123-582: The Actinocerida are an order of generally straight, medium to large cephalopods that lived during the early and middle Paleozoic , distinguished by a siphuncle composed of expanded segments that extend into the adjacent chambers, in which deposits formed within contain a system of radial canals and a narrow space along the inner side of the connecting ring known as a paraspatium. (Teichert 1964) Septal necks are generally short and cyrtochoanitic, some being recumbent, some hook shaped. Most grew to lengths of about 60 to 90 cm (2.0 to 3.0 ft) but some, like
246-405: A mantle with a significant cavity used for breathing and excretion , the presence of a radula (except for bivalves ), and the structure of the nervous system . Other than these common elements, molluscs express great morphological diversity, so many textbooks base their descriptions on a "hypothetical ancestral mollusc" (see image below). This has a single, " limpet -like" shell on top, which
369-427: A "shell vestige" or "gladius". The Incirrina have either a pair of rod-shaped stylets or no vestige of an internal shell, and some squid also lack a gladius. The shelled coleoids do not form a clade or even a paraphyletic group. The Spirula shell begins as an organic structure, and is then very rapidly mineralized. Shells that are "lost" may be lost by resorption of the calcium carbonate component. Females of
492-435: A 'rake' to comb up filaments from the sea floor. Others feed on macroscopic 'plants' such as kelp, rasping the plant surface with its radula. To employ this strategy, the plant has to be large enough for the mollusc to 'sit' on, so smaller macroscopic plants are not as often eaten as their larger counterparts. Filter feeders are molluscs that feed by straining suspended matter and food particles from water, typically by passing
615-399: A cloud of dark ink to confuse predators . This sac is a muscular bag which originated as an extension of the hindgut. It lies beneath the gut and opens into the anus, into which its contents – almost pure melanin – can be squirted; its proximity to the base of the funnel means the ink can be distributed by ejected water as the cephalopod uses its jet propulsion. The ejected cloud of melanin
738-451: A diversity of backgrounds. Experiments done in Dwarf chameleons testing these hypotheses showed that chameleon taxa with greater capacity for color change had more visually conspicuous social signals but did not come from more visually diverse habitats, suggesting that color change ability likely evolved to facilitate social signaling, while camouflage is a useful byproduct. Because camouflage
861-529: A flat fan shape with a mucus film between the individual tentacles, while another, Sepioteuthis sepioidea , has been observed putting the tentacles in a circular arrangement. Cephalopods have advanced vision, can detect gravity with statocysts , and have a variety of chemical 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
984-435: A group comprising cephalopods. Molluscus is used in classical Latin as an adjective only with nux ( nut ) to describe a particular type of soft nut. The use of mollusca in biological taxonomy by Jonston and later Linnaeus may have been influenced by Aristotle 's τὰ μαλάκια ta malákia (the soft ones; < μαλακός malakós "soft"), which he applied inter alia to cuttlefish . The scientific study of molluscs
1107-652: A gunshot-like popping noise, thought to function to frighten away potential predators. Cephalopods employ a similar method of propulsion despite their increasing size (as they grow) changing the dynamics of the water in which they find themselves. Thus their paralarvae do not extensively use their fins (which are less efficient at low Reynolds numbers ) and primarily use their jets to propel themselves upwards, whereas large adult cephalopods tend to swim less efficiently and with more reliance on their fins. Early cephalopods are thought to have produced jets by drawing their body into their shells, as Nautilus does today. Nautilus
1230-418: A hard shell use calcite (sometimes with traces of aragonite) to construct the eggshells. The shell consists of three layers: the outer layer (the periostracum ) made of organic matter, a middle layer made of columnar calcite , and an inner layer consisting of laminated calcite, often nacreous . In some forms the shell contains openings. In abalones there are holes in the shell used for respiration and
1353-453: A jet as a propulsion mechanism. Squids do not have the longitudinal muscles that octopus do. Instead, they have a tunic. This tunic is made of layers of collagen and it surrounds the top and the bottom of the mantle. Because they are made of collagen and not muscle, the tunics are rigid bodies that are much stronger than the muscle counterparts. This provides the squids some advantages for jet propulsion swimming. The stiffness means that there
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#17328007118421476-406: A length of 8 metres. They may terminate in a broadened, sucker-coated club. The shorter four pairs are termed arms , and are involved in holding and manipulating the captured organism. They too have suckers, on the side closest to the mouth; these help to hold onto the prey. Octopods only have four pairs of sucker-coated arms, as the name suggests, though developmental abnormalities can modify
1599-403: A maximum of 120,000 species. The total number of described species is difficult to estimate because of unresolved synonymy . In 1969, David Nicol estimated the probable total number of living mollusc species at 107,000 of which were about 12,000 fresh-water gastropods and 35,000 terrestrial . The Bivalvia would comprise about 14% of the total and the other five classes less than 2% of
