Chlorophyta is a division of green algae, informally waters of the Sargasso Sea. Many brown algae, such as called chlorophytes. The name is used in two very members of the order Fucales, commonly grow along different senses so that care is needed to determine the rocky seashores. Some members of the class are used as use by a particular author. In older classification food for humans. systems, it refers to a highly paraphyletic group of all Worldwide there are about 1500–2000 species of brown the green algae within the green plants (Viridiplantae), algae.[4] Some species are of sufficient commercial and thus includes about 7,000 species [4] [5] of mostly importance, such as Ascophyllum nodosum , that they aquatic photosynthetic eukaryotic organisms. Like the have become subjects of extensive research in their own land plants (bryophytes and tracheophytes), green algae right.[5] [4] contain chlorophylls a and b, and store food as starch Brown algae belong to a very large group, the in their plastids. Heterokontophyta, a eukaryotic group of organisms In newer classifications, it refers to one of the two distinguished most prominently by having chloroplasts clades making up the Viridiplantae, which are the surrounded by four membranes, suggesting an origin chlorophytes and the streptophytes or charophytes.[6][7] from a symbiotic relationship between a basal In this sense it includes only about 4,300 species.[3] eukaryote and another eukaryotic organism. Most brown algae contain the pigment fucoxanthin, which is responsible for the distinctive greenish-brown color that The red algae, or Rhodophyta ( / r o ʊ ˈ d ɒ f ɨ t ə / or / gives them their name. Brown algae are unique among ˌ r o ʊ d ə ˈ f a ɪ t ə /; from Greek: ῥόδον (rhodon) = rose + heterokonts in developing into multicellular forms with φυτόν (phyton) = plant, thus red plant), are one of the differentiated tissues, but they reproduce by means of oldest groups of eukaryotic algae,[2] and also one of the flagellated spores and gametes that closely resemble largest, with about 5,000–6,000 species [3] of mostly cells of other heterokonts. Genetic studies show their multicellular, marine algae, including many notable closest relatives to be the yellow-green algae. seaweeds. Other references indicate as many as 10,000 [4] species; more detailed counts indicate ~4,000 in ~600 Bryophyte is a traditional name used to refer to all genera (3,738 marine spp in 546 genera and 10 orders embryophytes (land plants) that do not have true (plus the unclassifiable); 164 freshwater spp in 30 [5] vascular tissue and are therefore called 'non-vascular genera in 8 orders). plants'.[1] Some bryophytes do have specialized tissues The red algae form a distinct group characterized by the for the transport of water; however since these do not following attributes: eukaryotic cells without flagella contain lignin, they are not considered to be true and centrioles, using floridean starch as food reserve, vascular tissue.[2] Currently bryophytes are thought not with phycobiliproteins as accessory pigments (giving to be a natural or monophyletic group; however the them their red color), and with chloroplasts lacking name is convenient and remains in use as a collective external endoplasmic reticulum and containing term for mosses, hornworts, and liverworts. Bryophytes unstacked thylakoids. [4] Most red algae are also produce enclosed reproductive structures (gametangia multicellular, macroscopic, marine, and have sexual and sporangia), but they produce neither flowers nor reproduction. seeds, reproducing via spores. The term bryophyte Many of the coralline algae, which secrete calcium comes from Greek βρύον - bryon, "tree-moss, oyster- carbonate and play a major role in building coral reefs, green" + φυτόν - fyton "plant". belong here. Red algae such as dulse (Palmaria palmata) and laver (nori/gim) are a traditional part of Vascular plants (also known as tracheophytes or European and Asian cuisine and are used to make other higher plants) are those plants that have lignified products like agar, carrageenans and other food [6] tissues for conducting water, minerals, and additives. photosynthetic products through the plant. Vascular plants include the clubmosses, Equisetum, ferns, The Phaeophyceae or brown algae (singular: alga), is gymnosperms (including conifers) and angiosperms (flowering plants). Scientific names for the group a large group of mostly marine multicellular algae, [2] [3] including many seaweeds of colder Northern include Tracheophyta and Tracheobionta. Hemisphere waters. They play an important role in Vascular plants are distinguished by two primary marine environments, both as food and for the habitats characteristics: they form. For instance Macrocystis, a kelp of the order 1. Vascular plants have vascular tissues which Laminariales, may reach 60 m in length, and forms circulate resources through the plant. This prominent underwater forests. Another example is feature allows vascular plants to evolve to a Sargassum, which creates unique habitats in the tropical larger size than non-vascular plants, which lack marattioid ferns , and ophioglossoid ferns . The term these specialized conducting tissues and are pteridophyte also refers to ferns and a few other therefore restricted to relatively small sizes. seedless vascular plants (see classification section 2. In vascular plants, the principal generation below). A pteridologist is a specialist in the study of phase is the sporophyte, which is usually diploid pteridophytes in a broader sense that includes the more with two sets of chromosomes per cell. Only the distantly related lycophytes. germ cells and gametophytes are haploid. By Ferns first appear in the fossil record 360 million years contrast, the principal generation