Sea anemones are a group of water-dwelling, predatory animals of the order Actiniaria. They are named for the anemone, a terrestrial flower, sea anemones are classified in the phylum Cnidaria, class Anthozoa, subclass Hexacorallia. Anthozoa often have large polyps that allow for digestion of larger prey, as cnidarians, sea anemones are related to corals, tube-dwelling anemones, and Hydra. Many sea anemones form symbiotic relationships with single-celled dinoflagellates, zooxanthellae or with green algae, that live within their cells. A sea anemone is a sessile polyp attached at the bottom to the surface beneath it by a foot, called a basal disc. Most are from 1.8 to 3 cm in diameter and they can have from a few tens to a few hundred tentacles. A few species are pelagic and are not attached to the bottom, they have a gas chamber within the pedal disc, allowing them to float upside down in the water. The mouth, the anus of the sea anemone, is in the middle of the oral disc surrounded by tentacles armed with many cnidocytes, cnidocytes contain stinging nematocysts, capsule-like organelles capable of everting suddenly, giving the phylum Cnidaria its name.
Each nematocyst contains a small venom vesicle filled with actinotoxins, a filament. This gives the anemone its characteristic sticky feeling, the sea anemone eats small fish and shrimp. The venom is a mix of toxins, including neurotoxins, that paralyzes the prey so the anemone can move it to the mouth for digestion inside the gastrovascular cavity, actinotoxins are highly toxic to prey species of fish and crustaceans. However, small banded fish in various colors, are not affected by their host anemones sting, several other species have similar adaptions and are unaffected. Most sea anemones are harmless to humans, but a few highly toxic species have caused injuries and are potentially lethal. A gastrovascular cavity functions as a stomach and possesses an opening to the outside. Waste and undigested matter is excreted through this opening, which can be described as an incomplete gut, the mouth is typically slit-like in shape, and bears a groove at one or both ends. The groove, termed a siphonophore, is ciliated, and helps to circulate water through the gastrovascular cavity, some anemones feed on small particles, which are caught with the aid of a mucus secretion and moving currents that are set up by the tentacles.
Most sea anemones are predacious, immobilizing their prey with the aid of their nematocysts, the mouth opens into a flattened pharynx. This consists of an in-folding of the wall, and is therefore lined by the animals epidermis
Arachnids are a class of joint-legged invertebrate animals, in the subphylum Chelicerata. The term is derived from the Greek word ἀράχνη, meaning spider, spiders are the largest order in the class. Almost all extant arachnids are terrestrial, living mainly on land, some inhabit freshwater environments and, with the exception of the pelagic zone, marine environments as well. They comprise over 100,000 named species, including spiders, harvestmen, mites, almost all adult arachnids have eight legs, and arachnids may be easily distinguished from insects by this fact, since insects have six legs. However, arachnids have two pairs of appendages that have become adapted for feeding and sensory perception. The first pair, the chelicerae, serve in feeding and defense, the next pair of appendages, the pedipalps, have been adapted for feeding, and/or reproductive functions. In Solifugae, the palps are quite leg-like, so that animals appear to have ten legs. The larvae of mites and Ricinulei have only six legs, a fourth pair usually appears when they moult into nymphs, mites are variable, as well as eight, there are adult mites with six or even four legs.
Arachnids are further distinguished from insects by the fact they do not have antennae or wings and their body is organized into two tagmata, called the prosoma, or cephalothorax, and the opisthosoma, or abdomen. The cephalothorax is derived from the fusion of the cephalon and the thorax, the abdomen is segmented in the more primitive forms, but varying degrees of fusion between the segments occur in many groups. It is typically divided into a preabdomen and postabdomen, although this is clearly visible in scorpions, and in some orders, such as the Acari. A telson is present in scorpions, where it has modified to a stinger. Like all arthropods, arachnids have an exoskeleton, and they have a structure of cartilage-like tissue, called the endosternite. The endosternite is even calcified in some Opiliones, most arachnids lack extensor muscles in the distal joints of their appendages. Spiders and whipscorpions extend their limbs hydraulically using the pressure of their hemolymph and some harvestmen extend their knees by the use of highly elastic thickenings in the joint cuticle.