1722-474: A muscle, which is why they can change their skin hue as rapidly as they do. Coloration is typically stronger in near-shore species than those living in the open ocean, whose functions tend to be restricted to disruptive camouflage . These chromatophores are found throughout the body of the octopus, however, they are controlled by the same part of the brain that controls elongation during jet propulsion to reduce drag. As such, jetting octopuses can turn pale because
1845-512: A novel mechanism for spectral discrimination in cephalopods was described. This relies on the exploitation of chromatic aberration (wavelength-dependence of focal length). Numerical modeling shows that chromatic aberration can yield useful chromatic information through the dependence of image acuity on accommodation. The unusual off-axis slit and annular pupil shapes in cephalopods enhance this ability by acting as prisms which are scattering white light in all directions. In 2015, molecular evidence
1968-399: A number of paired ganglia, the visceral cords serving the internal organs and the pedal ones serving the foot. Most pairs of corresponding ganglia on both sides of the body are linked by commissures (relatively large bundles of nerves). The ganglia above the gut are the cerebral, the pleural, and the visceral, which are located above the esophagus (gullet). The pedal ganglia, which control
2091-407: A pair of statocysts , which act as balance sensors. In gastropods, it secretes mucus as a lubricant to aid movement. In forms having only a top shell, such as limpets , the foot acts as a sucker attaching the animal to a hard surface, and the vertical muscles clamp the shell down over it; in other molluscs, the vertical muscles pull the foot and other exposed soft parts into the shell. In bivalves,
2214-471: A rare form of physiological color change which utilizes neural control of muscles to change the morphology of their chromatophores. This neural control of chromatophores has evolved convergently in both cephalopods and teleosts fishes. With the exception of the Nautilidae and the species of octopus belonging to the suborder Cirrina , all known cephalopods have an ink sac, which can be used to expel
2337-678: A rasping "tongue", the radula, and a complex digestive system in which exuded mucus and microscopic, muscle-powered "hairs" called cilia play various important roles. The generalized mollusc has two paired nerve cords , or three in bivalves . The brain , in species that have one, encircles the esophagus . Most molluscs have eyes , and all have sensors to detect chemicals, vibrations, and touch . The simplest type of molluscan reproductive system relies on external fertilization , but more complex variations occur. Nearly all produce eggs , from which may emerge trochophore larvae , more complex veliger larvae, or miniature adults. The coelomic cavity
2460-424: A risk of food poisoning, and many jurisdictions have regulations to reduce this risk. Molluscs have, for centuries, also been the source of important luxury goods, notably pearls , mother of pearl , Tyrian purple dye, and sea silk . Their shells have also been used as money in some preindustrial societies. A handful of mollusc species are sometimes considered hazards or pests for human activities. The bite of
2583-698: A second wutinocerid genus, Adamsoceras . Gonioceras is an offshoot of an early Armenoceras; Lambeoceras and Huronia are offshoots of a later Actinoceras . The Actinoceratidae extend into the Lower Silurian with Actinoceras ; the Armenoceratidae and Huroniidae extend into the Upper Silurian. The Ormoceratidae are possibly the most recent, extending into the Lower Devonian Gonioceras (Gonioceratidae)
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#17328007118422706-496: A shell (secondarily absent in a number of taxonomic groups, such as the nudibranchs ) that consists of mainly chitin and conchiolin (a protein hardened with calcium carbonate ), except the outermost layer, which in almost all cases is all conchiolin (see periostracum ). Molluscs never use phosphate to construct their hard parts, with the questionable exception of Cobcrephora . While most mollusc shells are composed mainly of aragonite , those gastropods that lay eggs with
2829-429: A shell-less subclass of cephalopods (squid, cuttlefish, and octopuses), have complex pigment containing cells called chromatophores which are capable of producing rapidly changing color patterns. These cells store pigment within an elastic sac which produces the color seen from these cells. Coleoids can change the shape of this sac, called the cytoelastic sacculus, which then causes changes in the translucency and opacity of
2952-456: A singular gill. Generally, the gills are rather like feathers in shape, although some species have gills with filaments on only one side. They divide the mantle cavity so water enters near the bottom and exits near the top. Their filaments have three kinds of cilia, one of which drives the water current through the mantle cavity, while the other two help to keep the gills clean. If the osphradia detect noxious chemicals or possibly sediment entering
3075-620: A startling array of fashions. As well as providing camouflage with their background, some cephalopods bioluminesce, shining light downwards to disguise their shadows from any predators that may lurk below. The bioluminescence is produced by bacterial symbionts; the host cephalopod is able to detect the light produced by these organisms. Bioluminescence may also be used to entice prey, and some species use colorful displays to impress mates, startle predators, or even communicate with one another. Cephalopods can change their colors and patterns in milliseconds, whether for signalling (both within
