Classification

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Taxonomy

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The science of classification and the nomenclature used. This modern classification system seeks to group organisms on the basis of evolutionary relationships. In this system, the bat, whale, horse, and humans are placed in the same class of animals because they all have descended from a common ancestor. There is much disagreement among biologists particularly with regard to groups of unicellular organisms. This method of classification takes into account anatomical and structural characteristics, modes of excretion, movement, digestion, genetic makeup, and biochemical capabilities. It proceeds from the largest, broadest group to the smaller, more specific subgroups.

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Classification and Subdivisions

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The modern scheme of taxonomy has 5 kingdoms for all living organisms. Each kingdom is divided into several major phyla (in the animal kingdom) or divisions (in other kingdoms). A phylum or division has several subphyla or subdivisions which are further divided into classes. Each class includes multiple orders. Orders are subdivided into families, and each family is made up of many genera (singular-genus). The species is the final, smallest subdivision. Organisms of the same species can mate with one another to produce fertile offspring. Example of the full classification of humans: Kingdom - Animal; Phylum - Chordata; Subphylum - Vertebrata; Class - Mammalia; Order - Primates; Family - Hominidae; Genus - Homo; Species - sapiens.

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Assignment of Scientific Names

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All organisms are assigned a scientific name consisting of the Genus and species name of that organism. Thus, humans are Homo sapiens, and the common housecat is Felis domestica. This follows a scheme originated by the biologist Carl Linn (Carolus Linnaeus).

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Modern Classification

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Biologists originally divided all living things into two categories: plants and animals. This division ignored a number of different organisms. One type of modern classification includes 5 kingdoms: Monera, Protista, Plantae, Fungi, and Animalia. Another classification system utilizes 3 kingdoms: Monera, Plantae (including fungi), and Animalia (including Protista).

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Viruses

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These are typically not considered to be living organisms. They cannot function outside of a host cell and are dependent upon the host's reproductive machinery to replicate. Thus, they have not been placed in any of the five kingdoms. They do not carry out physiological or biochemical processes outside of a host. They may be considered nonliving, although they are highly advanced parasites. They are capable of taking over their host's cellular machinery and directing the replication of the viral genome and protein coat. They have lytic and lysogenic life cycles. They contain either DNA or RNA and some esential enzymes surrounded by a protein coat. Ones that exclusively infect bacteria are called bacteriophages.

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Kingdom Monera (Monerans)

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AKA bacteria, are prokaryotic cells. They may exist as single cells or as aggregates of cells that stick together after division.

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Cyanobacteria

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AKA blue-green algae, living primarily in fresh water but also exist in marine environments. They possess a cell wall, and photosynthetic pigments, but have no flagella, true nucleus, chloroplasts, or mitochondria. They can withstand extreme temperatures, and are believed to be directly descended from the first organisms that developed photosynthetic capabilities.

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Other Bacteria

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These organisms are single celled prokaryotes with a single double-stranded circular loop of DNA which is not enclosed by a nuclear membrane. Almost all forms have cell walls. They play active roles in biogeochemical cycles; the recycling of various chemicals such as carbon, nitrogen, phosphorous, and sulfur. They may be classified by their morphological appearances; cocci (round), bacilli (rods), and spirilla (spiral). Some forms are duplexes (diplococci), clusters (staphylococci) and chains (streptococci). They are ubiquitous and many possess a wide variety of complex biochemical pathways.

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Monera (Monerans)

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These are prokaryotes. They lack a nucleus or any membrane-bound organelles. All of these are single celled organisms that reproduce asexually.

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Protista

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This kingdom contains primitive eukaryotic organisms with both plant-like and animal-like characteristics. These organisms are either single cells or colonies of similar cells with no differentiation of specialized tissues. Each of its cells possess the capability to carry out all of the life processes. This kingdom contains all simple eukaryotes that cannot be classified as plants or animals. For example, Euglena demonstrates the motility of animals and photosynthetic capabilities of plants.

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Fungi

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These organisms may be considered non-photosynthetic plants (i.e. they resemble plants in that they are multicellular, differentiated, and non-motile. They are not photosynthetic. They are either saprophytic (i.e. bread mold) or parasitic (i.e. athlete's foot fungus). Their modes of reproduction are varied and unique. Their cell walls are composed of chitin, not cellulose (as in plants).

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Plantae

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This kingdom includes multicellular organisms that exhibit differentiation of tissues, are non-motile and photosynthetic. Many of these organisms exhibit an alternation of generations and a distinct embryonic phase.

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Animalia

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This kingdom contains the multicellular, generally motile, heterotrophic organisms that have differentiated tissues (and organs in higher forms).