Scorpions and some harvestmen have evolved muscles that extend two leg joints at once, the equivalent joints of the pedipalps of scorpions though, are extended by elastic recoil. This type of system has almost certainly evolved from the book lungs. The excretory glands of arachnids include up to four pairs of coxal glands along the side of the prosoma, many arachnids have only one or the other type of excretory gland, although several do have both
Symmetry in biology
Symmetry in biology is the balanced distribution of duplicate body parts or shapes within the body of an organism. In nature and biology, symmetry is always approximate, for example plant leaves, while considered symmetrical, symmetry creates a class of patterns in nature, where the near-repetition of the pattern element is by reflection or rotation. The body plans of most multicellular organisms exhibit some form of symmetry, whether radial, bilateral, a small minority, notably among the sponges, exhibit no symmetry. Radially symmetric organisms resemble a pie where several cutting planes produce roughly identical pieces, such an organism exhibits no left or right sides. They have a top and a surface only. Most radially symmetric animals are symmetrical about an axis extending from the center of the oral surface, radial symmetry is especially suitable for sessile animals such as the sea anemone, floating animals such as jellyfish, and slow moving organisms such as starfish. Animals in the phyla cnidaria and echinodermata are radially symmetric, although many sea anemones and some corals have bilateral symmetry defined by a single structure, many flowers are radially symmetric or actinomorphic.
Roughly identical flower parts – petals and stamens occur at intervals around the axis of the flower. Many viruses have radial symmetries, their coats being composed of a small number of protein molecules arranged in a regular pattern to form polyhedrons, spheres. Tetramerism is a variant of radial symmetry found in jellyfish, which have four canals in an otherwise radial body plan, another variant of radial symmetry, means the organism is in five parts around a central axis, 72° apart. Among animals, only the echinoderms such as sea stars, sea urchins, being bilaterian animals, they initially develop with mirror symmetry as larvae, gain pentaradial symmetry later. Flowering plants show fivefold symmetry in many flowers and in various fruits and this is well seen in the arrangement of the five carpels in an apple cut transversely. Hexamerism is found in the corals and sea anemones which are divided into two based on their symmetry. The most common corals in the subclass Hexacorallia have a body plan, their polyps have sixfold internal symmetry.
Octamerism is found in corals of the subclass Octocorallia and these have polyps with eight tentacles and octameric radial symmetry. The octopus, has symmetry, despite its eight arms. Spherical symmetry occurs in an if it is able to be cut into two identical halves through any cut that runs through the organisms center. Organisms which show approximate spherical symmetry include the green alga Volvox
The annelids, known as the ringed worms or segmented worms, are a large phylum, with over 17,000 extant species including ragworms and leeches. The annelids are bilaterally symmetrical, coelomate, invertebrate organisms and they have parapodia for locomotion. Most textbooks still use the division into polychaetes, oligochaetes. Cladistic research since 1997 has radically changed this scheme, viewing leeches as a sub-group of oligochaetes and oligochaetes as a sub-group of polychaetes, in addition, the Pogonophora and Sipuncula, previously regarded as separate phyla, are now regarded as sub-groups of polychaetes. Annelids are considered members of the Lophotrochozoa, a super-phylum of protostomes that includes molluscs, flatworms, the basic annelid form consists of multiple segments. Each segment has the same sets of organs and, in most polychates, has a pair of parapodia that many use for locomotion. Septa separate the segments of many species, but are defined or absent in others. The septa of such species enable them to change the shapes of individual segments, although many species can reproduce asexually and use similar mechanisms to regenerate after severe injuries, sexual reproduction is the normal method in species whose reproduction has been studied.