3198-415: Is a hemocoel through which blood and coelomic fluid circulate and which encloses most of the other internal organs. These hemocoelic spaces act as an efficient hydrostatic skeleton . The blood of these molluscs contains the respiratory pigment hemocyanin as an oxygen -carrier. The heart consists of one or more pairs of atria ( auricles ), which receive oxygenated blood from the gills and pump it to
3321-539: Is a branch of malacology known as teuthology . Cephalopods became dominant during the Ordovician period, represented by primitive nautiloids . The class now contains two, only distantly related, extant subclasses: Coleoidea , which includes octopuses , squid , and cuttlefish ; and Nautiloidea , represented by Nautilus and Allonautilus . In the Coleoidea, the molluscan shell has been internalized or
3444-698: Is absent, whereas in the Nautiloidea, the external shell remains. About 800 living species of cephalopods have been identified. Two important extinct taxa are the Ammonoidea (ammonites) and Belemnoidea (belemnites). Extant cephalopods range in size from the 10 mm (0.3 in) Idiosepius thailandicus to the 700 kilograms (1,500 lb) heavy Colossal squid , the largest extant invertebrate . 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
3567-437: Is accordingly called malacology . The name Molluscoida was formerly used to denote a division of the animal kingdom containing the brachiopods , bryozoans , and tunicates , the members of the three groups having been supposed to somewhat resemble the molluscs. As now known, these groups have no relation to molluscs, and very little to one another, so the name Molluscoida has been abandoned. The most universal features of
3690-516: Is acute: training experiments have shown that the common octopus can distinguish the brightness, size, shape, and horizontal or vertical orientation of objects. The morphological construction gives cephalopod eyes the same performance as shark eyes; however, their construction differs, as cephalopods lack a cornea and have an everted retina. Cephalopods' eyes are also sensitive to the plane of polarization of light. Unlike many other cephalopods, nautiluses do not have good vision; their eye structure
3813-463: Is also capable of creating a jet by undulations of its funnel; this slower flow of water is more suited to the extraction of oxygen from the water. When motionless, Nautilus can only extract 20% of oxygen from the water. The jet velocity in Nautilus is much slower than in coleoids , but less musculature and energy is involved in its production. Jet thrust in cephalopods is controlled primarily by
Actinocerida - Misplaced Pages Continue
3936-529: Is an endemic snail species of the Eastern Alps . There is strong evidence for self-fertilization in the easternmost snail populations of this species. The most basic molluscan larva is a trochophore , which is planktonic and feeds on floating food particles by using the two bands of cilia around its "equator" to sweep food into the mouth, which uses more cilia to drive them into the stomach, which uses further cilia to expel undigested remains through
4059-718: Is antedated by Wutinoceras , its assumed primitive nature rather a derived condition. Actinocerids first appeared early in the Middle Ordovician, with the exception of the Georginidae, which are known from the Cassinian in Northern Australia. They reached their greatest diversity in the Middle Ordovician with more than 20 genera, then declined somewhat in the Late Ordovician and more so in
4182-546: Is any member of the molluscan class Cephalopoda / s ɛ f ə ˈ l ɒ p ə d ə / ( Greek plural κεφαλόποδες , kephalópodes ; "head-feet") such as a squid , octopus , cuttlefish , or nautilus . These exclusively marine animals are characterized by bilateral body symmetry , a prominent head, and a set of arms or tentacles ( muscular hydrostats ) modified from the primitive molluscan foot. Fishers sometimes call cephalopods " inkfish ", referring to their common ability to squirt ink . The study of cephalopods
4305-570: Is aragonite. As for other mollusc shells or coral skeletons, the smallest visible units are irregular rounded granules. Cephalopods, as the name implies, have muscular appendages extending from their heads and surrounding their mouths. These are used in feeding, mobility, and even reproduction. In coleoids they number eight or ten. Decapods such as cuttlefish and squid have five pairs. The longer two, termed tentacles , are actively involved in capturing prey; they can lengthen rapidly (in as little as 15 milliseconds ). In giant squid they may reach
4428-402: Is associated with the odontophore , a cartilaginous supporting organ. The radula is unique to the molluscs and has no equivalent in any other animal. Molluscs' mouths also contain glands that secrete slimy mucus , to which the food sticks. Beating cilia (tiny "hairs") drive the mucus towards the stomach, so the mucus forms a long string called a "food string". At the tapered rear end of
4551-427: Is highly developed, but lacks a solid lens . They have a simple " pinhole " eye through which water can pass. Instead of vision, the animal is thought to use olfaction as the primary sense for foraging , as well as locating or identifying potential mates. All octopuses and most cephalopods are considered to be color blind . Coleoid cephalopods (octopus, squid, cuttlefish) have a single photoreceptor type and lack
4674-594: Is limited to the Middle Ordovician, its quasi-lookalike Lambeoceras (Lamberoceratidae) to the uppermost Middle and Upper Ordovician. The wutinocerids are known only from the early Middle Ordovician (Whiterockian) and the polydesmiadids are restricted to about that time. Originating in the Ordovician, by the Devonian period actinocerids became rare; perhaps they were unable to compete with the more compact and maneuverable coiled nautiloids and ammonoids and cope with
