become organized into tissues (a tissue is agroup of similar cells organized to perform a common function). Some pro-tozoan colonial forms that have distinct somatic and reproductive cells mightbe placed at the cellular level of organization. Many authorities also placesponges at this level.

3. Cell-tissue level of organization. A step beyond the preceding is the aggregationof similar cells into definite patterns or layers, thus becoming a tissue. Someauthorities assign sponges to this level, although jellyfishes and their relatives(Cnidaria) more clearly demonstrate the tissue plan. Both groups are stilllargely of the cellular grade of organization because most of the cells are scattered and not organized into tissues. An excellent example of a tissue incnidarians is the nerve net, in which the nerve cells and their processes form a definite tissue structure, with the function of coordination.

4. Tissue-organ level of organization. Aggregation of tissues into organs is a furtherstep in complexity. Organs are usually made up of more than one kind of tissueand have a more specialized function than tissues. This is the organizationallevel of the flatworms (Platyhelminthes), in which there are a number of well-defined organs such as eyespots, digestive tract, and reproductive organs. Infact, the reproductive organs are well organized into a reproductive system.

5. Organ-system level of organization. When organs work together to performsome function we have the most complex level of organization—the organsystem. Systems are associated with basic body functions—circulation, respi-ration, digestion, and others. The simplest animals that show this type oforganization are nemertean worms, which have a complete digestive systemdistinct from the circulatory system. Most animal phyla demonstrate this typeof organization.

The term intercellular, meaning “between cells,” shouldnot be confused with the term intracellular, meaning“within cells.”



depot of materials for exchange and serve as a medium forextracellular reactions.We will describe the diversity of extra-cellular skeletal elements characteristic of the different groupsof animals in Chapters 10–20.

Types of TissuesA tissue is a group of similar cells, together with associated cellproducts, specialized for performance of a common function.The study of tissues is called histology (Gr. histos, tissue, +logos, discourse). All cells in metazoan animals take part in theformation of tissues. Sometimes cells of a tissue may be of sev-eral kinds, and some tissues have a great many intercellularmaterials.

During embryonic development, the germ layers becomedifferentiated into four kinds of tissues. These are epithelial,


f i g u r e 3.2Net cost of running for mammals of various sizes. Each point representsthe cost (measured in rate of oxygen consumption) of moving 1 g ofbody over 1 km. The cost decreases with increasing body size.

First eukaryote

Whale

Dinosaur

Amphibian

Fish

Eurypterid

Trilobite

Cyanobacteria

Bacteria

Origin of earth

1µm 10µm 100µm 1mm10mm 100mm 1cm 10cm 100cm 1m 10m 100m

Length

Bill

ion

year

s ag

oM

illio

n ye

ars

ago

4

3

2

1

800

600

400

300

Present

200

100

f i g u r e 3.1Graph showing evolution of length increasein the largest organisms present at differentperiods of life on earth. Note that bothscales are logarithmic.

White mouseKangaroo rat

Ground squirrel

DogHuman

Horse

10

1

0.1

0.0110 g 100 g 1 kg 10 kg 100 kg 1000 kg

Body size

ml O

2



connective (including vascular), muscular, and nervous tissues(figure 3.3).This is a surprisingly short list of only four basic tis-sue types that are able to meet the diverse requirements of ani-mal life.

Epithelial TissueAn epithelium (pl., epithelia) is a sheet of cells that covers anexternal or internal surface. Outside the body, epitheliumforms a protective covering. Inside, an epithelium lines allorgans of the body cavity, as well as ducts and passagewaysthrough which various materials and secretions move. Onmany surfaces epithelial cells are often modified into glandsthat produce lubricating mucus or specialized products suchas hormones or enzymes.

Epithelia are classified by cell form and number of celllayers. Simple epithelia (figure 3.4) are found in all metazoananimals, while stratified epithelia (figure 3.5) are mostlyrestricted to the vertebrates. All types of epithelia are sup-

ported by an underlying basement membrane, which is a con-densation of the ground substance of connective tissue. Bloodvessels never enter epithelial tissues, so they are dependent ondiffusion of oxygen and nutrients from underlying tissues.