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Kingdom Protista

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This kingdom contains organisms that are mostly unicellular, but there some colonial forms exist as well as some simple multicellular organisms that are neither plants nor animals. They are eukaryotes and possess a membrane-bound nucleus and organelles. The kingdom includes two major categories: protozoa and algae. The kingdom is divided into many phyla that fall primarily into these two categories.

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Protozoa

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Traditionally are considered single celled organisms that are heterotrophic, like little animals. This category includes a number of phyla. The rhizopods, including amoebas, move with cellular extensions called pseudopods. The ciliophors have cilia that are used for feeding and locomotion.

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Algae

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These organisms are primarily photosynthetic. They include the phytoplankton which are important sources of food for marine life. The Euglena may be considered an algal protist because it photosynthesizes. The Euglena can also act as a heterotroph and move about with its flagellum. The blue, green, and red algae can be multicellular and are sometimes placed in the animal kingdom.

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Protists Resembling Fungi

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The slime molds are often placed in the kingdom fungi, but they appear to be more directly related to the protists. They are arranged in a coenocytic (many nuclei) mass of protoplasm. The slime mold undergoes a unique life cycle containing animal-like and plant-like stages. These stages include fruiting bodies and unicellular flagellated spores. Slime molds reproduce asexually by sporulation.

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Kingdom Fungi

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These organisms are eukaryotes, primarily multicellular, and are heterotrophs. This differentiates them from the plant kingdom. They may be saprophytic, decomposing dead organic material, or parasitic. In either case, they absorb their food from their environment. They reproduce by asexual sporulation or by intricate sexual processes. Some varieties of Eumycophyta utilize extracellular digestion. Notable types are mushrooms, yeast, and lichens.

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Kingdom Plantae

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All of these organisms are multicellular, non-motile, and photosynthetic autotrophs.

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Differentiation of Tissues

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Plants have developed complex differentiated tissues to adapt to a terrestrial life. Photosynthetic tissue layers contain chloroplasts for carbohydrate manufacture. Supportive tissues provide mechanical support facilitating the typical upright radial construction of plants. Absorptive tissues like specialized rhizoids or complex roots project into soil for absorption of water and minerals. Conducting or vascular tissues include specialized "tubes" that transport water, minerals and nutrients to all parts of the plant. Waxy cuticles on exposed surfaces minimize loss of water while permitting the transmittance of light. Cells are in direct contact with the external environment by means of air spaces and stomata; therefore elaborate respiratory and excretory systems are unnecessary.

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Reproduction (Plants)

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Specialized sex organs in the gametophyte include the archegonium which produces eggs. The sporophyte contains a sporangium which manufactures spores. Plants undergo alternation of generations with a sexual gametophyte (haploid) stage followed by an asexual sporophyte (diploid) stage.

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Division Bryophyta

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These are simple plants with few specialized organs and tissues. They lack the water-conducting woody material (xylem) which functions in support in tracheophytes and retain flagellated sperm cells that must swim to the eggs. They never have become successful terrestrial plants and must live in moist places. They undergo alternation of generations. The gametophyte is the dominant generation; it is the main plant, larger and nutritionally independent. The sporophyte is small, short-lived, attached to the gametophyte, and grows from the archegonium. It resembles a hetertrophic parasite in that it obtains organic and inorganic material from the independent autotrophic gametophyte. There are two types: mosses and liverworts.

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Mosses

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Primitive bryophytes in which the sporophyte and gametophyte generations grow together. The gametophyte has a filamentous protonema (young moss plant) from which grows a vertical stem with radial leaves and a short sporophyte consisting of a food, stalk, and a capsule filled with spores. The sporophyte grows out of the archegonium at the tip of the gametophyte.

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Liverworts

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Flat, horizontal leaf-like plants with differentiated dorsal and ventral surfaces. The lower surface contains rhizoids, the middle provides for the storage of food, and the upper surface is photosynthetic.

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Division Tracheophyta

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Vascular plants (tracheophytes) are complex plants with a great degree of cell differentiation; they contain vascular tissues: xylem (water-conducting) and phloem (food-conducting). They have radial symmetry about a main vertical axis and are anchored by deep roots instead of rhizoids. Their extensive woody or non-woody support system allows them to grow to great heights. They have developed excellent provisions for water conservation (waxy surfaces) and gas exchange (stomata). Cellular water storage creates turgid cells. The sporophyte generation is dominant (in contrast to bryophytes). The gametophyte is short-lived, independent in primitive tracheophytes (fern), and small and parasitic in more advanced tracheophytes (seed plants). There are four divisions of vascular plants, three of which are almost extinct.

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Division Psilophyta (Psilophytes)

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These are the most primitive of the tracheophytes and contain rhizoids instead of roots and one vascular bundle (microphyll) in the leaves (i.e. Psilotum).