4797-449: Is made of proteins and chitin reinforced with calcium carbonate , and is secreted by a mantle covering the whole upper surface. The underside of the animal consists of a single muscular "foot". Although molluscs are coelomates , the coelom tends to be small. The main body cavity is a hemocoel through which blood circulates; as such, their circulatory systems are mainly open . The "generalized" mollusc's feeding system consists of
4920-418: Is more efficient, but in environments with little oxygen and in low temperatures, hemocyanin has the upper hand. The hemocyanin molecule is much larger than the hemoglobin molecule, allowing it to bond with 96 O 2 or CO 2 molecules, instead of the hemoglobin's just four. But unlike hemoglobin, which are attached in millions on the surface of a single red blood cell, hemocyanin molecules float freely in
5043-403: Is needed, compensating for their small size. However, organisms which spend most of their time moving slowly along the bottom do not naturally pass much water through their cavity for locomotion; thus they have larger gills, along with complex systems to ensure that water is constantly washing through their gills, even when the organism is stationary. The water flow is controlled by contractions of
Actinocerida - Misplaced Pages Continue
5166-439: Is no necessary muscle flexing to keep the mantle the same size. In addition, tunics take up only 1% of the squid mantle's wall thickness, whereas the longitudinal muscle fibers take up to 20% of the mantle wall thickness in octopuses. Also because of the rigidity of the tunic, the radial muscles in squid can contract more forcefully. The mantle is not the only place where squids have collagen. Collagen fibers are located throughout
5289-433: Is one of the largest invertebrates , surpassed in weight but not in length by the colossal squid . Freshwater and terrestrial molluscs appear exceptionally vulnerable to extinction. Estimates of the numbers of non-marine molluscs vary widely, partly because many regions have not been thoroughly surveyed. There is also a shortage of specialists who can identify all the animals in any one area to species. However, in 2004
5412-667: Is reduced. They have an open circulatory system and kidney-like organs for excretion. Good evidence exists for the appearance of gastropods, cephalopods , and bivalves in the Cambrian period, 541–485.4 million years ago. However, the evolutionary history both of molluscs' emergence from the ancestral Lophotrochozoa and of their diversification into the well-known living and fossil forms are still subjects of vigorous debate among scientists. Molluscs have been and still are an important food source for humans. Toxins that can accumulate in certain molluscs under specific conditions create
5535-457: Is referred to as a pseudomorph ). This strategy often results in the predator attacking the pseudomorph, rather than its rapidly departing prey. For more information, see Inking behaviors . The ink sac of cephalopods has led to a common name of "inkfish", formerly the pen-and-ink fish. Cephalopods are the only molluscs with a closed circulatory system. Coleoids have two gill hearts (also known as branchial hearts ) that move blood through
5658-420: Is secreted by a mantle covering the upper surface. The underside consists of a single muscular "foot". The visceral mass, or visceropallium, is the soft, nonmuscular metabolic region of the mollusc. It contains the body organs. The mantle cavity, a fold in the mantle, encloses a significant amount of space. It is lined with epidermis, and is exposed, according to habitat , to sea, fresh water or air. The cavity
5781-413: Is supplemented with fin motion; in the squid, the fins flap each time that a jet is released, amplifying the thrust; they are then extended between jets (presumably to avoid sinking). Oxygenated water is taken into the mantle cavity to the gills and through muscular contraction of this cavity, the spent water is expelled through the hyponome , created by a fold in the mantle. The size difference between
5904-553: Is the first evidence that cephalopod dermal tissues may possess the required combination of molecules to respond to light. Some squids have been shown to detect sound using their statocysts , but, in general, cephalopods are deaf. Most cephalopods possess an assemblage of skin components that interact with light. These may include iridophores, leucophores , chromatophores and (in some species) photophores . Chromatophores are colored pigment cells that expand and contract in accordance to produce color and pattern which they can use in
6027-423: Is the most complex of the invertebrates and their brain-to-body-mass ratio falls between that of endothermic and ectothermic vertebrates. Captive cephalopods have also been known to climb out of their aquaria, maneuver a distance of the lab floor, enter another aquarium to feed on captive crabs, and return to their own aquarium. The brain is protected in a cartilaginous cranium. The giant nerve fibers of
6150-554: Is transmitted to humans by water snail hosts, and affects about 200 million people. Snails and slugs can also be serious agricultural pests, and accidental or deliberate introduction of some snail species into new environments has seriously damaged some ecosystems . The words mollusc and mollusk are both derived from the French mollusque , which originated from the post-classical Latin mollusca , from mollis , soft, first used by J. Jonston (Historiæ Naturalis, 1650) to describe