The minority of living polychaetes whose reproduction and lifecycles are known produce trochophore larvae, Oligochaetes are full hermaphrodites and produce a ring-like cocoon around their bodies, in which the eggs and hatchlings are nourished until they are ready to emerge. Earthworms are Oligochaetes that support terrestrial food chains both as prey and in some regions are important in aeration and enriching of soil, in addition to improving soil fertility, annelids serve humans as food and as bait. Scientists observe annelids to monitor the quality of marine and fresh water, although blood-letting is no longer in favor with doctors, some leech species are regarded as endangered species because they have been over-harvested for this purpose in the last few centuries. Ragworms jaws are now being studied by engineers as they offer a combination of lightness. Since annelids are soft-bodied, their fossils are rare – mostly jaws, although some late Ediacaran fossils may represent annelids, the oldest known fossil that is identified with confidence comes from about 518 million years ago in the early Cambrian period.
Fossils of most modern mobile polychaete groups appeared by the end of the Carboniferous, there are over 22,000 living annelid species, ranging in size from microscopic to the Australian giant Gippsland earthworm and Amynthas mekongianus, which can both grow up to 3 metres long. As their name suggests, they have multiple chetae per segment, polychaetes have parapodia that function as limbs, and nuchal organs that are thought to be chemosensors. Most are marine animals, although a few live in fresh water. These have few or no chetae per segment, and no nuchal organs or parapodia, the clitellates are sub-divided into, which includes earthworms. Oligochaetes have a pad in the roof of the mouth
A cephalopod is any member of the molluscan class Cephalopoda. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, fishermen sometimes call them inkfish, referring to their common ability to squirt ink. The study of cephalopods is a branch of known as teuthology. Cephalopods became dominant during the Ordovician period, represented by primitive nautiloids, the class now contains two, only distantly related, extant subclasses, which includes octopuses and cuttlefish, and Nautiloidea, represented by Nautilus and Allonautilus. In the Coleoidea, the shell has been internalized or is absent, whereas in the Nautiloidea. About 800 living species of cephalopods have been identified, two important extinct taxa are the Ammonoidea and Belemnoidea. There are over 800 extant species of cephalopod, although new species continue to be described, an estimated 11,000 extinct taxa have been described, although the soft-bodied nature of cephalopods means they are not easily fossilised.
Cephalopods are found in all the oceans of Earth, none of them can tolerate freshwater, but the brief squid, Lolliguncula brevis, found in Chesapeake Bay, is a notable partial exception in that it tolerates brackish water. Cephalopods are thought to be unable to live in due to multiple biochemical constraints. Cephalopods occupy most of the depth of the ocean, from the plain to the sea surface. Their diversity is greatest near the equator and decreases towards the poles, Cephalopods are widely regarded as the most intelligent of the invertebrates, and have well developed senses and large brains. The nervous system of cephalopods is the most complex of the invertebrates, the brain is protected in a cartilaginous cranium. Cephalopods have known to climb out of their aquaria, maneuver a distance of the lab floor, enter another aquarium to feed on the crabs. Cephalopods are social creatures, when isolated from their own kind, some cephalopods are able to fly through the air for distances of up to 50 m.
While cephalopods are not particularly aerodynamic, they achieve these impressive ranges by jet-propulsion, the animals spread their fins and tentacles to form wings and actively control lift force with body posture. Cephalopods have advanced vision, can detect gravity with statocysts, and have a variety of sense organs. Octopuses use their arms to explore their environment and can use them for depth perception, most cephalopods rely on vision to detect predators and prey, and to communicate with one another. Consequently, cephalopod vision is acute, training experiments have shown that the octopus can distinguish the brightness, shape
Turtles are reptiles of the order Testudines characterised by a special bony or cartilaginous shell developed from their ribs and acting as a shield. Turtle may refer to the order as a whole or to fresh-water, the order Testudines includes both extant and extinct species. The earliest known members of this date from 157 million years ago, making turtles one of the oldest reptile groups. Of the 327 known species alive today, some are highly endangered, turtles are ectotherms—animals commonly called cold-blooded—meaning that their internal temperature varies according to the ambient environment. However, because of their metabolic rate, leatherback sea turtles have a body temperature that is noticeably higher than that of the surrounding water. Turtles are classified as amniotes, along with reptiles, birds. Like other amniotes, turtles breathe air and do not lay eggs underwater, Chelonia is based on the Greek word χελώνη chelone tortoise, denoting armor or interlocking shields, testudines, on the other hand, is based on the Latin word testudo tortoise.