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Division Lycophyta (Lycophytes)

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These belong to an ancient subdivision, have roots, are non-woody and contain microphyll leaves (i.e. club mosses).

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Division Sphenophyta (Sphenophytes)

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These tracheophytes possess roots, microphyll leaves, and hallow jointed stems. Whorls of leaves occur on each joint (i.e. equisetum (horsetail)).

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Division Pterophyta (Pterophytes)

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This is the largest division and includes the familiar fern. They evolved from early psilopsids. They contain large leaves (megaphylls) that possess many vascular bundles. Ferns grow lengthwise, not in diameter, and contain xylem as tracheids, not vessels. They do not produce seeds and their short-lived gametophyte generation possess heart-shaped leaves. Sperm are flagellated and thus require water or moisture to facilitate fertilization. The fern's leaves are part of the sporophyte generation. They grow from an underground stem called the rhizome. Sporangium on the underside of the leaves produce monoploid spores that germinate to form gametophytes.

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Division Coniferophyta (Conifers)

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These are the largest grouping of gymnosperms, which are naked-seeded plants. They include cycads, pines, spruce, and firs. They have cones, spiral clusters of modified leaves. There are two different types of cones: large female cones, whose sporangia produce megaspores, and small male cones, whose sporangia produce microspores. The gametophyte stage is short-lived and microscopic. The male microspore produces pollen which can be carried by the wind; thus, the requirement of a water environment for flagellated sperm is eliminated and they are truly terrestrial. Sperm nuclei fertilize the egg with the aid of a pollen tube and the embryo develops within the exposed seed. The presence of a specialized cambium tissue allows for secondary growth - secondary xylem (wood) and secondary phloem. They can grow in diameter as well as in length and are woody, not herbaceous (green with soft stems) plants. Most are evergreens (non-deciduous).

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Division Anthophyta

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This division includes the flowering plants known as angiosperms. They have covered seeds and are the most abundant of all plants. They have flowers, not cones, as their principal reproductive structure. The anther of the male stamen produces microspores (pollen grains) while the ovary of the female pistil produces megaspores. Successful pollination results in the germination of pollen tubes that aid in fertilization of female eggs in the gametophyte. The embryo develops into a seed within the ovary. The ovary eventually ripens into fruit - the means by which the seeds are dispersed. Xylem conducting cells are in the form of vessels as well as tracheids allowing for better conduction of water. There are two subclasses of angiosperms: dicotyledons (dicots) and monocotyledons (monocots).

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Dicotyledons (Dicots)

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These have "net veined" leaves and vascular bundles about a ring within the central cylinder. They contain two cotyledons (seed leaves) within a seed. Many have cambium; and can be woody. They have flower parts in multiples of four or five. Some examples are the maple and apple trees, potatoes, carrots, goldenrods, and buttercups.

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Monocotyledons (Monocots)

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These contain leaves with parallel veins, scattered vascular bundles, and seeds with single cotyledons (seed leaves). Most do not possess cambium and therefore are non-woody (herbaceous). They contain flower parts in multiples of three. Some examples include grasses such as wheat, corn, rye, and rice. Others include sugar cane, pineapple, irises, bananas, orchids, and palms (woody monocots).

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Kingdom Animalia (Metazoa) / Differentiation of tissues, organs, and organ systems

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Simple multicellular animals such as sponges, coelenterates, and flatworms have minimal differentiation. Most of their cells are in direct contact with the outside environment. In these organisms, few systems (such as the digestive and reproductive) are required to support the life processes. In more advanced animals, specialized tissues and systems facilitate digestion, locomotion, circulation, message conduction (nervous system), and support.

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Alimentation

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All animals, except some parasites like the tapeworm, ingest bulkfoods (holotrophic), digest them, and then eliminate the remains.

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Locomotion

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All animals employ some form of locomotion to acquire nutrients. Some are sessile (stationary) and create currents to trap food. This is important for protection, mate selection, and reproduction.

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Bilateral Symmetry

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Most animals have right and left sides that are mirror images. The head is directed anteriorly. However, some animals, such as the echinoderms and cnidarians, are radially symmetrical.

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Nervous System

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Animals possess a system enabling them to receive stimuli and control their actions. They have sense organs, specialized conductors, and higher brain centers for coordination and learning.

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Chemical Coordinating System

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Animals secrete chemicals (hormones) which operate in conjunction with the nervous system to maintain a steady state or homeostasis.

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Porifera (Sponges)

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These organisms have two layers of cells, pores, are sessile, and have a low degree of cellular respiration.