6273-538: Is unclear. The Upper Cambrian Protactinocerida have been suggested as being ancestral but none are known to have gone beyond the near end of the Cambrian extinction, which makes any connection hypothetical. Polydesmia was once thought to be the ancestral form of the actinocerids, and was derived from the ellesmeroceriid Bathmoceras . However, it turned out based on a reassessment of Lower Ordovician and Whiterockian formations in northeastern China that Polydesmia
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#17328007118426396-426: Is unknown, but chromatophores are under the control of neural pathways, allowing the cephalopod to coordinate elaborate displays. Together, chromatophores and iridophores are able to produce a large range of colors and pattern displays. Cephalopods utilize chromatophores' color changing ability in order to camouflage themselves. Chromatophores allow Coleoids to blend into many different environments, from coral reefs to
6519-400: Is used for multiple adaptive purposes in cephalopods, color change could have evolved for one use and the other developed later, or it evolved to regulate trade offs within both. Color change is widespread in ectotherms including anoles, frogs, mollusks, many fish, insects, and spiders. The mechanism behind this color change can be either morphological or physiological. Morphological change is
6642-417: Is usually mixed, upon expulsion, with mucus , produced elsewhere in the mantle, and therefore forms a thick cloud, resulting in visual (and possibly chemosensory) impairment of the predator, like a smokescreen . However, a more sophisticated behavior has been observed, in which the cephalopod releases a cloud, with a greater mucus content, that approximately resembles the cephalopod that released it (this decoy
6765-775: The Early Silurian ; made a slight come back in the Middle Silurian but not to Late Ordovician numbers; and declined more or less steadily from the Late Silurian into the Devonian. Three major lineages began the Middle Ordovician, the Actinoceratidae, Armenoceratidae, and Ormoceratidae. The Actinoceratidae and Armenoceratidae are most likely derived from Wutinoceras and the Ormoceratidae from
6888-487: The IUCN Red List of Threatened Species included nearly 2,000 endangered non-marine molluscs. For comparison, the great majority of mollusc species are marine, but only 41 of these appeared on the 2004 Red List. About 42% of recorded extinctions since the year 1500 are of molluscs, consisting almost entirely of non-marine species. Because of the great range of anatomical diversity among molluscs, many textbooks start
7011-405: The blue-ringed octopus is often fatal, and that of Octopus apollyon causes inflammation that can last over a month. Stings from a few species of large tropical cone shells of the family Conidae can also kill, but their sophisticated, though easily produced, venoms have become important tools in neurological research. Schistosomiasis (also known as bilharzia, bilharziosis, or snail fever)
7134-462: The coelom , a small cavity that surrounds the heart, into which they shed ova or sperm . The nephridia extract the gametes from the coelom and emit them into the mantle cavity. Molluscs that use such a system remain of one sex all their lives and rely on external fertilization . Some molluscs use internal fertilization and/or are hermaphrodites , functioning as both sexes; both of these methods require more complex reproductive systems. C. obtusus
7257-583: The sparkling enope squid ( Watasenia scintillans ). It achieves color vision with three photoreceptors , which are based on the same opsin , but use distinct retinal molecules as chromophores: A1 (retinal), A3 (3-dehydroretinal), and A4 (4-hydroxyretinal). The A1-photoreceptor is most sensitive to green-blue (484 nm), the A2-photoreceptor to blue-green (500 nm), and the A4-photoreceptor to blue (470 nm) light. In 2015,
7380-400: The ventricle , which pumps it into the aorta (main artery ), which is fairly short and opens into the hemocoel. The atria of the heart also function as part of the excretory system by filtering waste products out of the blood and dumping it into the coelom as urine . A pair of metanephridia ("little kidneys") to the rear of and connected to the coelom extracts any re-usable materials from
7503-472: The Georginidae but don't become well established until the beginning of the early Whiterockian Stage (Dapingian) of the Middle Ordovician (Flower 1868,1976) The Georginidae, introduced and described by Mary Wade in 1977 (Wade 1988), based on the genus Georgina , are known from the upper Canadian Coolibah Formation of the Georgina Basin in Northern Australia. How the Georginidae relate to older stocks
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#17328007118427626-665: The Huroniidae of the Silurian grew significantly larger. The Actinocerida inhabited shallow to quite deep waters, where they alternated between swimming and lying on the bottom. They were predatory, and able to control their buoyancy to a greater degree than their contemporaries. The derivation of the Actinocerida remains enigmatic. They first appear late in the Early Ordovician (Cassinian Stage, late Floian) with
7749-556: The ability to determine color by comparing detected photon intensity across multiple spectral channels. When camouflaging themselves, they use their chromatophores to change brightness and pattern according to the background they see, but their ability to match the specific color of a background may come from cells such as iridophores and leucophores that reflect light from the environment. They also produce visual pigments throughout their body and may sense light levels directly from their body. Evidence of color vision has been found in
