C L A S S I F I C A T I O N1078

The classification system used in this book is basedon both the commonly recognized six-kingdom sys-tem and the newer, three-domain system. A kingdomis a group of related phyla, whereas a domain is alarger-scale grouping that can encompass kingdoms.In the three-domain system, all living organisms areclassified into one of three domains based on cellu-lar similarities. Two of the three domains consist ofprokaryotes, and one domain consists of eukaryotes.The table below compares the two systems ofclassification.

The domain Bacteria aligns with the kingdomEubacteria. The domain Archaea aligns with thekingdom Archaebacteria. Both archaea and bacteriaare prokaryotic microbes, although the two groupsdiffer significantly.

The third domain, Eukarya, consists of all of theeukaryotic organisms. The four kingdoms that alignwith the domain Eukarya are Animalia (animals),Plantae (plants), Fungi (fungi), and Protista(protists).

The information on the following pages isconveniently organized into commonly recognizedsubgroups. However, not all of the existing sub-groups are presented here, and all classifications aresometimes debated and revised by the scientificcommunity. For example, biologists have proposedseveral new kingdoms to replace Protista, becausethe subgroups of Protista are no longer thought tobe strongly related. Also, scientists are still uncer-tain about the number of species in each group.

CLASSIFICATIONReference

PROKARYOTES EUKARYOTES

Domain DomainBacteria Archaea

Domain Eukarya

Kingdom Kingdom Kingdom Kingdom Kingdom KingdomEubacteria Archaebacteria Protista Fungi Plantae Animalia

Kingdom Eubacteria

All bacteria are prokaryotic and lack membrane-bound organelles. Most are unicellular and repro-duce by fission. Most species are heterotrophic, butsome are photosynthetic or chemosynthetic. Morethan 4,000 living, described species of bacteria exist.

Proteobacteria Alpha Proteobacteria many species, such asRhizobium sp., are parasitic or mutualistic bacteriathat live within eukaryotic hosts

Beta Proteobacteria diverse modes of nutrition;some species, such as Nitrosomonas sp., are impor-tant in the nitrogen cycle

Gamma Proteobacteria some species are photo-synthetic; some species derive energy by reducingammonia and hydrogen sulfide; some species, suchas Escherichia coli, are enteric

Delta Proteobacteria some species, such asBdellovibrio sp., are predators of other bacteria

Epsilon Proteobacteria many species, such asHelicobacter pylori, are pathogenic

Gram-Positive BacteriaMost, but not all, members of this diverse group areGram-positive. One subgroup, the actinomycetes, isthe source of many antibiotics. Many species, includ-ing actinomycetes, can cause disease.

SpirochetesThese long, spiral cells have flagellated ends. Somecause serious diseases such as syphilis.

ChlamydiaThis group includes obligate internal parasities.Some cause diseases.

CyanobacteriaThis group includesphotosynthetic bacteria which are common on land and in water. The chloro-plasts of some protistsprobably evolved from cyanobacteria.

DOMAIN BACTERIA

Lyngbya sp.,a cyanobacterium

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C L A S S I F I C A T I O N 1079

Reference

This phylogenetic diagram showshypotheses of the evolutionaryrelationships between all major groups ofliving organisms. Scientists are uncertainabout some of these groupings. Dashedlines indicate relationships that are veryuncertain or strongly debated. Forupdates on phylogenetic information,visit go.hrw.com and type in thekeyword HM6 Phylo.

Eubacteria (bacteria)

Archaebacteria (archaea)

Bryophyta (mosses)

Pteridophyta (ferns)

Coniferophyta (conifers)

Anthophyta (flowering plants)

Fungi (fungi)

Porifera (sponges)

Cnidaria (cnidarians)

Nematoda (roundworms)

Arthropoda (arthropods)

Platyhelminthes (flatworms)

Annelida (segmented worms)

Mollucsa (mollusks)

Echinodermata (echinoderms)

Cephalochordata (lancelets)

Cephalaspidomorphi (lampreys)

Sarcopterygii (lobe-finned fishes)

Amphibia (amphibians)

Mammalia (mammals)

Reptilia (reptiles)

Aves (birds)

Protista (protists)

Prokaryo

tes

Green algae and plants

Land plants

Vascular plantsEukaryotes

Animals

Deuterostomes

Vertebrates

Amniotes

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Kingdom Protista

Kingdom Protista is a diverse group of eukaryotes

that are not plants, fungi, or animals. Traditionally,

species have been classified in Protista whenever

their characteristics do not clearly match with those

of one of the other eukaryotic kingdoms.