phase in non- ago in the Carboniferous but many of the current vascular plants is usually the gametophyte, families and species did not appear until roughly 145 which is haploid with one set of chromosomes million years ago in the early Cretaceous (after per cell. In these plants, generally only the spore flowering plants came to dominate many stalk and capsule are diploid. environments). One possible mechanism for the presumed switch from Ferns are not of major economic importance, but some emphasis on the haploid generation to emphasis on the are grown or gathered for food, as ornamental plants, diploid generation is the greater efficiency in spore for remediating contaminated soils, and have been the dispersal with more complex diploid structures. In other subject of research for their ability to remove some words, elaboration of the spore stalk enabled the chemical pollutants from the air. Some are significant production of more spore and the ability to release it weeds. They also play a role in mythology, medicine, higher and to broadcast it farther. Such developments and art. may include more photosynthetic area for the spore- bearing structure, the ability to grow independent roots, woody structure for support, and more branching. The spermatophytes (from the Greek word "Σπερματόφυτα") (also known as phanerogams) Water transport happens in either xylem or phloem: comprise those plants that produce seeds. They are a xylem carries water and inorganic solutes upward subset of the embryophytes or land plants. The living toward the leaves from the roots, while phloem carries spermatophytes form five groups: organic solutes throughout the plant. • cycads , a subtropical and tropical group of plants with a large crown of compound leaves Lycopodiopsida is a class of plants often loosely and a stout trunk, grouped as the fern allies. Traditionally the group • Ginkgo , a single living species of tree, included not only the clubmosses and firmosses, but also the spikemosses (Selaginella and relatives) and the • conifers , cone-bearing trees and shrubs, quillworts (Isoetes and relatives). However, the latter • gnetophytes , woody plants in the genera are now usually separated off into a separate class, Gnetum, Welwitschia, and Ephedra, and Isoetopsida. • angiosperms , the flowering plants, a large group Clubmosses are thought to be structurally similar to the including many familiar plants in a wide variety earliest vascular plants, with small, scale-like leaves, of habitats. homosporous spores borne in sporangia at the bases of In addition to the taxa listed above, the fossil record the leaves, branching stems (usually dichotomous), and contains evidence of many extinct taxa of seed plants. generally simple form. The so-called "seed ferns" (Pteridospermae) were one The Class Lycopodiopsida as interpreted here contains of the earliest successful groups of land plants, and a single living order, the Lycopodiales, and a single forests dominated by seed ferns were prevalent in the extinct order, the Drepanophycales. late Paleozoic. Glossopteris was the most prominent tree genus in the ancient southern supercontinent of Gondwana during the Permian period. By the Triassic A fern is any one of a group of about 12,000 species of period, seed ferns had declined in ecological plants belonging to the botanical group known as [3] importance, and representatives of modern Pteridophyta. Unlike mosses, they have xylem and gymnosperm groups were abundant and dominant phloem (making them vascular plants). They have through the end of the Cretaceous, when angiosperms stems, leaves, and roots like other vascular plants. Ferns radiated. Another Late Paleozoic group of probable reproduce via spores and have neither seeds nor spermatophytes were the gigantopterids. flowers. By far the largest group of ferns are the leptosporangiate ferns , but ferns as defined here (also The gymnosperms are a group of seed-bearing plants called monilophytes) include horsetails, whisk ferns, that includes conifers, cycads, Ginkgo, and Gnetales. The term "gymnosperm" comes from the Greek word The male gametophyte in angiosperms is significantly gymnospermos (γυμνόσπερμος), meaning "naked reduced in size compared to those of gymnosperm seed seeds", after the unenclosed condition of their seeds plants. The smaller pollen decreases the time from (called ovules in their unfertilized state). Their naked pollination — the pollen grain reaching the female plant condition stands in contrast to the seeds or ovules of — to fertilization of the ovary; in gymnosperms, flowering plants (angiosperms), which are enclosed fertilization can occur up to a year after pollination, during pollination. Gymnosperm seeds develop either whereas, in angiosperms, the fertilization begins very on the surface of scale- or leaf-like appendages of soon after pollination. The shorter time leads to cones, or at the end of short stalks (Ginkgo). angiosperm plants' setting seeds sooner and faster than The gymnosperms and angiosperms together comprise gymnosperms, which is a distinct evolutionary the spermatophytes or seed plants. By far the largest advantage. group of living gymnosperms are the conifers (pines, • Closed carpel enclosing the ovules (carpel or cypresses, and relatives), followed by cycads, Gnetales carpels and accessory parts may become the (Gnetophyta, Ephedra and Welwitschia), and Ginkgo (a fruit) single living species). The closed carpel of angiosperms also allows adaptations to specialized pollination syndromes and The flowering plants (angiosperms), also known as controls. This helps to prevent self-fertilization, thereby Angiospermae or Magnoliophyta, are the most maintaining increased diversity. Once the ovary is diverse group of land plants. Angiosperms are