Connective TissueConnective tissues are a diverse group of tissues that serve var-ious binding and supportive functions.They are so widespreadin the body that removal of other tissues would still leave thecomplete form of the body clearly apparent. Connective tissueis made up of relatively few cells, a great many extracellularfibers, and a fluid, known as ground substance (also calledmatrix), in which the fibers are embedded.We recognize sev-eral different types of connective tissue.Two kinds of connec-tive tissue proper occur in vertebrates. Loose connectivetissue is composed of fibers and both fixed and wanderingcells suspended in a syrupy ground substance. Dense con-nective tissue, such as tendons and ligaments, is composed


f i g u r e 3.3Types of tissues in a vertebrate, showing examples of where different tissues are located in a frog.

Stratified epitheliumin epidermis

Areolar connectivetissue in dermis

Reproductivetissue(testes)

Skeletal muscle tissuein voluntary muscles

Smooth muscletissue in intestinal wall

Columnar epithelium inlining of stomach

Cardiac muscletissue in heart

Blood tissue invascular system

Nervous tissuein brain

Bone tissue



largely of densely packed fibers (figure 3.6).Much of the fibroustissue of connective tissue is composed of collagen (Gr. kolla,glue, + genos, descent), a protein material of great tensilestrength. Collagen is the most abundant protein in the animalkingdom, found in animal bodies wherever both flexibility andresistance to stretching are required. The connective tissue ofinvertebrates, as in vertebrates, consists of cells, fibers, andground substance,but usually it is not as elaborately developed.

Other types of connective tissue include blood, lymph,and tissue fluid (collectively considered vascular tissue),com-posed of distinctive cells in a watery ground substance, theplasma. Vascular tissue lacks fibers under normal conditions.Cartilage is a semirigid form of connective tissue with closelypacked fibers embedded in a gel-like ground substance(matrix). Bone is a calcified connective tissue containing cal-cium salts organized around collagen fibers.


Simplesquamousepithelial

cellBasementmembrane Nucleus

Freesurface

Simple squamous epithelium

Simple cuboidal epithelium

Simple columnar epithelium

Simple squamous epithelium, composed of flattened cells that form a continuous delicatelining of blood capillaries, lungs, and other surfaces where it permits the passive diffusion of gasesand tissue fluids into and out of cavities.

Simple cuboidal epithelial cell

Basementmembrane

Lumen(free space)

Simple cuboidal epithelium is composed of short, boxlike cells. Cuboidal epithelium usuallylines small ducts and tubules, such as those of the kidney and salivary glands, and may have activesecretory or absorptive functions.

Epithelial cells

Basementmembrane Nuclei

Microvilli oncell surface

Simple columnar epithelium resembles cuboidal epithelium but the cells are taller and usu-ally have elongate nuclei. This type of epithelium is found on highly absorptive surfaces such asthe intestinal tract of most animals. The cells often bear minute, fingerlike projections calledmicrovilli that greatly increase the absorptive surface. In some organs, such as the female repro-ductive tract, the cells are ciliated.

f i g u r e 3.4Types of simple epithelium



Muscular TissueMuscle is the most common tissue of most animals. It origi-nates (with few exceptions) from mesoderm,and its unit is thecell or muscle fiber, specialized for contraction. Whenviewed with a light microscope, striated muscle appearstransversely striped (striated), with alternating dark and lightbands (figure 3.7). In vertebrates we recognize two types ofstriated muscle: skeletal and cardiac muscle. A third kind ofmuscle is smooth (or visceral) muscle, which lacks the charac-teristic alternating bands of the striated type (figure 3.7).Unspecialized cytoplasm of muscles is called sarcoplasm,and contractile elements within the fiber are myofibrils.

Nervous TissueNervous tissue is specialized for reception of stimuli and con-duction of impulses from one region to another. Two basictypes of cells in nervous tissue are neurons (Gr. nerve), the

basic functional unit of the nervous system, and neuroglia(nu-rog´le-a; Gr. nerve, + glia, glue), a variety of nonnervouscells that insulate neuron membranes and serve various sup-portive functions. Figure 3.8 shows the functional anatomy ofa typical nerve cell.

Animal Body PlansAs pointed out in the prologue to this chapter, the diversity ofanimal body form is constrained by ancestral history, habitat,and way of life. Although a worm that adopts a parasitic life inthe intestine of a vertebrate will look and function very differ-ently from a free-living member of the same group, both sharedistinguishing hallmarks of the phylum.

Major evolutionary innovations in the forms of animalsinclude multicellularity, bilateral symmetry, “tube-within-a-tube”plan,and eucoelomate (true coelom) body plan.These advance-ments with their principal alternatives can be arranged in abranching pattern, as shown in figure 3.9.