Because protists are not defined clearly, are so

diverse, and are so poorly understood, the classifica-

tion of protists is problematic. Many scientists have

proposed groupings and names that differ from

those listed here. In fact, three or more new king-

doms have been proposed to replace Protista.

However, scientists have not yet clearly favored any

particular new system.

Protists can be unicellular or multicellular. Most

have chromosomes, mitochondria, and internal com-

partments. Some have chloroplasts and conduct

photosynthesis, and some have cell walls. They may

reproduce sexually, asexually, or both. They occur in

many environments, including water, soil, and inside

of other organisms. More than 40,000 living,

described species exist.

Animal-like Protists (includes Protozoa and Sarcomastigophora)

Sarcodina or Rhizopoda amoebas; unicellular, het-

erotrophic, and amorphous; move by using cytoplas-

mic extensions, or pseudopodia

Radiozoa or Actinipoda radiolarians; unicellular,

heterotrophic, and marine; have pored shells

through which cytoplasmic threads project

Ciliophora ciliates; unicellular, heterotrophic, and

complex; have rows of cilia and two types of nuclei

Mastigophora or Kinetoplastida mastigophorans;

have at least one flagellum; most are unicellular and

heterotrophic; some cause disease

Apicomplexa or Sporozoa unicellular, het-

erotrophic, and spore-forming; have complex life

cycles; adults are nonmotile parasites; undergo

asexual and sexual reproduction

Kingdom Archaebacteria

This kingdom is made up of prokaryotes called

archaea. Many archaea live in extreme environ-

ments. They differ from eubacteria in cell wall and

cell membrane structure. Genetic similarities

between archaea and eukaryotes suggest that

archaea are more closely related to eukaryotes

than to bacteria. Fewer than 100 living, described

species exist.

Extreme Halophiles

Extreme halophiles inhabit environments of very

high salinity, such as the Dead Sea and Great Salt

Lake (salinity 15 to 20 percent). Many are aerobic.

Methanogens

Methanogens are anaerobic methane producers that

inhabit soil, swamps, and the digestive tracts of ani-

mals—particularly the tracts of grazing mammals

such as cattle. Most use CO2

as a carbon source.

Methanogens produce

nearly 2 trillion kilograms

(2 billion tons) of methane

gas annually.

Thermoacidophiles

Thermoacidophiles inhabit

hot, acidic areas, tolerate

extreme heat, and require

sulfur. Most are anaerobic.

C L A S S I F I C A T I O N1080

CLASSIFICATION, continuedReference

DOMAIN ARCHAEA

DOMAIN EUKARYA

An amoeba is anexample of a sarcodine.

This undersea hydrothermal ventis home to thermoacidophiles.

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Kingdom Protista, continued

Plantlike ProtistsChlorophyta green algae; unicellular, colonial, mul-ticellular, and photosynthetic; contain chlorophylls a and b; their chloroplasts are similar to those ofplants; scientists think that plants descended fromgreen algae

Euglenophyta euglenoids; unicellular, photosyn-thetic, and heterotrophic; reproduce asexually; mostspecies live in fresh water; chloroplasts resemblethe chloroplasts of green algae and may haveevolved from the same symbiotic bacteria fromwhich green algae chloroplasts evolved; some areconsidered to be animal-like and are classified inEuglenozoa

Bacillariophyta diatoms; unicellular and photosyn-thetic; secrete a unique shell that is made of opalinesilica and that resembles a lidded box; have chloro-plasts that resemble the chloroplasts of brown algae;contain chlorophylls a and c and fucoxanthin

Phaeophyta brown algae; multicellular and photo-synthetic; contain chlorophylls a and c and fucoxan-thin (the source of their brownish color)

Dinoflagellata unicellular; heterotrophic andautotrophic species; most species are marine andare enclosed by two cellulose plates; most speciescontain carotenoids and chlorophylls a and c.; some-times grouped in Alveolata with the animal-like pro-tist groups Apicomplexa and Ciliata

Rhodophyta red algae; multicellular and photosyn-thetic; most are marine and contain chlorophyll aand phycobilins; chloroplasts probably evolved fromsymbiotic cyanobacteria

Funguslike ProtistsMyxomycota plasmodial slime molds; het-erotrophic; individuals stream as part of a multinu-cleate mass of cytoplasm; can produce spores thatgive rise to new individuals; sometimes grouped withother slime molds in Mycetozoa within Protozoa