Turtle may either refer to the order as a whole, or to particular turtles that make up a form taxon that is not monophyletic, the meaning of the word turtle differs from region to region. In North America, all chelonians are commonly called turtles, including terrapins, in Great Britain, the word turtle is used for sea-dwelling species, but not for tortoises. The term tortoise usually refers to any land-dwelling, non-swimming chelonian, most land-dwelling chelonians are in the Testudinidae family, only one of the 14 extant turtle families. Terrapin is used to describe several species of small, hard-shell turtles, typically found in brackish waters. Some languages do not have this distinction, as all of these are referred to by the same name, for example, in Spanish, the word tortuga is used for turtles and terrapins. A sea-dwelling turtle is tortuga marina, a freshwater species tortuga de río, the largest living chelonian is the leatherback sea turtle, which reaches a shell length of 200 cm and can reach a weight of over 900 kg.
Freshwater turtles are generally smaller, but with the largest species, the Asian softshell turtle Pelochelys cantorii, a few individuals have been reported up to 200 cm. This dwarfs even the better-known alligator snapping turtle, the largest chelonian in North America and they became extinct at the same time as the appearance of man, and it is assumed humans hunted them for food. The only surviving giant tortoises are on the Seychelles and Galápagos Islands and can grow to over 130 cm in length, the largest ever chelonian was Archelon ischyros, a Late Cretaceous sea turtle known to have been up to 4.6 m long. The smallest turtle is the speckled padloper tortoise of South Africa and it measures no more than 8 cm in length and weighs about 140 g. Two other species of turtles are the American mud turtles
Bryozoa, are a phylum of aquatic invertebrate animals. Typically about 0.5 millimetres long, they are filter feeders that sieve food particles out of the using a retractable lophophore. Most marine species live in tropical waters, but a few occur in oceanic trenches, one class lives only in a variety of freshwater environments, and a few members of a mostly marine class prefer brackish water. Over 4,000 living species are known, one genus is solitary and the rest are colonial. The phylum was originally called Polyzoa, but this term was superseded by Bryozoa in 1831, another group of animals discovered subsequently, whose filtering mechanism looked similar, was included in Bryozoa until 1869, when the two groups were noted to be very different internally. The more recently discovered group was given the name Entoprocta, while the original Bryozoa were called Ectoprocta, Bryozoa has remained the more widely used term for the latter group. Individuals in bryozoan colonies are called zooids, since they are not fully independent animals, all colonies contain autozooids, which are responsible for feeding and excretion.
Colonies of some classes have various types of non-feeding specialist zooids, some of which are hatcheries for fertilized eggs, the class Cheilostomata have the largest number of species, possibly because they have the widest range of specialist zooids. A few species can creep very slowly by using spiny defensive zooids as legs, autozooids supply nutrients to non-feeding zooids by channels that vary between classes. Zooids have no special excretory organs, and the polypides of autozooids are scrapped when the polypides become overloaded by waste products, in autozooids the gut is U-shaped, with the mouth inside the crown of tentacles and the anus outside it. Colonies take a variety of forms, including fans, the Cheilostomata produce mineralized exoskeletons and form single-layered sheets which encrust over surfaces. Zooids of all the species are simultaneous hermaphrodites. Although those of marine species function first as males and as females. All species emit sperm into the water, some release ova into the water, while others capture sperm via their tentacles to fertilize their ova internally.
