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 classification of protists is problematic. Many scientists have proposed groupings and names that differ from those listed here. In fact, three or more new kingdoms 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 compartments. 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, heterotrophic, and amorphous; move by using cytoplasmic 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 Protista, continued
Chlorophyta green algae; unicellular, colonial, multicellular, and photosynthetic; contain chlorophylls a and b; their chloroplasts are similar to those of plants; scientists think that plants descended from green algae
Euglenophyta euglenoids; unicellular, photosynthetic, and heterotrophic; reproduce asexually; most species live in fresh water; chloroplasts resemble the chloroplasts of green algae and may have evolved from the same symbiotic bacteria from which green algae chloroplasts evolved; some are considered to be animal-like and are classified in Euglenozoa
Bacillariophyta diatoms; unicellular and photosynthetic; secrete a unique shell that is made of opaline silica and that resembles a lidded box; have chloroplasts 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 fucoxanthin (the source of their brownish color)
Dinoflagellata unicellular; heterotrophic and autotrophic species; most species are marine and are enclosed by two cellulose plates; most species contain carotenoids and chlorophylls a and c.; sometimes grouped in Alveolata with the animal-like pro-tist groups Apicomplexa and Ciliata
Rhodophyta red algae; multicellular and photosynthetic; most are marine and contain chlorophyll a and phycobilins; chloroplasts probably evolved from symbiotic cyanobacteria
Myxomycota plasmodial slime molds; heterotrophic; individuals stream as part of a multinucleate mass of cytoplasm; can produce spores that give rise to new individuals; sometimes grouped with other slime molds in Mycetozoa within Protozoa
Oomycota or Pseudofungi water molds, white rusts, and downy mildews; unicellular heterotrophs; parasites or decomposers; cell walls consist of cellulose; sometimes grouped in Stramenopila with Bacillariophyta, Chrysophyta, and Phaeophyta
Dictyostelida cellular slime molds; heterotrophic cells that take on different forms depending on food availability; cells are usually amoeba-shaped but may aggregate into a moving mass called a slug; a slug may produce spores
Dictyostelium sp., a cellular slime mold
Dictyostelium sp., a cellular slime mold
DOMAIN EUKARYA, continued
Fungi are heterotrophic by absorption and eukary-otic. Most are multicellular and are composed of filaments called hyphae. Although all fungi that are yeasts are unicellular, some are dimorphic, or have both hyphal and unicellular stages. Hyphae are multinucleate, have divisions called septae between cells, and have chitinous cell walls. Most fungi reproduce sexually and asexually. Some lack a sexual stage or are difficult to classify and are labeled Fungi Imperfecti, or Deuteromycetes. About 70,000 living, described species of fungi exist.
Zygomycetes, such as bread molds, usually lack septae. Some are terrestrial. Some are parasitic. Fusion of hyphae leads to formation of a zygote, which divides by meiosis when it germinates.
Ascomycetes—such as brewer's and baker's yeasts, molds, morels, and truffles—include terrestrial, marine, and freshwater species. Septae are usually perforated. Fusion of the hyphae forms a dense, interwoven mass that contains characteristic microscopic reproductive structures called asci (singular, ascus). Many fungi formerly classified as Deuteromycetes are now classified as Ascomycota.
Basidiomycetes reproduce sexually. Hyphae usually have incomplete septae. Fusion of the hyphae forms a mushroom, a densely interwoven structure that contains characteristic microscopic reproductive structures called basidia (singular, basidium). Basidiomycota includes mushrooms, toadstools, shelf fungi, rusts, and smuts.
Plants are multicellular eukaryotes whose cell walls contain cellulose. Most are autotrophic and terrestrial and contain tissues and organs. Plants contain chlorophylls a and b in plastids and have a life cycle in which sexual and asexual reproduction alternate. About 280,000 living, described species exist.
Bryophytes, or mosses, are nonvascular plants whose gametophytes are larger than their sporo-phytes. The sporophytes grow on the gametophytes. Bryophytes have simple conducting tissue and lack roots, stems, and leaves.
Hepatophytes, or liverworts, are nonvascular plants whose gametophytes are larger than their sporo-phytes. The sporophytes grow on the gametophytes and lack stomata, roots, stems, and leaves.
Anthocerophytes, or hornworts, are nonvascular plants whose gametophytes are larger than their sporophytes. The sporophytes grow on the gameto-phytes, have stomata, and lack roots, stems, and leaves.
Pteridophytes, or ferns, are seedless vascular plants whose sporophytes are larger than their small, flat, independent gametophytes. The sporophytes have roots and stems. The lower surfaces of the leaves produce spores.
Lycophytes, such as club mosses, are seedless vascular plants whose sporophytes are larger than their small, flat, independent gametophytes. The sporophytes resemble the gametophytes of mosses; they produce spores in cones and have roots, stems, and leaves.