Oomycota or Pseudofungi water molds, whiterusts, and downy mildews; unicellular heterotrophs;parasites or decomposers; cell walls consist of cellu-lose; sometimes grouped in Stramenopila withBacillariophyta, Chrysophyta, and Phaeophyta

Dictyostelida cellular slime molds; heterotrophiccells that take on different forms depending on foodavailability; cells are usually amoeba-shaped butmay aggregate into a moving mass called a slug; aslug may produce spores

Reference

Corallina sp., a red alga

Dictyostelium sp., a cellular slime mold

Volvox sp., a green alga

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Kingdom Fungi

Fungi are heterotrophic by absorption and eukary-otic. Most are multicellular and are composed of fila-ments called hyphae. Although all fungi that areyeasts are unicellular, some are dimorphic, or haveboth hyphal and unicellular stages. Hyphae are multi-nucleate, have divisions called septae between cells,and have chitinous cell walls. Most fungi reproducesexually and asexually. Some lack a sexual stage orare difficult to classify and are labeled FungiImperfecti, or Deuteromycetes. About 70,000 living,described species of fungi exist.

Phylum ZygomycotaZygomycetes, such as bread molds, usually lackseptae. Some are terrestrial. Some are parasitic.Fusion of hyphae leads to formation of a zygote,which divides by meiosis when it germinates.

Phylum AscomycotaAscomycetes—such as brewer’s and baker’s yeasts,molds, morels, and truffles—include terrestrial,marine, and freshwater species. Septae are usuallyperforated. Fusion of the hyphae forms a dense,interwoven mass that contains characteristic micro-scopic reproductive structures called asci (singular,ascus). Many fungi formerly classified asDeuteromycetes are now classified as Ascomycota.

PhylumBasidiomycotaBasidiomycetes reproducesexually. Hyphae usuallyhave incomplete septae.Fusion of the hyphaeforms a mushroom, adensely interwoven struc-ture that contains charac-teristic microscopicreproductive structurescalled basidia (singular,basidium). Basidiomycotaincludes mushrooms,toadstools, shelf fungi,rusts, and smuts.

Kingdom Plantae

Plants are multicellular eukaryotes whose cell wallscontain cellulose. Most are autotrophic and terres-trial and contain tissues and organs. Plants containchlorophylls a and b in plastids and have a life cyclein which sexual and asexual reproduction alternate.About 280,000 living, described species exist.

Phylum BryophytaBryophytes, or mosses, are nonvascular plantswhose gametophytes are larger than their sporo-phytes. The sporophytes grow on the gametophytes.Bryophytes have simple conducting tissue and lackroots, stems, and leaves.

Phylum HepatophytaHepatophytes, or liverworts, are nonvascular plantswhose gametophytes are larger than their sporo-phytes. The sporophytes grow on the gametophytesand lack stomata, roots, stems, and leaves.

Phylum AnthocerophytaAnthocerophytes, or hornworts, are nonvascularplants whose gametophytes are larger than theirsporophytes. The sporophytes grow on the gameto-phytes, have stomata, and lack roots, stems, andleaves.

Morchella sp., a mushroom

CLASSIFICATION, continuedReference

Marchantia sp., a liverwort

DOMAIN EUKARYA, continued

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Phylum PteridophytaPteridophytes, or ferns, are seedless vascular plantswhose sporophytes are larger than their small, flat,independent gametophytes. The sporophytes haveroots and stems. The lower surfaces of the leavesproduce spores.

Phylum LycophytaLycophytes, such as club mosses, are seedless vas-cular plants whose sporophytes are larger than theirsmall, flat, independent gametophytes. The sporo-phytes resemble the gametophytes of mosses; theyproduce spores in cones and have roots, stems, and leaves.

Phylum SphenophytaSphenophytes, such as horsetails, are seedless vas-cular plants whose sporophytes are larger than theirsmall, flat, independent gametophytes. The sporo-phytes produce spores in cones and have roots,leaves, and jointed stems.

Phylum PsilotophytaPsilophytes, such as whisk ferns, are seedless vascu-lar plants whose sporophytes are larger than theirsmall, flat, independent gametophytes. The sporo-phytes lack leaves, have roots and stems, and pro-duce spores in sporangia at stem tips.

Phylum ConiferophytaConifers are gymnosperms such as pines, spruces,firs, larches, and yews. Most have sporophytes thatare evergreen trees or shrubs that have needlelikeor scalelike leaves. Microscopic gametophytesdevelop from spores produced within cones on thesporophytes.