seed- fertilized, the carpel and some surrounding tissues producing plants like the gymnosperms and can be develop into a fruit. This fruit often serves as an distinguished from the gymnosperms by a series of attractant to seed-dispersing animals. The resulting synapomorphies (derived characteristics). These cooperative relationship presents another advantage to characteristics include flowers, endosperm within the angiosperms in the process of dispersal. seeds, and the production of fruits that contain the • Reduced female gametophyte, seven cells with seeds. eight nuclei The ancestors of flowering plants diverged from The reduced female gametophyte, like the reduced male gymnosperms around 245–202 million years ago, and gametophyte, may be an adaptation allowing for more the first flowering plants known to exist are from 140 rapid seed set, eventually leading to such flowering million years ago. They diversified enormously during plant adaptations as annual herbaceous life-cycles, the Lower Cretaceous and became widespread around allowing the flowering plants to fill even more niches. 100 million years ago, but replaced conifers as the • Endosperm dominant trees only around 60–100 million years ago. In general, endosperm formation begins after fertilization and before the first division of the zygote. • Flowers Endosperm is a highly nutritive tissue that can provide The flowers, which are the reproductive organs of food for the developing embryo, the cotyledons, and flowering plants, are the most remarkable feature sometimes the seedling when it first appears. distinguishing them from other seed plants. Flowers aid These distinguishing characteristics taken together have angiosperms by enabling a wider range of adaptability made the angiosperms the most diverse and numerous and broadening the ecological niches open to them. land plants and the most commercially important group This has allowed flowering plants to largely dominate to humans. The major exception to the dominance of terrestrial ecosystems. terrestrial ecosystems by flowering plants is the • Stamens with two pairs of pollen sacs coniferous forest. Stamens are much lighter than the corresponding organs of gymnosperms and have contributed to the Monocotyledons, also known as monocots, are one of diversification of angiosperms through time with two major groups of flowering plants (or angiosperms) adaptations to specialized pollination syndromes, such that are traditionally recognized, the other being as particular pollinators. Stamens have also become dicotyledons, or dicots. Monocot seedlings typically modified through time to prevent self-fertilization, have one cotyledon (seed-leaf), in contrast to the two which has permitted further diversification, allowing cotyledons typical of dicots. Monocots have been angiosperms eventually to fill more niches. recognized at various taxonomic ranks, and under • Reduced male parts, three cells various names (see below). The APG II system recognises a clade called "monocots" but does not assign it to a taxonomic rank. According to the IUCN there are 59,300 species of Aside from cotyledon number, other broad differences monocots.[1] The largest family in this group (and in the have been noted between monocots and dicots, flowering plants as a whole) by number of species are although these have proven to be differences primarily the orchids (family Orchidaceae), with more than between monocots and eudicots. Many early-diverging 20,000 species.[2] In agriculture the majority of the dicot groups have "monocot" characteristics such as biomass produced comes from monocots.[3] The true scattered vascular bundles, trimerous flowers, and non- grasses, family Poaceae (Gramineae), are the most tricolpate pollen.[2] In addition, some monocots have economically important family in this group. These dicot characteristics such as reticulated leaf veins.[2] include all the true grains (rice, wheat, maize, etc.), the Feature In monocots In dicots pasture grasses, sugar cane, and the bamboos. True grasses have evolved to become highly specialised for Number of parts in threes in fours or fives wind pollination. Grasses produce much smaller of each flower (flowers are (tetramerous or flowers, which are gathered in highly visible plumes trimerous) pentamerous) (inflorescences). Other economically important monocot families are the palm family (Arecaceae), Number of one three banana family (Musaceae), ginger family furrows or pores (Zingiberaceae) and the onion family Alliaceae, which in pollen includes such ubiquitously used vegetables as onions and garlic. Number of one two Many plants cultivated for their blooms are also from cotyledons the monocot group, notably lilies, daffodils, irises, (leaves in the amaryllis, orchids, cannas, bluebells and tulips. seed) Arrangement of scattered in concentric vascular bundles circles The dicotyledons, also known as dicots, are a group of in the stem flowering plants whose seed typically has two embryonic leaves or cotyledons. There are around Roots are develop from the 199,350 species within this group.