Stratified squamous epithelium

Transitional epithelium—stretched

Free surface

Stratified squamousepithelial cell

Nuclei

Basementmembrane

Stratified squamous epitheliumconsists of two to many layers of cellsadapted to withstand mild mechanicalabrasion. The basal layer of cells under-goes continuous mitotic divisions,producing cells that are pushed towardthe surface where they are sloughed offand replaced by new cells beneath them.This type of epithelium lines the oralcavity, esophagus, and anal canal of manyvertebrates, and vagina of mammals.

f i g u r e 3.5Types of stratified epithelium

Free surfaceBasementmembrane

Connectivetissue

Nucleus Transitionalepithelial cell

Transitional epithelium—unstretched

Transitional epithelium is a type of stratifiedepithelium specialized to accommodate great stretching. This type of epithelium is found in the urinary tract and bladder of vertebrates. In therelaxed state it appears to be four or five celllayers thick, but when stretched out it appearsto have only two or three layers of extremelyflattened cells.




Collagen fiber Elastic fiberNucleus Nucleus Fibers

Loose connective tissue, also called areolar connective tissue, is the“packing material” of the body that anchors blood vessels, nerves, andbody organs. It contains fibroblasts that synthesize the fibers andground substance of connective tissue and wandering macrophages thatphagocytize pathogens or damaged cells. The different fiber typesinclude strong collagen fibers (thick and violet in micrograph) and elasticfibers (black and branching in micrograph) formed of the protein elastin.Adipose (fat) tissue is considered a type of loose connective tissue.

Dense connective tissue forms tendon, ligaments, and fasciae (fa´sha),the latter arranged as sheets or bands of tissue surrounding skeletalmuscle. In tendon (shown here) the collagenous fibers are extremelylong and tightly packed together.

Lacuna MatrixChondrocyte

Cartilage is a vertebrate connective tissue composed of a firm gelground substance (matrix) containing cells (chondrocytes) living in smallpockets called lacunae, and collagen or elastic fibers (depending on thetype of cartilage). In hyaline cartilage shown here, both collagen fibersand ground substance are stained uniformly purple, and cannot bedistinguished one from the other. Because cartilage lacks a blood supply,all nutrients and waste materials must diffuse through the ground sub-stance from surrounding tissues.

Centralcanal

Osteocytesin lacunae

Mineralizedmatrix

Bone, strongest of vertebrate connective tissues, contains mineralizedcollagen fibers. Small pockets (lacunae) within the matrix contain bonecells, called osteocytes. The osteocytes communicate with blood vesselsthat penetrate into bone by means of a tiny network of channels calledcanaliculi. Unlike cartilage, bone undergoes extensive remodeling duringan animal’s life, and can repair itself following even extensive damage.

f i g u r e 3.6Types of connective tissue



Animal SymmetrySymmetry refers to balanced proportions, or correspondencein size and shape of parts on opposite sides of a median plane.Spherical symmetry means that any plane passing throughthe center divides the body into equivalent,or mirrored,halves(figure 3.10). This type of symmetry is found chiefly among

some protozoan groups and is rare in metazoans. Sphericalforms are best suited for floating and rolling.

Radial symmetry (figure 3.10) applies to forms thatcan be divided into similar halves by more than two planespassing through the longitudinal axis. These are the tubular,vase, or bowl shapes in which one end of the longitudinal axisis usually the mouth. Examples are the hydras, jellyfishes, sea


Skeletal muscle fiber Nucleus Striations

Nucleus of cardiacmuscle cell

Note striations Intercalated discs(special junctionsbetween cells)

Nuclei of smooth muscle cells

Skeletal muscle is a type of striated muscle found in both invertebrates and verte-brates. It is composed of extremely long, cylindrical fibers, which are multinucleatecells that may reach from one end of the muscle to the other. Viewed through thelight microscope, the cells appear to have a series of stripes, called striations, runningacross them. Skeletal muscle is called voluntary muscle (in vertebrates) because it con-tracts when stimulated by nerves under conscious cerebral control.

Cardiac muscle is another type of striated muscle found only in the vertebrateheart. The cells are much shorter than those of skeletal muscle and have only onenucleus per cell (uninucleate). Cardiac muscle tissue is a branching network of fiberswith individual cells interconnected by junctional complexes called intercalated discs.Cardiac muscle is called involuntary muscle because it does not require nerve activityto stimulate contraction. Instead, heart rate is controlled by specialized pacemakercells located in the heart itself. However, autonomic nerves from the brain may alterpacemaker activity.