In some species the larvae have large yolks, go to feed, others produce larvae that have little yolk but swim and feed for a few days before settling. After settling, all larvae undergo a metamorphosis that destroys. Freshwater species produce statoblasts that lie dormant until conditions are favorable, predators of marine bryozoans include nudibranchs, sea urchins, crustaceans and starfish. Freshwater bryozoans are preyed on by snails and fish, in Thailand, many populations of one freshwater species have been wiped out by an introduced species of snail
Ammonoids are an extinct group of marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs are more related to living coleoids than they are to shelled nautiloids such as the living Nautilus species. The earliest ammonites appear during the Devonian, and the last species died out during the Cretaceous–Paleogene extinction event, Ammonites are excellent index fossils, and it is often possible to link the rock layer in which a particular species or genus is found to specific geologic time periods. Their fossil shells usually take the form of planispirals, although there were some helically spiraled and nonspiraled forms, the name ammonite, from which the scientific term is derived, was inspired by the spiral shape of their fossilized shells, which somewhat resemble tightly coiled rams horns. Pliny the Elder called fossils of these animals ammonis cornua because the Egyptian god Ammon was typically depicted wearing rams horns, often the name of an ammonite genus ends in -ceras, which is Greek for horn.
The topology of the septa, especially around the rim, results in the various suture patterns found, three major types of suture patterns are found in the Ammonoidea, Goniatitic - numerous undivided lobes and saddles, typically 8 lobes around the conch. This pattern is characteristic of the Paleozoic ammonoids, ceratitic - lobes have subdivided tips, giving them a saw-toothed appearance, and rounded undivided saddles. This suture pattern is characteristic of Triassic ammonoids and appears again in the Cretaceous pseudoceratites, ammonitic - lobes and saddles are much subdivided, subdivisions are usually rounded instead of saw-toothed. Ammonoids of this type are the most important species from a point of view. This suture type is characteristic of Jurassic and Cretaceous ammonoids, the siphuncle in most ammonoids by far is a narrow tubular structure that runs along the outer rim, known as the venter, connecting the chambers of the phragmocone to the body or living chamber. This distinguishes them from living nautiloides and typical Nautilida, the word siphuncle comes from the New Latin siphunculus, meaning little siphon.
Originating from within the bactritoid nautiloids, the ammonoid cephalopods first appeared in the Devonian, while nearly all nautiloids show gently curving sutures, the ammonoid suture line is variably folded, forming saddles and lobes. In subsequent taxonomies, these are regarded as orders within the subclass Ammonoidea. Because ammonites and their relatives are extinct, little is known about their way of life. Their soft body parts are rarely preserved in any detail. Nonetheless, much has been worked out by examining ammonoid shells, synchrotron analysis of an aptychophoran ammonite revealed remains of isopod and mollusc larvae in its buccal cavity, indicating at least this kind of ammonite fed on plankton. They may have avoided predation by squirting ink, much like modern cephalopods, the soft body of the creature occupied the largest segments of the shell at the end of the coil. The smaller earlier segments were walled off and the animal could maintain its buoyancy by filling them with gas, the smaller sections of the coil would have floated above the larger sections
Ascidiacea is a paraphyletic class in the subphylum Tunicata of sac-like marine invertebrate filter feeders. Ascidians are characterized by an outer tunic made of the polysaccharide cellulose. Ascidians are found all over the world, usually in water with salinities over 2. 5%. While members of the Thaliacea and Larvacea swim freely like plankton, sea squirts are sessile animals, they remain attached to their substratum, such as rocks. Sea squirts feed by taking in water through a tube, the oral siphon, the water enters the mouth and pharynx, flows through mucus-covered gill slits into a water chamber called the atrium, exits through the atrial siphon. Sea squirts are rounded or cylindrical animals ranging from about 0.5 to 10 cm in size, one end of the body is always firmly fixed to rock, coral, or some similar solid surface. The lower surface is pitted or ridged, and in some species has root-like extensions that help the animal grip onto the surface, the body wall is covered by a smooth thick tunic, which is often quite rigid.