Phylum CycadophytaCycads are gymnosperms and seed-bearing vascularplants. The sporophytes are evergreen trees andshrubs that have palmlike leaves. Microscopic game-tophytes develop from spores produced in cones onseparate sporophytes.

Phylum GinkgophytaGinkgophyta contains one species: a gymnospermcalled Ginkgo biloba. The sporophyte is a deciduoustree that has fan-shaped leaves and fleshy seeds.Microscopic gametophytes develop from spores pro-duced on separate sporophytes.

Phylum GnetophytaGnetophytes are gymnosperms and seed-bearingvascular plants whose sporophytes are shrubs orvines that have some angiosperm features.Microscopic gametophytes develop from sporesproduced in cones on the sporophytes.

Phylum AnthophytaAnthophytes are angiosperms, or flowering plants,and are seed-bearing vascular plants whose sporo-phytes are flowering trees, shrubs, herbs, or vines.Microscopic gametophytes develop from spores pro-duced in flower reproductive structures.

Class Monocotyledones monocots; embryos haveone cotyledon; flower parts in multiples of three; leafveins parallel; includes grasses, sedges, lilies, irises,palms, and orchids

Class Dicotyledones dicots; embryos have twocotyledons; flower parts in multiples of two, four, orfive; includes most flowering plants, such as daisies,legumes, hardwood trees, shrubs, and vines

Dicksonia antartica, a fern Ginko biloba, a ginkophyte

Helianthus annuus, a dicot

Reference

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Kingdom Animalia

Animals are multicellular, eukaryotic, and het-erotrophic organisms. Most obtain nutrition byingestion and have specialized tissues. Many havecomplex organs and organ systems. None have cellwalls or chloroplasts. Most undergo sexual repro-duction. They are aquatic or terrestrial. More than 1 million living, described species exist.

Phylum PoriferaPoriferans, or sponges, are asymmetrical and lacktissues and organs. Their body wall consists of twolayers of interdependent cells. Poriferans undergosexual and asexual reproduction. Most are marine.

Phylum CnidariaCnidarians are radially sym-metrical and are marine orfreshwater. Most are gelati-nous and have distinct tis-sues and a baglike body oftwo cell layers.

Class Hydrozoa hydrasClass Cubozoa box jelliesClass Scyphozoa jellyfishClass Anthozoa sea anemones and corals

Phylum CtenophoraCtenophores, or comb jellies, are radially symmetri-cal, marine, transparent, and gelatinous.

Phylum PlatyhelminthesFlatworms are bilaterally symmetrical.

Class Turbellaria planariansClasses Trematoda and Monogenea flukesClass Cestoda tapeworms

Phylum NematodaNematodes, or roundworms, are parasitic, unseg-mented, long, and slender pseudocoelomates.

Phylum RotiferaRotifers are free-living, aquatic pseudocoelomates.

Phylum MolluscaMollusks are soft-bodied protostomes whose three-part body consists of a foot, a visceral mass, and amantle. They are terrestrial, freshwater, or marine.

Class Polyplacophora chitonsClass Gastropoda gastropodsClass Bivalvia bivalvesClass Cephalopoda cephalopods

Phylum AnnelidaAnnelids are serially segmented, bilaterally symmet-rical protostomes.

Class Polychaeta bristle wormsClass Oligochaeta earthwormsClass Hirudinea leeches

Phylum ArthropodaArthropods are bilaterally symmetrical, segmentedprotostomes that are aerial, terrestrial, or aquaticand have paired, jointed appendages and a chitinousexoskeleton.

Subphylum Chelicerata includes arachnidsSubphylum Crustacea crustaceansSubphylum Myriapoda centipedes and millipedesSubphylum Hexapoda insects and parainsecta

Phylum EchinodermataEchinoderms are marine deuterostomes. Adults haveradial symmetry and a five-part body plan. Mosthave a water-vascular system and tube feet.

Class Crinoidea sea lilies and feather starsClass Ophiuroidea brittle stars and basket starsClass Echinoidea sea urchins and sand dollarsClass Holothuroidea sea cucumbersClass Asteroidea sea stars

Phylum ChordataChordates are bilat-erally symmetrical,aquatic or terres-trial deuterostomesthat have a noto-chord, a dorsalnerve cord, pharyn-geal slits, and a tail.

Subphylum Urochordata tunicatesSubphylum Cephalochordata lanceletsSubphylum Vertebrata vertebrates

Cribrinopsis fernadli, asea anemone

CLASSIFICATION, continuedReference

DOMAIN EUKARYA, continued

Ambystoma cingulatum, a vertebrate

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