Smooth muscle is nonstriated muscle found in both invertebrates and vertebrates.Smooth muscle cells are long, tapering strands, each containing a single nucleus.Smooth muscle is the most common type of muscle in invertebrates in which it servesas body wall musculature and lines ducts and sphincters. In vertebrates, smooth mus-cle cells are organized into sheets of muscle circling the walls of the alimentary canal,blood vessels, respiratory passages, and urinary and genital ducts. Smooth muscle istypically slow acting and can maintain prolonged contractions with very little energyexpenditure. Its contractions are involuntary and unconscious. The principal functionsof smooth muscles are to push the material in a tube, such as the intestine, along itsway by active contractions or to regulate the diameter of a tube, such as a blood ves-sel, by sustained contraction.

f i g u r e 3.7Types of muscle tissue



urchins, and some sponges. A variant form is biradial symme-try, in which, because of some part that is single or pairedrather than radial, only one or two planes passing through thelongitudinal axis produce mirrored halves. Sea walnuts (phy-lum Ctenophora), which are more or less globular in form buthave a pair of tentacles, are an example.Radial and biradial ani-mals are usually sessile, freely floating, or weakly swimming.The two phyla that are primarily radial, Cnidaria andCtenophora, are called the Radiata. Echinoderms (sea starsand their kin) are primarily bilateral animals (their larvae arebilateral) that have become secondarily radial as adults.

Bilateral symmetry applies to animals that can bedivided along a sagittal plane into two mirrored portions, rightand left halves (figure 3.10). The appearance of bilateral sym-metry in animal evolution was a major innovation because

bilateral animals are much better fitted for directional (forward)movement than are radially symmetrical animals. Bilateral ani-mals are collectively called Bilateria. Bilateral symmetry isstrongly associated with cephalization, discussed on page 65.

Some convenient terms used for locating regions of ani-mal bodies (Figure 3.11) are anterior, used to designate thehead end;posterior, the opposite or tail end;dorsal, the backside; and ventral, the front or belly side. Medial refers to themidline of the body; lateral refers to the sides.Distal parts arefarther from the middle of the body; proximal parts arenearer. A frontal plane (also sometimes called coronal plane)divides a bilateral body into dorsal and ventral halves by run-ning through the anteroposterior axis and the right-left axis atright angles to the sagittal plane, the plane dividing an ani-mal into right and left halves. A transverse plane (also called


Dendrites: receive stimulifrom other neurons

Cell body

Nucleus

NucleolusAxon hillock

Schwann cell: forms insulatingsheath around many vertebrateperipheral nerves

Nodes of Ranvier: these interruptionsin Schwann cell insulation allow actionpotentials to leap from node to node

Collateralaxon

Synaptic terminals: release neurotransmitterchemicals into synapse when actionpotential arrives

Directionofconduction

Axon: transmits electrical impulsesfrom cell body to synaptic terminals

f i g u r e 3.8Functional anatomy of a neuron. From the nucleated body, or soma,extend one or more dendrites (Gr. dendron, tree), which receive elec-trical impulses from receptors or other nerve cells, and a single axonthat carries impulses away from the cell body to other nerve cells or toan effector organ. The axon is often called a nerve fiber. Nerves areseparated from other nerves or from effector organs by specializedjunctions called synapses.




Eumetazoans

Bilateral symmetry

Coelom from splitting of mesodermalbands, spiral cleavage

Enterocoelomate body planCoelom from mesodermalpouches, radial cleavage

Ancestral unicellular organism

Unicellular Multicellular

Cell aggregate

No germ layers, no true tissuesor organs, intracellular digestion

Germ layers, true tissues,mouth, digestive cavity

Radial symmetry Bilateral symmetry

(Mesozoa, sponges)

(Radiate animals)

Acoelomate body plan Tube-within-a-tube

Flow-through digestivetube; body cavity between

gut and body wallNemertean body plan Flatworm body plan

Mouth opening into blind sacdigestive tube, no circulatory system

(Platyhelminths)Complete digestive tractand circulatory system

Pseudocoelomate body planCavity derived from blastocoel,

no peritoneal lining

Eucoelomate body plan

Coelom derived from mesodermand lined with peritoneum

(Nematodes, rotifers, etc.)