The tunic consists of cellulose along with proteins and calcium salts, unlike the shells of molluscs, the tunic is composed of living tissue, and often has its own blood supply. In some colonial species, the tunics of adjacent individuals are fused into a single structure, the upper surface of the animal, opposite to the part gripping the substratum, has two openings, or siphons. When removed from the water, the animal often violently expels water from these siphons, the body itself can be divided into up to three regions, although these are not clearly distinct in most species. The pharyngeal region contains the pharynx, while the abdomen contains most of the bodily organs. In many sea squirts, the postabdomen, or even the entire abdomen, are absent, as its name implies, the pharyngeal region is occupied mainly by the pharynx. The large buccal siphon opens into the pharynx, acting like a mouth, the pharynx itself is ciliated and contains numerous perforations, or stigmata, arranged in a grid-like pattern around its circumference.
The beating of the cilia sucks water through the siphon, a long ciliated groove, or endostyle, runs along one side of the pharynx, and a projecting ridge along the other. The endostyle may be homologous with the gland of vertebrates. The pharynx is surrounded by an atrium, through water is expelled through a second, usually smaller. Cords of connective tissue cross the atrium to maintain the shape of the body. The outer body wall consists of connective tissue, muscle fibres, the pharynx forms the first part of the digestive system
A frog is any member of a diverse and largely carnivorous group of short-bodied, tailless amphibians composing the order Anura. The oldest fossil proto-frog appeared in the early Triassic of Madagascar, Frogs are widely distributed, ranging from the tropics to subarctic regions, but the greatest concentration of species diversity is in tropical rainforests. There are approximately 4,800 recorded species, accounting for over 85% of extant amphibian species and they are one of the five most diverse vertebrate orders. The body plan of a frog is generally characterized by a stout body, protruding eyes, cleft tongue, limbs folded underneath. Besides living in water and on dry land, the adults of some species are adapted for living underground or in trees. The skin of the frog is glandular, with secretions ranging from distasteful to toxic, Frogs skins vary in colour from well-camouflaged dappled brown and green to vivid patterns of bright red or yellow and black to advertise toxicity and warn off predators.
Frogs typically lay their eggs in water, the eggs hatch into aquatic larvae called tadpoles that have tails and internal gills. They have highly specialized rasping mouth parts suitable for herbivorous, omnivorous or planktivorous diets, the life cycle is completed when they metamorphose into adults. A few species deposit eggs on land or bypass the tadpole stage, adult frogs generally have a carnivorous diet consisting of small invertebrates, but omnivorous species exist and a few feed on fruit. Frogs are extremely efficient at converting what they eat into body mass and they are an important food source for predators and part of the food web dynamics of many of the worlds ecosystems. The skin is semi-permeable, making them susceptible to dehydration, so they live in moist places or have special adaptations to deal with dry habitats. Frogs are valued as food by humans and have many roles in literature, symbolism. Frog populations have declined significantly since the 1950s, more than one third of species are considered to be threatened with extinction and over one hundred and twenty are believed to have become extinct since the 1980s.
The number of malformations among frogs is on the rise and a fungal disease. Conservation biologists are working to understand the causes of these problems, the name frog derives from Old English frogga, abbreviated to frox and frosc, probably deriving from Proto-Indo-European preu = to jump. About 88% of amphibian species are classified in the order Anura and these include around 4,810 species in 33 families, of which the Leptodactylidae and Ranidae are the richest in species. The use of the common frog and toad has no taxonomic justification. From a classification perspective, all members of the order Anura are frogs, there are numerous exceptions to this rule
Diatoms are a major group of algae, and are among the most common types of phytoplankton. Diatoms are unicellular, although they can form colonies in the shape of filaments or ribbons, zigzags, the first diatom formally described in scientific literature, the colonial Bacillaria paradoxa, was found in 1783 by Danish naturalist Otto Friedrich Müller. Diatoms are producers within the food chain, a unique feature of diatom cells is that they are enclosed within a cell wall made of silica called a frustule. These frustules show a diversity in form, but are usually almost bilaterally symmetrical. The symmetry is not perfect since one of the valves is slightly larger than the other, fossil evidence suggests that they originated during, or before, the early Jurassic period. Only male gametes of centric diatoms are capable of movement by means of flagella, diatom communities are a popular tool for monitoring environmental conditions and present, and are commonly used in studies of water quality. More than 200 genera of living diatoms are known, with an estimated 100,000 extant species, diatoms are a widespread group and can be found in the oceans, in fresh water, in soils, and on damp surfaces.