Sphenophytes, such as horsetails, are seedless vascular plants whose sporophytes are larger than their small, flat, independent gametophytes. The sporophytes produce spores in cones and have roots, leaves, and jointed stems.
Psilophytes, such as whisk ferns, are seedless vascular plants whose sporophytes are larger than their small, flat, independent gametophytes. The sporophytes lack leaves, have roots and stems, and produce spores in sporangia at stem tips.
Conifers are gymnosperms such as pines, spruces, firs, larches, and yews. Most have sporophytes that are evergreen trees or shrubs that have needlelike or scalelike leaves. Microscopic gametophytes develop from spores produced within cones on the sporophytes.
Cycads are gymnosperms and seed-bearing vascular plants. The sporophytes are evergreen trees and shrubs that have palmlike leaves. Microscopic gametophytes develop from spores produced in cones on separate sporophytes.
Ginkgophyta contains one species: a gymnosperm called Ginkgo biloba. The sporophyte is a deciduous tree that has fan-shaped leaves and fleshy seeds. Microscopic gametophytes develop from spores produced on separate sporophytes.
Gnetophytes are gymnosperms and seed-bearing vascular plants whose sporophytes are shrubs or vines that have some angiosperm features. Microscopic gametophytes develop from spores produced in cones on the sporophytes.
Anthophytes are angiosperms, or flowering plants, and are seed-bearing vascular plants whose sporophytes are flowering trees, shrubs, herbs, or vines. Microscopic gametophytes develop from spores produced in flower reproductive structures.
Class Monocotyledones monocots; embryos have one cotyledon; flower parts in multiples of three; leaf veins parallel; includes grasses, sedges, lilies, irises, palms, and orchids
Class Dicotyledones dicots; embryos have two cotyledons; flower parts in multiples of two, four, or five; includes most flowering plants, such as daisies, legumes, hardwood trees, shrubs, and vines
DOMAIN EUKARYA, continued
Animals are multicellular, eukaryotic, and heterotrophic organisms. Most obtain nutrition by ingestion and have specialized tissues. Many have complex organs and organ systems. None have cell walls or chloroplasts. Most undergo sexual reproduction. They are aquatic or terrestrial. More than 1 million living, described species exist.
Poriferans, or sponges, are asymmetrical and lack tissues and organs. Their body wall consists of two layers of interdependent cells. Poriferans undergo sexual and asexual reproduction. Most are marine.
Cnidarians are radially symmetrical and are marine or freshwater. Most are gelatinous and have distinct tissues and a baglike body of two cell layers.
Class Hydrozoa hydras Class Cubozoa box jellies Class Scyphozoa jellyfish Class Anthozoa sea anemones and corals
Ctenophores, or comb jellies, are radially symmetrical, marine, transparent, and gelatinous.
Flatworms are bilaterally symmetrical.
Class Turbellaria planarians
Classes Trematoda and Monogenea flukes
Class Cestoda tapeworms
Nematodes, or roundworms, are parasitic, unseg-mented, long, and slender pseudocoelomates.
Rotifers are free-living, aquatic pseudocoelomates.
Mollusks are soft-bodied protostomes whose three-part body consists of a foot, a visceral mass, and a mantle. They are terrestrial, freshwater, or marine.
Class Polyplacophora chitons Class Gastropoda gastropods Class Bivalvia bivalves Class Cephalopoda cephalopods
Annelids are serially segmented, bilaterally symmetrical protostomes.
Class Polychaeta bristle worms Class Oligochaeta earthworms Class Hirudinea leeches
Arthropods are bilaterally symmetrical, segmented protostomes that are aerial, terrestrial, or aquatic and have paired, jointed appendages and a chitinous exoskeleton.
Subphylum Chelicerata includes arachnids Subphylum Crustacea crustaceans Subphylum Myriapoda centipedes and millipedes Subphylum Hexapoda insects and parainsecta
Echinoderms are marine deuterostomes. Adults have radial symmetry and a five-part body plan. Most have a water-vascular system and tube feet.
Class Crinoidea sea lilies and feather stars Class Ophiuroidea brittle stars and basket stars Class Echinoidea sea urchins and sand dollars Class Holothuroidea sea cucumbers Class Asteroidea sea stars
Chordates are bilaterally symmetrical, aquatic or terrestrial deuterostomes that have a noto-chord, a dorsal nerve cord, pharyn- Ambystoma cingulatum, geal slits, and a tail. a vertebrate
Subphylum Urochordata tunicates Subphylum Cephalochordata lancelets Subphylum Vertebrata vertebrates
THE GEOLOGIC TIME SCALE Reference
The Geologic Time Scale
Beginning of interval (mya*)
Characteristics from geologic and fossil evidence
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