Schizocoelomate body plan

Annelid body plan Vertebratebody plan

Molluscan body plan

Soft, segmented body Arthropod body plan

Soft, unsegmented bodywith mantle, usually

a shellBilateral symmetry,

jointed endoskeleton,specialized dorsal nervous

system, modified schizocoelEchinoderm body plan

Segmented body,exoskeleton,

jointed appendages

Secondary radial symmetry,endoskeletal plates

f i g u r e 3.9Architectural patterns of animals. These basic bodyplans have been variouslymodified during evolutionarydescent to fit animals to agreat variety of habitats.Ectoderm is shown in gray,mesoderm in red, and endo-derm in yellow.



a cross section) would cut through a dorsoventral and a right-left axis at right angles to both the sagittal and frontal planesand would result in anterior and posterior portions.

Developmental Patterns in Bilateral AnimalsThe expression of an animal’s body plan is based on an inher-ited pattern of development. In sexual multicellular organisms,the extremely complex process of embryonic development ofan animal from egg to fully differentiated adult is highly pre-dictable and virtually flawless.When developmental alterationsdo appear during speciation events, changes tend to be con-fined to end stages of development. The organization of eggs

and early cleavage stages remain stubbornly resistant tochange because any deviation introduced at an early stagewould be ruinous to the entire course of development. Never-theless, several times in the history of life just such radicaltransformations did occur to herald the appearance of com-pletely new designs for living, such as new classes, phyla, oreven major divisions within the animal kingdom.

Patterns of CleavageOne of the most fundamental aspects of animal develop-ment indicating evolutionary relationships is symmetry ofcleavage. Cleavage is the initial process of development fol-lowing fertilization of the egg by a sperm: dividing a fertil-


Bilateral symmetryRadial symmetrySpherical symmetry

f i g u r e 3.10The planes of symmetry as illustrated by spherically, radially, and bilaterally symmetrical animals.

Sagittal plane

Dorsal

Posterior

Frontal plane

Ventral

Anterior

Transverse plane

f i g u r e 3.11Descriptive terms used to identify positions on thebody of a bilaterally symmetrical animal.



ized egg, now called a zygote, into a large number of cells,called blastomeres.

Zygotes of many animals cleave by one of two patterns:radial or spiral (figure 3.12). In radial cleavage, the cleavageplanes are symmetrical to the polar axis and produce tiers, orlayers, of cells on top of each other. Radial cleavage is also saidto be regulative because each blastomere of the early embryo,if separated from the others, can adjust or “regulate” its devel-opment into a complete and well-proportioned (though possi-bly smaller) embryo (Figure 3.13).

Spiral cleavage, found in several phyla, differs fromradial in several ways. Rather than an egg dividing parallel or

perpendicular to the animal-vegetal axis, it cleaves oblique tothis axis and typically produces a quartet of cells that come tolie not on top of each other but in the furrows between thecells (see figure 3.12). In addition,spirally cleaving eggs tend topack their cells tightly together much like a cluster of soapbubbles, rather than just lightly contacting each other as dothose in many radially cleaving embryos. Spirally cleavingembryos also differ from radial embryos in having a mosaicform of development, in which the organ-forming determi-nants in the egg cytoplasm become strictly localized in theegg, even before the first cleavage division. The result is that ifthe early blastomeres are separated, each will continue todevelop for a time as though it were still part of the whole.Each forms a defective, partial embryo (figure 3.13). A curiousfeature of most spirally cleaving embryos is that at about the29-cell stage a blastomere called the 4d cell is formed that willgive rise to all mesoderm of the embryo.

The importance of these two cleavage patterns extendswell beyond the differences we have described. They signal afundamental dichotomy, an early evolutionary divergence ofbilateral metazoan animals into two separate lineages. Spiralcleavage is found in annelids,molluscs, and several other inver-tebrate phyla; all are included in the Protostomia (“mouthfirst”) division of the animal kingdom (see the illustrationinside the front cover of this book). The name Protostomiarefers to the formation of the mouth from the first embryolog-ical opening, the blastopore.Radial cleavage is characteristic ofthe Deuterostomia (“mouth second”) division of the animalkingdom, a grouping that includes echinoderms (sea stars andtheir kin),chordates,and hemichordates. In Deuterostomia, theblastopore usually becomes the anus, while the mouth formssecondarily. Other distinguishing developmental hallmarks ofthese two divisions are summarized in figure 3.14.