7872-452: The acidity of the organic shell matrix (see Mollusc shell ); shell-forming cephalopods have an acidic matrix, whereas the gladius of squid has a basic matrix. The basic arrangement of the cephalopod outer wall is: an outer (spherulitic) prismatic layer, a laminar (nacreous) layer and an inner prismatic layer. The thickness of every layer depends on the taxa. In modern cephalopods, the Ca carbonate
7995-434: The air for distances of up to 50 metres (160 ft). While cephalopods are not particularly aerodynamic, they achieve these impressive ranges by jet-propulsion; water continues to be expelled from the funnel while the organism is in the air. The animals spread their fins and tentacles to form wings and actively control lift force with body posture. One species, Todarodes pacificus , has been observed spreading tentacles in
8118-412: The anus. New tissue grows in the bands of mesoderm in the interior, so the apical tuft and anus are pushed further apart as the animal grows. The trochophore stage is often succeeded by a veliger stage in which the prototroch , the "equatorial" band of cilia nearest the apical tuft, develops into the velum ("veil"), a pair of cilia-bearing lobes with which the larva swims. Eventually, the larva sinks to
8241-454: The appearance of their surroundings is notable given that cephalopods' vision is monochromatic. Cephalopods also use their fine control of body coloration and patterning to perform complex signaling displays for both conspecific and intraspecific communication. Coloration is used in concert with locomotion and texture to send signals to other organisms. Intraspecifically this can serve as a warning display to potential predators. For example, when
8364-530: The arrival of jawed fish. The Actinocerida contain nine families; the Georginidae, Wutinoceratidae, Polydesmiidae, Armenoceratidae, Ormoceratidae, Actinoceratidae, Gonioceratidae, Lambeoceratide, and Huroniidae. The Carbactinoceratidae, included in the taxonomy in the Treatise, (Vol K) have been removed to the Pseudorthocerida. Cephalopods A cephalopod / ˈ s ɛ f ə l ə p ɒ d /
8487-403: The bloodstream. Cephalopods exchange gases with the seawater by forcing water through their gills, which are attached to the roof of the organism. Water enters the mantle cavity on the outside of the gills, and the entrance of the mantle cavity closes. When the mantle contracts, water is forced through the gills, which lie between the mantle cavity and the funnel. The water's expulsion through
8610-517: The body cavity; others, like some fish, accumulate oils in the liver; and some octopuses have a gelatinous body with lighter chloride ions replacing sulfate in the body chemistry. Squids are the primary sufferers of negative buoyancy in cephalopods. The negative buoyancy means that some squids, especially those whose habitat depths are rather shallow, have to actively regulate their vertical positions. This means that they must expend energy, often through jetting or undulations, in order to maintain
8733-672: The body structure of molluscs are a mantle with a significant body cavity used for breathing and excretion , and the organization of the nervous system. Many have a calcareous shell. Molluscs have developed such a varied range of body structures, finding synapomorphies (defining characteristics) to apply to all modern groups is difficult. The most general characteristic of molluscs is they are unsegmented and bilaterally symmetrical. The following are present in all modern molluscs: Other characteristics that commonly appear in textbooks have significant exceptions: Estimates of accepted described living species of molluscs vary from 50,000 to
8856-582: The brain is unable to achieve both controlling elongation and controlling the chromatophores. Most octopuses mimic select structures in their field of view rather than becoming a composite color of their full background. Evidence of original coloration has been detected in cephalopod fossils dating as far back as the Silurian ; these orthoconic individuals bore concentric stripes, which are thought to have served as camouflage. Devonian cephalopods bear more complex color patterns, of unknown function. Coleoids,
8979-401: The capillaries of the gills . A single systemic heart then pumps the oxygenated blood through the rest of the body. Like most molluscs, cephalopods use hemocyanin , a copper-containing protein, rather than hemoglobin , to transport oxygen. As a result, their blood is colorless when deoxygenated and turns blue when bonded to oxygen. In oxygen-rich environments and in acidic water, hemoglobin
9102-436: The cavity by entering not only through the orifices, but also through the funnel. Squid can expel up to 94% of the fluid within their cavity in a single jet thrust. To accommodate the rapid changes in water intake and expulsion, the orifices are highly flexible and can change their size by a factor of twenty; the funnel radius, conversely, changes only by a factor of around 1.5. Some octopus species are also able to walk along
9225-561: The cell. By rapidly changing multiple chromatophores of different colors, cephalopods are able to change the color of their skin at astonishing speeds, an adaptation that is especially notable in an organism that sees in black and white. Chromatophores are known to only contain three pigments, red, yellow, and brown, which cannot create the full color spectrum. However, cephalopods also have cells called iridophores, thin, layered protein cells that reflect light in ways that can produce colors chromatophores cannot. The mechanism of iridophore control
9348-399: The cephalopod mantle have been widely used for many years as experimental material in neurophysiology ; their large diameter (due to lack of myelination ) makes them relatively easy to study compared with other animals. Many cephalopods are social creatures; when isolated from their own kind, some species have been observed shoaling with fish. Some cephalopods are able to fly through