They dominate over phytoplankton in nutrient-rich coastal waters and during oceanic spring blooms since they can more rapidly than phytoplankton. Most live pelagically in open water, although some live as surface films at the water-sediment interface and they are especially important in oceans, where they contribute an estimated 45% of the total oceanic primary production of organic material. Spatial distribution of phytoplankton species is restricted both horizontally and vertically. Though most are microscopic, some species of diatoms can reach up to 2 mm in length, diatoms belong to a large group called the heterokonts, including both autotrophs and heterotrophs. Their yellowish-brown chloroplasts are typical of heterokonts, having four membranes, individuals usually lack flagella, but they are present in male gametes of the centric diatoms and have the usual heterokont structure, except they lack the hairs characteristic in other groups. Most diatoms are nonmotile, as their relatively dense cell walls cause them to readily sink, planktonic forms in open water usually rely on turbulent mixing of the upper layers by the wind to keep them suspended in sunlit surface waters.
Some species actively regulate their buoyancy with intracellular lipids to counter sinking, a feature of diatoms is the urea cycle, which links them evolutionarily to animals. This was discovered in research carried out by Andrew Allen, Chris Bowler and their study showed that while diatoms and animals use the urea cycle for different ends, they are seen to be evolutionally linked in such a way that animals and plants are not. Diatom cells are contained within a unique cell wall known as a frustule made up of two valves called thecae, that typically overlap one another. The biogenic silica composing the wall is synthesised intracellularly by the polymerisation of silicic acid monomers. This material is extruded to the cell exterior and added to the wall
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic environments, they are predominantly marine species. Their distinguishing feature is cnidocytes, specialized cells that use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance, sandwiched between two layers of epithelium that are one cell thick. They have two basic forms, swimming medusae and sessile polyps, both of which are radially symmetrical with mouths surrounded by tentacles that bear cnidocytes. Both forms have a single orifice and body cavity that are used for digestion and respiration, many cnidarian species produce colonies that are single organisms composed of medusa-like or polyp-like zooids, or both. Cnidarians activities are coordinated by a nerve net and simple receptors. Several free-swimming species of Cubozoa and Scyphozoa possess balance-sensing statocysts, not all cnidarians reproduce sexually, with many species having complex life cycles of asexual polyp stages and sexual medusae.
Some, omit either the polyp or the medusa stage, Cnidarians were formerly grouped with ctenophores in the phylum Coelenterata, but increasing awareness of their differences caused them to be placed in separate phyla. Staurozoa have recently been recognised as a class in their own rather than a sub-group of Scyphozoa. Most cnidarians prey on organisms ranging in size from plankton to animals several times larger than themselves, but many obtain much of their nutrition from dinoflagellates, many are preyed on by other animals including starfish, sea slugs and turtles. Many scleractinian corals—which form the foundation for coral reefs—possess polyps that are filled with zooxanthellae. While reef-forming corals are almost entirely restricted to warm and shallow waters, other cnidarians can be found at great depths, in polar regions. Recent phylogenetic analyses support monophyly of cnidarians, as well as the position of cnidarians as the group of bilaterians. Cnidarians form a phylum that are more complex than sponges, about as complex as ctenophores, and less complex than bilaterians.
Cnidarians are distinguished from all other animals by having cnidocytes that fire like harpoons and are used mainly to capture prey, in some species, cnidocytes can be used as anchors. Hence and ctenophores have traditionally been labelled diploblastic, along with sponges, both cnidarians and ctenophores have a type of muscle that, in more complex animals, arises from the middle cell layer. As a result, some recent text books classify ctenophores as triploblastic, adult cnidarians appear as either swimming medusae or sessile polyps, and many hydrozoan species are known to alternate between the two forms. Both are radially symmetrical, like a wheel and a tube respectively, since these animals have no heads, their ends are described as oral and aboral