Radial cleavage Spiral cleavage

A B

f i g u r e 3.12Radial and spiral cleavage patterns shown at two-, four-, and eight-cellstages. A, Radial cleavage, typical of echinoderms, chordates, and hemi-chordates. B, Spiral cleavage, typical of molluscs, annelids, and otherprotostomes. Arrows indicate clockwise movements of small cells(micromeres) following division of large cells (macromeres).

Regulative(Sea urchin)

Mosaic(Mollusc)

Separate blastomeres Separate blastomeres

Normal larvae (plutei)

Normallarva

Defective larvae

f i g u r e 3.13Regulative and mosaic development. A, Regulativedevelopment. If the early blastomeres of a sea urchinembryo are separated, each will develop into a completelarva. B, Mosaic development. When the blastomeres of a mollusc embryo are separated, each gives rise to apartial, defective larva.

A B



Body CavitiesAnother major developmental event affecting the body plan ofbilateral animals was evolution of a coelom, a fluid-filled cav-ity between the outer body wall and the gut (see figure 3.9). Acoelom also provides a tube-within-a-tube arrangement thatallows much greater body flexibility than is possible in animalslacking an internal body cavity. A coelom also provides spacefor visceral organs and permits greater size and complexity byexposing more cells to surface exchange. A fluid-filled coelomadditionally serves as a hydrostatic skeleton in some forms,especially many worms, aiding in such activities as movementand burrowing.

As shown in figure 3.9, the presence of a coelom is a keyfactor in the evolution of bilateral metazoan body planes.

Acoelomate BilateriaMany bilateral animals do not have a true coelom. In fact, flat-worms and a few others have no body cavity surrounding thegut (figure 3.15, top); they are “acoelomate” (Gr. a, without, +koiloma, cavity). The region between the ectodermal epider-mis and the endodermal digestive tract is completely filledwith mesoderm in the form of a spongy mass of space-fillingcells called parenchyma.

Pseudocoelomate BilateriaNematodes and several other phyla have a body cavity sur-rounding the gut called a pseudocoel and its possessors havea tube-within-a-tube arrangement as do animals with a true


1 blastomereexcised

1 blastomereexcised

4-cellembryo

4-cellembryo

2normallarvae

Gut

4 Mosaic embryo 4 Regulative embryo

3 Coelom forms by splitting (schizocoelous) 3 Coelom forms by outpocketing (enterocoelous)

2 Spiral cleavage

Blastopore Mouth

2 Radial cleavage

1 Blastopore becomes mouth, anus forms secondarily Blastopore becomes anus, mouth forms secondarily1

Futureintestine

FutureAnus

Futuremouth

FutureintestineBlastopore

Blastocoel

Developmentarrested

Mesoderm

Blastocoel Coelom

Pocketof gut Mesoderm

PROTOSTOME DEUTEROSTOME

Anus

f i g u r e 3.14Developmental tendencies of protostomes and deuterostomes. These tendencies are much modified in some groups, for example, the vertebrates. Cleavage in mammals is rotational ratherthan radial; in reptiles, birds, and many fishes the cleavage is discoidal. Vertebrates have alsoevolved a derived form of coelom formation that is basically schizocoelous.

Eucoelomate

Mesodermalorgan

Mesodermalperitoneum

Mesentery

Pseudocoelomate

Acoelomate

Ectoderm

Ectoderm

Mesodermalorgan

Pseudocoel(from blastocoel)

Gut (endoderm)

Mesoderm (muscle)

Gut (endoderm)Mesodermalorgan

Ectoderm

Parenchyma(mesoderm)

Gut

Coelom

f i g u r e 3.15Acoelomate, pseudocoelomate, and eucoelomatebody plans.




coelom. However, unlike a true coelom, apseudocoel is derived from the embryonicblastocoel and is in fact a persistent blasto-coel. It lacks a peritoneum, a thin cellularmembrane derived from mesoderm that, inanimals with a true coelom, lines the bodycavity.

Eucoelomate BilateriaThe remaining bilateral animals possess atrue coelom lined with mesodermal peri-toneum (figure 3.15,bottom).A true coelomarises within the mesoderm itself and maybe formed by one of two methods, schizo-coelous or enterocoelous (figure 3.16), orby modifying these methods.The two termsare descriptive, for schizo comes from theGreek schizein, meaning to split; entero isderived from the Greek enteron, meaninggut; and coelous comes from the Greek koi-los, meaning hollow or cavity. In schizo-coelous formation the coelom arises, as theword implies, from splitting of mesodermalbands that originate from cells in the blasto-pore region. (Mesoderm is one of three pri-mary germ layers that appear very early in the developmentof all bilateral animals, lying between the innermost endo-derm and outermost ectoderm.) In enterocoelous formation,the coelom comes from pouches of the archenteron, or prim-itive gut.