9471-491: The depth of the ocean, from the abyssal plains to the sea surface, and have also been found in the hadal zone . Their diversity is greatest near the equator (~40 species retrieved in nets at 11°N by a diversity study) and decreases towards the poles (~5 species captured at 60°N). Cephalopods are widely regarded as the most intelligent of the invertebrates and have well developed senses and large brains (larger than those of gastropods ). The nervous system of cephalopods
9594-469: The expansion of the mantle at the end of the jet. In some tests, the collagen has been shown to be able to begin raising mantle pressure up to 50ms before muscle activity is initiated. These anatomical differences between squid and octopuses can help explain why squid can be found swimming comparably to fish while octopuses usually rely on other forms of locomotion on the sea floor such as bipedal walking, crawling, and non-jetting swimming. Nautiluses are
9717-421: The foot is adapted for burrowing into the sediment; in cephalopods it is used for jet propulsion, and the tentacles and arms are derived from the foot. Most molluscs' circulatory systems are mainly open , except for cephalopods , whose circulatory systems are closed . Although molluscs are coelomates , their coeloms are reduced to fairly small spaces enclosing the heart and gonads. The main body cavity
9840-422: The foot, are below the esophagus and their commissure and connectives to the cerebral and pleural ganglia surround the esophagus in a circumesophageal nerve ring or nerve collar . The acephalic molluscs (i.e., bivalves) also have this ring but it is less obvious and less important. The bivalves have only three pairs of ganglia— cerebral, pedal, and visceral— with the visceral as the largest and most important of
9963-413: The form of jetting. The composition of these mantles differs between the two families, however. In octopuses, the mantle is made up of three muscle types: longitudinal, radial, and circular. The longitudinal muscles run parallel to the length of the octopus and they are used in order to keep the mantle the same length throughout the jetting process. Given that they are muscles, it can be noted that this means
10086-456: The funnel can be used to power jet propulsion. If respiration is used concurrently with jet propulsion, large losses in speed or oxygen generation can be expected. The gills, which are much more efficient than those of other mollusks, are attached to the ventral surface of the mantle cavity. There is a trade-off with gill size regarding lifestyle. To achieve fast speeds, gills need to be small – water will be passed through them quickly when energy
10209-475: The living molluscs. In 2009, Chapman estimated the number of described living mollusc species at 85,000. Haszprunar in 2001 estimated about 93,000 named species, which include 23% of all named marine organisms. Molluscs are second only to arthropods in numbers of living animal species — far behind the arthropods' 1,113,000 but well ahead of chordates ' 52,000. About 200,000 living species in total are estimated, and 70,000 fossil species, although
10332-468: The mantle cavity, the gills' cilia may stop beating until the unwelcome intrusions have ceased. Each gill has an incoming blood vessel connected to the hemocoel and an outgoing one to the heart. Molluscs use intracellular digestion . Most molluscs have muscular mouths with radulae , "tongues", bearing many rows of chitinous teeth, which are replaced from the rear as they wear out. The radula primarily functions to scrape bacteria and algae off rocks, and
10455-472: The maximum diameter of the funnel orifice (or, perhaps, the average diameter of the funnel) and the diameter of the mantle cavity. Changes in the size of the orifice are used most at intermediate velocities. The absolute velocity achieved is limited by the cephalopod's requirement to inhale water for expulsion; this intake limits the maximum velocity to eight body-lengths per second, a speed which most cephalopods can attain after two funnel-blows. Water refills
10578-422: The most neurologically advanced of all invertebrates —and either the giant squid or the colossal squid is the largest known extant invertebrate species. The gastropods ( snails , slugs and abalone ) are by far the most diverse class and account for 80% of the total classified molluscan species. The four most universal features defining modern molluscs are a soft body composed almost entirely of muscle ,
10701-404: The non threatening herbivorous parrotfish to approach unaware prey. The octopus Thaumoctopus mimicus is known to mimic a number of different venomous organisms it cohabitates with to deter predators. While background matching, a cephalopod changes its appearance to resemble its surroundings, hiding from its predators or concealing itself from prey. The ability to both mimic other organisms and match
10824-427: The number of arms expressed. Mollusca Mollusca is a phylum of protostomic invertebrate animals , whose members are known as molluscs or mollusks ( / ˈ m ɒ l ə s k s / ). Around 76,000 extant species of molluscs are recognized, making it the second-largest animal phylum after Arthropoda . The number of additional fossil species is estimated between 60,000 and 100,000, and
10947-524: The oceans, from the seashores to the abyssal zone , but some form a significant part of the freshwater fauna and the terrestrial ecosystems . Molluscs are extremely diverse in tropical and temperate regions, but can be found at all latitudes . About 80% of all known mollusc species are gastropods. Cephalopoda such as squid , cuttlefish , and octopuses are among the most neurologically advanced of all invertebrates. The giant squid , which until recently had not been observed alive in its adult form,