[1] Flowering plants adventitious radicle that are not dicotyledons are monocotyledons, typically having one embryonic leaf. Arrangement of parallel reticulate Dicotyledons are not a monophyletic group, and major leaf veins therefore the names "dicotyledons" and "dicots" are, Secondary absent often present strictly speaking, deprecated. However, the vast majority of "dicots" do form a monophyletic group growth called the eudicots or tricolpates. These may be distinguished from all other flowering plants by the structure of their pollen. Other dicotyledons and monocotyledons have monosulcate pollen, or forms derived from it, whereas eudicots have tricolpate pollen, or derived forms, the pollen having three or Sponges are animals of the phylum Porifera ( / p ɒ more pores set in furrows called colpi. ˈ r ɪ f ər ə /; meaning "pore bearer").They are multicellular organisms which have bodies full of pores and channels Traditionally the dicots have been called the allowing water to circulate through them, consist of Dicotyledones (or Dicotyledoneae), at any rank. If jelly-like mesohyl sandwiched between two thin layers treated as a class, as in the Cronquist system , they may of cells. While all animals have unspecialized cells that be called the Magnoliopsida after the type genus can transform into specialized cells, sponges are unique Magnolia. In some schemes, the eudicots are treated as in having some specialized cells, but can also have a separate class, the Rosopsida (type genus Rosa), or as specialized cells that can transform into other types, several separate classes. The remaining dicots often migrating between the main cell layers and the (palaeodicots) may be kept in a single paraphyletic mesohyl in the process. Sponges do not have nervous, class, called Magnoliopsida, or further divided. digestive or circulatory systems. Instead, most rely on maintaining a constant water flow through their bodies Compared to Monocotyledons to obtain food, oxygen and remove wastes. The shapes of their bodies are adapted for maximal efficiency of water flow. Water enters through the central cavity, deposits nutrients, and leaves through a hole called the sponges. However it is uncertain which group of osculum. All sponges are sessile aquatic animals. sponges is closest to Eumetazoa, as both calcareous Although there are freshwater species, the great sponges and a subgroup of demosponges called majority are marine (salt water) species, ranging from Homoscleromorpha have been nominated by different tidal zones to depths exceeding 8,800 metres (5.5 mi). researchers. In addition, a study in 2008 suggested the While most of the approximately 5,000–10,000 known earliest animals may have been similar to modern comb species feed on bacteria and other food particles in the jellies. water, some host photosynthesizing micro-organisms as The few species of demosponge that have entirely soft endosymbionts and these alliances often produce more fibrous skeletons with no hard elements have been used food and oxygen than they consume. A few species of by humans over thousands of years for several sponge that live in food-poor environments have purposes, including as padding and as cleaning tools. become carnivores that prey mainly on small By the 1950s, though, these had been overfished so crustaceans.[1] heavily that the industry almost collapsed, and most Most species use sexual reproduction, releasing sperm sponge-like materials are now synthetic. Sponges and cells into the water to fertilize ova that in some species their microscopic endosymbionts are now being are released and in others are retained by the "mother". researched as possible sources of medicines for treating The fertilized eggs form larvae which swim off in a wide range of diseases. Dolphins have been observed search of places to settle. Sponges are known for using sponges as tools while foraging. regenerating from fragments that are broken off, although this only works if the fragments include the right types of cells. A few species reproduce by Cnidaria ( / n a ɪ ˈ d ɛ ər i ə / with a silent c) is a phylum budding. When conditions deteriorate, for example as [4] temperatures drop, many freshwater species and a few containing over 10,000 species of animals found marine ones produce gemmules, "survival pods" of exclusively in aquatic and mostly marine environments. unspecialized cells that remain dormant until conditions Their distinguishing feature is cnidocytes, specialized improve and then either form completely new sponges cells that they use mainly for capturing prey. Their or recolonize the skeletons of their parents. bodies consist of mesoglea, a non-living jelly-like substance, sandwiched between two layers of The mesohyl functions as an endoskeleton in most epithelium that are mostly one cell thick. They have sponges, and is the only skeleton in soft sponges that two basic body forms: swimming medusae and sessile encrust hard surfaces such as rocks. More commonly, polyps, both of which are radially symmetrical with the mesohyl is stiffened by mineral spicules, by spongin mouths surrounded by tentacles that bear cnidocytes. fibers or both. Demosponges use spongin, and in many Both forms have a single orifice and body cavity that species, silica spicules and in some species, calcium are used for digestion and respiration. Many cnidarian carbonate exoskeletons. Demosponges constitute about species produce colonies that are single organisms 90% of all known sponge species, including all composed of medusa-like or polyp-like zooids, or both. freshwater ones, and have the widest range of habitats. Cnidarians' activities are coordinated by a decentralized Calcareous sponges, which have calcium carbonate nerve net and simple receptors. Several free-swimming spicules and, in some species, calcium carbonate Cubozoa and Scyphozoa possess balance-sensing exoskeletons, are restricted to relatively shallow marine statocysts, and some have simple eyes. Not all waters where production of calcium carbonate is cnidarians reproduce sexually. Many have complex easiest. The fragile glass sponges, with "scaffolding" of lifecycles with asexual polyp stages and sexual silica spicules, are restricted to polar regions and the medusae, but some omit either the polyp or the medusa ocean depths where predators are rare. Fossils of all of stage. these types have been found in rocks dated from 580 million years ago. In addition Archaeocyathids, whose Cnidarians were for a long time