Once development is complete, the results of schizo-coelous and enterocoelous formations are indistinguishable.Both give rise to a true coelom lined with a mesodermal peri-toneum (Gr.peritonaios, stretched around) and having mesen-teries in which the visceral organs are suspended.

Metamerism (Segmentation)Metamerism is a serial repetition of similar body segmentsalong the longitudinal axis of the body. Each segment is calleda metamere, or somite. In forms such as earthworms andother annelids, in which metamerism is most clearly repre-sented, the segmental arrangement includes both external andinternal structures of several systems. There is repetition ofmuscles, blood vessels, nerves, and the setae of locomotion.Some other organs, such as those of sex, may be repeated in

Schizocoelous

Blastocoel(fluid filled)

Endoderm

Archenteron(embryonic gut)

Ectoderm

BlastoporeEarlymesoderm cells

BlastocoelGut

Split inmesoderm

Developingcoelom

Enterocoelous

BlastocoelEctoderm

BlastocoelGut

Earlymesodermal

pouch

Archenteron(embryonic gut)

BlastoporeEndoderm

Separationof pouches

from gut

Developingcoelom

f i g u r e 3.16Types of mesoderm and coelomic formation. In schizocoelous formation, the mesodermoriginates from the wall of the archenteron near the blastopore and proliferates into a band oftissue which splits to form the coelom. In enterocoelous formation, most mesoderm originatesas a series of pouches from the archenteron; these pinch off and enlarge to form the coelom. Inboth formations the coeloms expand to obliterate the blastocoel.

only a few somites. Evolutionary changes have obscured muchof the segmentation in many animals including humans.

True metamerism is found in only three phyla: Annelida,Arthropoda, and Chordata (figure 3.17), although superficialsegmentation of the ectoderm and the body wall may be foundamong many diverse groups of animals.

CephalizationDifferentiation of a head end is called cephalization and isfound only in bilaterally symmetrical animals.Concentration ofnervous tissue and sense organs in a head bestows obviousadvantages to an animal moving through its environment headfirst. This is the most efficient positioning of instruments forsensing the environment and responding to it. Usually themouth of an animal is located on the head as well, since somuch of an animal’s activity is concerned with procuring food.Cephalization is always accompanied by differentiation alongan anteroposterior axis (polarity). Polarity usually involvesgradients of activities between limits, such as between theanterior and the posterior ends.



From the relatively simple organisms thatmark the beginnings of life on earth, animalevolution has progressed through a history ofever more intricately organized forms. Adap-tive modifications for different niche require-ments within a phyletic line of ancestry,however, have been constrained by theancestral pattern of body architecture.

Cells became integrated into tissues,tissues into organs, and organs into systems.Whereas a unicellular organism carries outall life functions within the confines of a sin-gle cell, a complex multicellular animal is anorganization of subordinate units that areunited at successive levels. One correlate ofincreased body complexity is an increase inbody size, which offers certain advantages,

such as more effective predation, reducedenergy cost of locomotion, and improvedhomeostasis.

A metazoan body consists of cells,most of which are functionally specialized;body fluids, divided into intracellular andextracellular fluid compartments; and extra-cellular structural elements, which arefibrous or formless elements that serve vari-ous structural functions in the extracellularspace. Cells of metazoans develop into vari-ous tissues made up of similar cells perform-ing common functions. Basic tissue typesare nervous, connective, epithelial, and mus-cular. Tissues are organized into larger func-tional units called organs, and organs areassociated to form systems.


f i g u r e 3.17Segmented phyla. These three phyla illustrate an important principle innature—metamerism, or repetition of structural units. Metamerism ishomologous in annelids and arthropods, but chordates have derivedtheir segmentation independently. Segmentation brings more varied specialization because segments, especially in arthropods, have becomemodified for different functions.

Chordata

Arthropoda

Annelida

Every organism has an inherited bodyplan that may be described in terms ofbroadly inclusive characteristics, such as sym-metry, presence or absence of body cavities,partitioning of body fluids, presence orabsence of segmentation, degree of cephal-ization, and type of nervous system.