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Cnidarians

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AKA coelenterates, contain a digestive sac that is sealed at one end (gastrovascular cavity). Two layers of cells are present - the ectoderm and the endoderm. They have many specialized features including tentacles, stinging cells, and net nerves (e.g. hydra, jellyfish, sea anemone, and coral).

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Platyhelminthes (Flatworms)

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These organisms are ribbonlike, bilaterally symmetrical, and possess three layers of cells including a solid mesoderm. They do not have circulatory systems and their nervous system consists of eyes, an anterior brain ganglion, and a pair of longitudinal nerve cords.

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Nematoda (Round Worms)

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These organisms possess long digestive tubes and an anus. A solid mesoderm is present. They lack circulatory systems and possess nerve cords and an anterior nerve ring (e.g., hookworm, trichina, free-living soil nematodes).

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Annelida (Segmented Worms)

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These organisms possess a coelom (a true body cavity) contained in the mesoderm. They have well defined systems including nervous, circulatory, and excretory (e.g. earthworms, leeches).

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Mollusca (Muollusks)

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These organisms are softbodied and possess mantles that often secrete calcareous (calcium carbonate) exoskeletons. They breathe by gills and contain chambered hearts, blood sinuses, and a pair of ventral nerve cords (e.g., clams, snails, and squid).

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Arthropoda

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These organisms have jointed appendages, chitinous exoskeletons, and open circulatory systems (sinuses). The three most important classes are insects, arachnids, and crustaceans.

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Insects

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These arthropoda possess three pairs of legs, spiracles, and tracheal tubes designed for breathing outside of an aquatic environment.

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Arachnids

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These arthropoda have four pairs of legs and "book lungs" (e.g. scorpion, spiders).

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Crustaceans

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These arthropoda have a segmented body with a variable number of appendages and possess gills (e.g. lobster, crayfish, shrimp).

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Echinoderms

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These organisms are spiny, radially symmetric, contain a water-vascular system, and possess the capacity for regeneration of parts. There is evolutionary evidence suggesting a link between echinoderms and chordates (e.g. starfish and sea urchin).

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Chordates

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These organisms are characterized by a stiff dorsal rod called the notochord, present at some stage of embryologic development. They have paired gill slits and a tail extending beyond the anus at some point during development. The lancelets and tunicates (like amphioxus) are chordates, but not vertebrates.

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Vertebrates

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The most advanced subphylum of the chordates. They include amphibians, reptiles, birds, fish, and mammals. They also possess bones called vertebrae, which form the backbone. Bony vertebrae replace the notochord of the embryo and protect the nerve cord; a bony case protects the brain (the skull). They can be divided into 5 classes: Fish, amphibia, reptiles, birds, and mammals.

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Fish

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All of these organisms possess a two-chambered heart, gills, and utilize external fertilization for reproduction.

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Jawless Fish

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Eel-like, retain the notochord throughout life, have a cartilaginous internal skeleton, no jaws, and a sucking mouth. They include the class Agnatha and include the lamprey and hagfish.

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Cartilaginous Fish

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Possess jaws and teeth. A reduced notochord exists as segments between cartilaginous vertebrae and includes sharks.

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Bony Fish

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The most prevalent type of fish. They have scales and lack a notochord in the adult form. During development cartilage is replaced by a bony skeleton and includes sturgeon, trout, and tuna.

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Amphibia

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The larval stage, known as the tadpole, is found in water, has gills, a tail, and no legs. The adult lives on land, has lungs, two pairs of legs, no tail, a three-chambered heart, no scales, and utilizes external fertilization. Eggs are laid in water with a jelly-like secretion. Examples include the frog, salamander, toad, and newt.

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Reptiles

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These animals are terrestrial. They breathe air by means of the lungs, lay leathery eggs and utilize internal fertilization. They are cold-blooded (poikilothermic) and have scales and a three-chambered heart. Examples include the turtle, lizard, snake, and crocodile.

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Birds

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These animals have a four-chambered heart, are warm-blooded (homeothermic) and their eggs are surrounded by shells. Examples include the hen and the eagle.

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Mammals

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These animals are warm-blooded and feed their offspring with milk produced in the mammary glands. There are three classes: Monetremes, Marsupials, and Placental.

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Monetremes

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These mammals lay leathery eggs, have horny bills, and milk (mammary) glands with numerous opening but no nipples. Examples include the duckbill platypus and spiny anteater.

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Marsupials

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These mammals are pouched. The embryo begins development in the uterus and completes development while attached to nipples in the abdominal pouch. Examples include the kangaroo and opossum.

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Placental Mammals

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These mammals have embryos that develop fully in the uterus. The placenta attaches the embryo to the uterine wall and provides for the exchange of food, oxygen, and waste material. Examples include the bat, whale, mouse, and man.