grouped with fossils are common in rocks from 530 to 490 million Ctenophores in the phylum Coelenterata, but increasing years ago, are now regarded as a type of sponge. awareness of their differences caused them to be placed in separate phyla. Cnidarians are classified into four The sponge's closest single-celled relatives are thought main groups: the almost wholly sessile Anthozoa (sea to be choanoflagellates, which strongly resemble the anemones, corals, sea pens); swimming Scyphozoa cells sponges use to drive their water flow systems and (jellyfish); Cubozoa (box jellies); and Hydrozoa, a capture most of their food. Sponges are generally diverse group that includes all the freshwater cnidarians agreed, also, to not form a monophyletic group, in other as well as many marine forms, and has both sessile words do not include all and only the descendants of a members such as Hydra and colonial swimmers such as common ancestor, because Eumetazoa (more complex the Portuguese Man o' War. Staurozoa have recently animals) are thought to be descendants of a subgroup of been recognised as a class in their own right rather than a sub-group of Scyphozoa, and there is debate about πλατύ, platy, meaning "flat" and ἕλμινς (root: ἑλμινθ-), whether Myxozoa and Polypodiozoa are cnidarians or helminth-, meaning worm)[2] are a phylum of relatively closer to bilaterians (more complex animals). simple bilaterian, unsegmented, soft-bodied Most cnidarians prey on organisms ranging in size from invertebrate animals. Unlike other bilaterians, they have plankton to animals several times larger than no body cavity, and no specialized circulatory and themselves, but many obtain much of their nutrition respiratory organs, which restricts them to flattened from endosymbiotic algae, and a few are parasites. shapes that allow oxygen and nutrients to pass through Many are preyed upon by other animals including their bodies by diffusion. starfish, sea slugs, fish and turtles. Coral reefs, whose In traditional zoology texts Platyhelminthes are divided polyps are rich in endosymbiotic algae, support some of into Turbellaria, which are mostly non-parasitic animals the world's most productive ecosystems, and protect such as planarians, and three entirely parasitic groups: vegetation in tidal zones and on shorelines from strong Cestoda, Trematoda and Monogenea. Turbellarians are currents and tides. While corals are almost entirely mostly predators, and live in water or in shaded, humid restricted to warm, shallow marine waters, other terrestrial environments such as leaf litter. Cestodes cnidarians live in the depths, in polar seas and in (tapeworms) and trematodes (flukes) have complex life- freshwater. cycles, with mature stages that live as parasites in the Fossil cnidarians have been found in rocks formed digestive systems of fish or land vertebrates, and about 580 million years ago, and other fossils show that intermediate stages that infest secondary hosts. The corals may have been present shortly before 490 million eggs of trematodes are excreted from their main hosts, years ago and diversified a few million years later. whereas adult cestodes generate vast numbers of Fossils of cnidarians that do not build mineralized hermaphroditic, segment-like proglottids which detach structures are very rare. Scientists currently think that when mature, are excreted and then release eggs. cnidarians, ctenophores and bilaterians are more closely Unlike the other parasitic groups, the monogeneans are related to calcareous sponges than these are to other external parasites infesting aquatic animals, and their sponges, and that anthozoans are the evolutionary larvae metamorphose into the adult form after attaching "aunts" or "sisters" of other cnidarians, and the most to a suitable host. closely related to bilaterians. Recent analyses have Because they do not have internal body cavities, for concluded that cnidarians, although considered more over a century Platyhelminthes were regarded as a "primitive" than bilaterians, have a wider range of primitive stage in the evolution of bilaterians (animals genes. with bilateral symmetry and hence with distinct front Jellyfish stings killed several hundred people in the and rear ends). However, analyses since the mid-1980s 20th century, and cubozoans are particularly dangerous. have separated out one sub-group, the Acoelomorpha, On the other hand, some large jellyfish are considered a as basal bilaterians, in other words closer to the original delicacy in eastern and southern Asia. Coral reefs have bilaterians than to any other modern groups. The long been economically important as providers of remaining Platyhelminthes form a monophyletic group, fishing grounds, protectors of shore buildings against in other words one that contains all and only currents and tides, and more recently as centers of descendants of a common ancestor that is itself a tourism. However, they are vulnerable to over-fishing, member of the group. The redefined Platyhelminthes is mining for construction materials, pollution, and part of the Lophotrochozoa, one of the three main damage caused by tourism. groups of more complex bilaterians. These analyses have also concluded that the redefined Platyhelminthes, Hydrozoa (hydrozoans) are a taxonomic class of very excluding Acoelomorpha, consists of two monophyletic small, predatory animals which can be solitary or sub-groups, Catenulida and Rhabditophora, and that colonial and which mostly live in saltwater. A few Cestoda, Trematoda and Monogenea form a genera within this class live in freshwater. Hydrozoans monophyletic sub-group within one branch of the are related to jellyfish and corals and belong to the Rhabditophora. Hence the traditional platyhelminth phylum Cnidaria. sub-group "Turbellaria" is