Based on several developmental char-acteristics, bilateral metazoans are dividedinto two major groups. The Protostomia arecharacterized by spiral cleavage, mosaicdevelopment, and the mouth forming at or near the embryonic blastopore. TheDeuterostomia are characterized by radialcleavage, regulative development, and themouth forming secondarily to the anus andnot from the blastopore.

rev i ew q u e s t i o n s

1. Name the five levels of organization inanimal complexity and explain howeach successive level is more complexthan the one preceding it.

2. Can you suggest why, during the evo-lutionary history of animals, there hasbeen a tendency for maximum bodysize to increase? Do you think itinevitable that complexity should

increase along with body size? Why or why not?

3. What are the meanings of the terms“parenchyma”and “stroma”as theyrelate to body organs?

s u m m a r y




4. Body fluids of eumetazoan animals areseparated into fluid “compartments.”Name these compartments andexplain how compartmentalizationmay differ in animals with open andclosed circulatory systems.

5. What are the four major types oftissues in the body of a metazoan?

6. How would you distinguish betweensimple and stratified epithelium? Whatcharacteristic of stratified epitheliummight explain why it, rather than sim-ple epithelium is found lining the oralcavity, esophagus, and vagina?

7. What are the three elements presentin all connective tissue? Give someexamples of the different types of con-nective tissue.

8. What are three different kinds of mus-cle found among animals? Explainhow each is specialized for particularfunctions.

9. Describe the principal structural andfunctional features of a neuron.

10. Match the animal group with its bodyplan:___ Unicellular a. Nematode___ Cell aggregate b. Vertebrate___ Blind sac, c. Protozoan

acoelomate d. Flatworm___ Tube-within-a-tube, e. Sponge

pseudocoelomate f. Arthropod___ Tube-within-a-tube, g. Nemertean

eucoelomate

11. Distinguish among spherical, radial,biradial, and bilateral symmetry.

12. Use the following terms to identifyregions on your body and on the bodyof a frog: anterior, posterior, dorsal,ventral, lateral, distal, proximal.

13. How would frontal, sagittal, and trans-verse planes divide your body?

14. What is the difference between radialand spiral cleavage?

15. What are the distinguishing develop-mental hallmarks of the two majorgroups of bilateral metazoans, the Pro-tostomia and the Deuterostomia?

16. What is meant by metamerism? Namethree phyla showing metamerism.

s e l e c t e d re fe re n c e s

See also general references on p. 406.Arthur,W. 1997. The origin of animal body

plans. Cambridge, United Kingdom,Cambridge University Press. Explores thegenetic, developmental, and population-level processes involved in the evolutionof the 35 or so body plans that arose inthe geological past.

Bonner, J. T. 1988. The evolution of complex-ity by means of natural selection. Prince-ton, New Jersey, Princeton UniversityPress. Levels of complexity in organismsand how size affects complexity.

Grene, M. 1987. Hierarchies in biology. Amer.Sci. 75:504–510 (Sept.–Oct.). The term“hierarchy” is used in many differentsenses in biology. The author points out

that current evolutionary theory carriesthe hierarchical concept beyond theDarwinian restriction to the two levelsof gene and organism.

Kessel, R. G., and R. H. Kardon. 1979. Tissuesand organs: a text-atlas of scanning elec-tron microscopy. San Francisco,W. H. Free-man & Company.Collection of excellentscanning electron micrographs with text.

McGowan, C. 1999. A practical guide to ver-tebrate mechanics. New York, CambridgeUniversity Press. Using many examplesfrom his earlier book, Diatoms todinosaurs, 1994, the author describesprinciples of biomechanics that underliefunctional anatomy. Includes practicalexperiments and laboratory exercises.

Radinsky, L. B. 1987. The evolution of verte-brate design. Chicago, University ofChicago Press. A lucid functional analy-sis of vertebrate body plans and theirevolutionary transformations over time.

Welsch, U., and V. Storch. 1976. Comparativeanimal cytology and histology. London,Sidgwick & Jackson. Comparative histol-ogy with good treatment ofinvertebrates.

Willmer, P. 1990. Invertebrate relationships:patterns in animal evolution. Cambridge,Cambridge University Press. Chapter 2 isan excellent discussion of animal sym-metry, developmental patterns, origin of body cavities, and segmentation.

c u s t o m we b s i t e

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