now regarded as paraphyletic Some examples of hydrozoans are the Freshwater Jelly since it excludes the wholly parasitic groups although (Craspedacusta sowerbyi), the freshwater polyps these are descended from one group of "turbellarians". (Hydra), Obelia, the Portuguese Man o' War (Physalia Over half of all known flatworm species are parasitic, physalis), the chondrophores (Porpitidae), "air fern" and some do enormous harm to humans and their (Sertularia argenta) and the pink-hearted hydroids livestock. Schistosomiasis, caused by one genus of (Tubularia). trematodes, is the second most devastating of all human The flatworms, known in scientific literature as diseases caused by parasites, surpassed only by malaria. Platyhelminthes or Plathelminthes (from the Greek Neurocysticercosis, which arises when larvae of the pork tapeworm Taenia solium penetrate the central "super-phylum" of protostomes that also includes nervous system, is the major cause of acquired epilepsy molluscs, brachiopods, flatworms and nemerteans. worldwide. The threat of platyhelminth parasites to The basic annelid form consists of multiple segments, humans in developed countries is rising because of each of which has the same sets of organs and, in most organic farming, the popularity of raw or lightly cooked polychaetes, a pair of parapodia that many species use foods, and imports of food from high-risk areas. In less for locomotion. Septa separate the segments of many developed countries, people often cannot afford the fuel species, but are poorly-defined or absent in some, and required to cook food thoroughly, and poorly designed Echiura and Sipuncula show no obvious signs of water-supply and irrigation projects increase the segmentation. In species with well-developed septa, the dangers presented by poor sanitation and unhygienic blood circulates entirely within blood vessels, and the farming. vessels in segments near the front ends of these species Two planarian species have been used successfully in are often built up with muscles to act as hearts. The the Philippines, Indonesia, Hawaii, New Guinea and septa of these species also enable them to change the Guam to control populations of the imported giant shapes of individual segments, which facilitates African snail Achatina fulica , which was displacing movement by peristalsis ("ripples" that pass along the native snails. However, there is now concern that these body) or by undulations that improve the effectiveness planarians may themselves become a serious threat to of the parapodia. In species with incomplete septa or native snails. In North-west Europe there are concerns none, the blood circulates through the main body cavity about the spread of the New Zealand planarian without any kind of pump, and there is a wide range of Arthurdendyus triangulatus , which preys on locomotory techniques – some burrowing species turn earthworms. their pharynges inside out to drag themselves through The Aschelminthes (also known as the sediment. Aeschelminthes), closely associated with Although many species can reproduce asexually and use similar mechanisms to regenerate after severe the Platyhelminthes, are an obsolete injuries, sexual reproduction is the normal method in phylum of pseudocoelomate and other species whose reproduction has been studied. The similar animals that are no longer minority of living polychaetes whose reproduction and considered closely related and have been lifecycles are known produce trochophore larvae, which promoted to phyla in their own right. The live as plankton and then sink and metamorphose into miniature adults. Oligochaetes are full hermaphrodites term Aschelminth is now generally only and produce a ring-like cocoon round their bodies, in used as an informal name for any member which the eggs and hatchlings are nourished until they of the approximately ten different are ready to emerge. invertebrate phyla formerly included within Earthworms support terrestrial food chains both as prey Aschelminthes. and by aerating and enriching soil. The burrowing of marine polychaetes, which may constitute up to a third of all species in near-shore environments, encourages the development of ecosystems by enabling water and The annelids (also called "ringed worms"), formally oxygen to penetrate the sea floor. In addition to called Annelida (from French annelés "ringed ones", improving soil fertility, annelids serve humans as food ultimately from Latin anellus "little ring"[2]), are a large and as bait. Scientists observe annelids to monitor the phylum of segmented worms, with over 17,000 modern quality of marine and fresh water. Although blood- species including ragworms, earthworms and leeches. letting is no longer in favor with doctors, some leech They are found in marine environments from tidal species are regarded as endangered species because zones to hydrothermal vents, in freshwater, and in moist they have been over-harvested for this purpose in the terrestrial environments. Although most textbooks still last few centuries. Ragworms' jaws are now being use the traditional division into polychaetes (almost all studied by engineers as they offer an exceptional marine), oligochaetes (which include earthworms) and combination of lightness and strength. leech-like species, research since 1997 has radically Since annelids are soft-bodied, their fossils are rare – changed this scheme, viewing leeches as a sub-group of mostly jaws and the mineralized tubes that some of the oligochaetes and oligochaetes as a sub-group of species secreted. Although some late Ediacaran fossils polychaetes. In addition, the Pogonophora, Echiura and may represent annelids, the oldest known fossil that is Sipuncula, previously regarded as separate phyla, are identified with confidence comes from about 518 now regarded as sub-groups of polychaetes. Annelids million years ago in the early Cambrian period. Fossils are considered members of the Lophotrochozoa, a of most modern mobile polychaete groups appeared by the end of the Carboniferous, about 299 million years nephridia ("kidneys") are important parts of the ago. Scientists disagree about whether some body reproductive system as well as the circulatory and fossils from the mid Ordovician, about 472 to 461 excretory systems; in bivalves, the gills both "breathe" million years ago, are the remains of oligochaetes, and and produce a water current in the mantle cavity, which the earliest certain fossils of the group appear in the is important for excretion and reproduction. Tertiary period, which began 65 million years ago. There is good evidence for the appearance of The Mollusca (pronounced / mə ˈ l ʌ skə /), common name gastropods, cephalopods and bivalves in the Cambrian molluscs or mollusks[note 1] (pronounced / ˈ m ɒ ləsks /), is period 542 to 488.3 million years ago. However the a large phylum of invertebrate animals. There are evolutionary history both of molluscs' emergence from around 85,000 recognized extant species of molluscs. the ancestral Lophotrochozoa and of their Mollusca is the largest marine phylum, comprising diversification into the well-known living and fossil about 23% of all the named marine organisms. forms are still subjects of vigorous debate among Numerous molluscs also live in freshwater and scientists. terrestrial habitats. Molluscs are highly diverse, not Molluscs have been and still are an important food only in size and in anatomical structure, but also in source for anatomically modern humans. However behaviour and in habitat. The phylum is typically there is a risk of food-poisoning from toxins that divided into nine or ten taxonomic classes, of which accumulate in molluscs under certain conditions, and two are entirely extinct. Cephalopod molluscs such as many countries have regulations that aim to minimize squid, cuttlefish and octopus are among the most this risk. Molluscs have for centuries also been the neurologically advanced of all invertebrates – and source of important luxury goods, notably pearls, either the giant squid or the colossal squid is the largest mother of pearl, Tyrian purple dye, and sea silk. Their known invertebrate species. The gastropods (snails and shells have also been used as a money in some pre- slugs) are by far the most numerous molluscs in terms industrial societies. of classified species, and account for 80% of the total. Mollusc species can also represent hazards or pests for Molluscs have such a varied range of body structures human activities. The bite of the blue-ringed octopus is that it is difficult to find defining characteristics that often fatal, and that of Octopus apollyon causes apply to all modern groups. The two most universal inflammation that can last for over a month. Stings features are a mantle with a significant cavity used for from a few species of large tropical cone shells can also breathing and excretion, and the structure of the kill, but their sophisticated though easily produced nervous system. As a result of this wide diversity, many venoms have become important tools in neurological textbooks base their descriptions on a hypothetical research. Schistosomiasis (also known as bilharzia, "generalized mollusc". This has a single, "limpet-like" bilharziosis or snail fever) is transmitted to humans via shell on top, which is made of proteins and chitin water snail hosts, and affects about 200 million people. reinforced with calcium carbonate, and is secreted by a Snails and slugs can also be serious agricultural pests, mantle that covers the whole upper surface. The and accidental or deliberate introduction of some snail underside of the animal consists of a single muscular species into new environments has seriously damaged "foot". Although molluscs are coelomates, the coelom some ecosystems. is very small, and the main body cavity is a hemocoel through which blood circulates – molluscs' circulatory An arthropod is an invertebrate animal having an systems are mainly open. The "generalized" mollusc's exoskeleton (external skeleton), a segmented body, and feeding system consists of a rasping "tongue" called a jointed appendages. Arthropods are members of the radula and a complex digestive system in which exuded phylum Arthropoda (from Greek ἄ ρθρον árthron, mucus and microscopic, muscle-powered "hairs" called "joint", and ποδός podós "foot", which together mean cilia play various important roles. The "generalized "jointed feet"), and include the insects, arachnids, mollusc" has two paired nerve cords, or three in crustaceans, and others. Arthropods are characterized bivalves. The brain, in species that have one, encircles by their jointed limbs and cuticles, which are mainly the esophagus. Most molluscs have eyes, and all have made of α-chitin; the cuticles of crustaceans are also sensors that detect chemicals, vibrations and touch. The biomineralized with calcium carbonate. The rigid simplest type of molluscan reproductive system relies cuticle inhibits growth, so arthropods replace it on external fertilization, but there are more complex periodically by molting. The arthropod body plan variations. All produce eggs, from which may emerge consists of repeated segments, each with a pair of trochophore larvae, more complex veliger larvae, or appendages. It is so versatile that they have been miniature adults. compared to Swiss Army knives, and it has enabled them to become the most species-rich members of all A striking feature of molluscs is the use of the same ecological guilds in most environments. They have over organ for multiple functions. For example: the heart and a million described species, making up more than 80% of all described living animal species, and are one of cycloneuralians (or their constituent clades) in a only two animal groups that are very successful in dry superphylum Ecdysozoa. Overall however, the basal environments – the other being the amniotes. They relationships of Metazoa are not yet well resolved. range in size from microscopic plankton up to forms a Likewise, the relationships between various arthropod few meters long. groups are still actively debated. Arthropods' primary internal cavity is a hemocoel, Arthropods contribute to the human food supply both which accommodates their internal organs and through directly as food, and more importantly as pollinators of which their blood circulates; they have open circulatory crops. Some specific species are known to spread systems. Like their exteriors, the internal organs of severe disease to humans, livestock, and crops. arthropods are generally built of repeated segments. Echinoderms (Phylum Echinodermata) are a phylum Their nervous system is "ladder-like", with paired of marine animals. Echinoderms are found at every ventral nerve cords running through all segments and ocean depth, from the intertidal zone to the abyssal forming paired ganglia in each segment. Their heads zone. Aside from the hard-to-classify Arkarua, the first are formed by fusion of varying numbers of segments, definitive members of the phylum appeared near the and their brains are formed by fusion of the ganglia of start of the Cambrian period. these segments and encircle the esophagus. The respiratory and excretory systems of arthropods vary, The phylum contains about 7,000 living species, depending as much on their environment as on the making it the second-largest grouping of subphylum to which they belong. deuterostomes, after the chordates. Echinoderms are also the largest phylum that has no freshwater or Their vision relies on various combinations of terrestrial representatives. compound eyes and pigment-pit ocelli: in most species the ocelli can only detect the direction from which light The word is derived from the Greek ἐχινοδέρματα is coming, and the compound eyes are the main source (echinodermata), plural of ἐχινόδερμα (echinoderma), of information, but the main eyes of spiders are ocelli "spiny skin" from ἐχινός (echinos), "sea-urchin", that can form images and, in a few cases, can swivel to originally "hedgehog,"[1] and δέρμα (derma), "skin".[2][3] track prey. Arthropods also have a wide range of The echinoderms are important both biologically and chemical and mechanical sensors, mostly based on geologically: biologically because few other groupings modifications of the many setae (bristles) that project are so abundant in the biotic desert of the deep sea, as through their cuticles. well as the shallower oceans, and geologically as their Arthropods' methods of reproduction and development ossified skeletons are major contributors to many are diverse; all terrestrial species use internal limestone formations, and can provide valuable clues as fertilization, but this is often by indirect transfer of the to the geological environment. Further, it is held by sperm via an appendage or the ground, rather than by some[citation needed] that the radiation of echinoderms was direct injection. Aquatic species use either internal or responsible for the Mesozoic revolution of marine life. external fertilization. Almost all arthropods lay eggs, but scorpions give birth to live young after the eggs have hatched inside the mother. Arthropod hatchlings Chordates (phylum Chordata) are animals which are vary from miniature adults to grubs and caterpillars that either vertebrates or one of several closely related lack jointed limbs and eventually undergo a total invertebrates. They are united by having, for at least metamorphosis to produce the adult form. The level of some period of their life cycle, a notochord, a hollow maternal care for hatchlings varies from nonexistent to dorsal nerve cord, pharyngeal slits, an endostyle, and a the prolonged care provided by scorpions. post-anal tail. The phylum Chordata consists of three The versatility of the arthropod modular body plan has subphyla: Tunicata, represented by tunicates; made it difficult for zoologists and paleontologists to Cephalochordata, represented by lancelets; and classify them and work out their evolutionary ancestry, Craniata, which includes Vertebrata. The Hemichordata which dates back to the Cambrian period. From the late have been presented as a fourth chordate subphylum, 1950s to late 1970s, it was thought that arthropods were but they are now usually treated as a separate phylum. polyphyletic, that is, there was no single arthropod Tunicate larvae have both a notochord and a nerve cord ancestor. Now they are generally regarded as which are lost in adulthood. Cephalochordates have a monophyletic. Historically, the closest evolutionary notochord and a nerve cord (but no brain or specialist relatives of arthropods were considered to be annelid sensory organs) and a very simple circulatory system. worms, as both groups have segmented bodies. This Craniates are the only sub-phylum whose members hypothesis is by now largely rejected, with annelids and have skulls. In all craniates except for hagfish, the molluscs forming the superphylum Lophotrochozoa. dorsal hollow nerve cord is surrounded with Many analyses support a placement of arthropods with cartilaginous or bony vertebrae and the notochord is generally reduced; hence, hagfish are not regarded as vertebrates. The chordates and three sister phyla, the Hemichordata, the Echinodermata and the Xenoturbellida, make up the deuterostomes, one of the two superphyla that encompass all fairly complex animals.