11070-413: The octopus Callistoctopus macropus is threatened, it will turn a bright red brown color speckled with white dots as a high contrast display to startle predators. Conspecifically, color change is used for both mating displays and social communication. Cuttlefish have intricate mating displays from males to females. There is also male to male signaling that occurs during competition over mates, all of which are
11193-443: The octopus genus Argonauta secrete a specialized paper-thin egg case in which they reside, and this is popularly regarded as a "shell", although it is not attached to the body of the animal and has a separate evolutionary origin. The largest group of shelled cephalopods, the ammonites , are extinct, but their shells are very common as fossils . The deposition of carbonate, leading to a mineralized shell, appears to be related to
11316-444: The octopus must actively flex the longitudinal muscles during jetting in order to keep the mantle at a constant length. The radial muscles run perpendicular to the longitudinal muscles and are used to thicken and thin the wall of the mantle. Finally, the circular muscles are used as the main activators in jetting. They are muscle bands that surround the mantle and expand/contract the cavity. All three muscle types work in unison to produce
11439-497: The only extant cephalopods with a true external shell. However, all molluscan shells are formed from the ectoderm (outer layer of the embryo); in cuttlefish ( Sepia spp.), for example, an invagination of the ectoderm forms during the embryonic period, resulting in a shell ( cuttlebone ) that is internal in the adult. The same is true of the chitinous gladius of squid and octopuses. Cirrate octopods have arch-shaped cartilaginous fin supports , which are sometimes referred to as
11562-440: The other muscle fibers in the mantle. These collagen fibers act as elastics and are sometimes named "collagen springs". As the name implies, these fibers act as springs. When the radial and circular muscles in the mantle contract, they reach a point where the contraction is no longer efficient to the forward motion of the creature. In such cases, the excess contraction is stored in the collagen which then efficiently begins or aids in
11685-411: The part of the mantle cavity, is swept by the outgoing "lane" of the current created by the gills. Carnivorous molluscs usually have simpler digestive systems. As the head has largely disappeared in bivalves, the mouth has been equipped with labial palps (two on each side of the mouth) to collect the detritus from its mucus. The cephalic molluscs have two pairs of main nerve cords organized around
11808-442: The posterior and anterior ends of this organ control the speed of the jet the organism can produce. The velocity of the organism can be accurately predicted for a given mass and morphology of animal. Motion of the cephalopods is usually backward as water is forced out anteriorly through the hyponome, but direction can be controlled somewhat by pointing it in different directions. Some cephalopods accompany this expulsion of water with
11931-407: The product of chromatophore coloration displays. There are two hypotheses about the evolution of color change in cephalopods. One hypothesis is that the ability to change color may have evolved for social, sexual, and signaling functions. Another explanation is that it first evolved because of selective pressures encouraging predator avoidance and stealth hunting. For color change to have evolved as
12054-548: The proportion of undescribed species is very high. Many taxa remain poorly studied. Molluscs are the largest marine phylum, comprising about 23% of all the named marine organisms . They are highly diverse, not just in size and anatomical structure, but also in behaviour and habitat , as numerous groups are freshwater and even terrestrial species. The phylum is typically divided into 7 or 8 taxonomic classes , of which two are entirely extinct . Cephalopod molluscs, such as squid , cuttlefish , and octopuses , are among
12177-431: The radial and circular mantle cavity muscles. The gills of cephalopods are supported by a skeleton of robust fibrous proteins; the lack of mucopolysaccharides distinguishes this matrix from cartilage. The gills are also thought to be involved in excretion, with NH 4 being swapped with K from the seawater. While most cephalopods can move by jet propulsion, this is a very energy-consuming way to travel compared to
12300-440: The release of egg and sperm, in the nautilus a string of tissue called the siphuncle goes through all the chambers, and the eight plates that make up the shell of chitons are penetrated with living tissue with nerves and sensory structures. The body of a mollusc has a ventral muscular foot, which is adapted to different purposes (locomotion, grasping the substratum, burrowing or feeding) in different classes. The foot carries
12423-441: The result of a change in the density of pigment containing cells and tends to change over longer periods of time. Physiological change, the kind observed in cephalopod lineages, is typically the result of the movement of pigment within the chromatophore, changing where different pigments are localized within the cell. This physiological change typically occurs on much shorter timescales compared to morphological change. Cephalopods have
12546-439: The result of natural selection different parameters would have to be met. For one, you would need some phenotypic diversity in body patterning among the population. The species would also need to cohabitate with predators which rely on vision for prey identification. These predators should have a high range of visual sensitivity, detecting not just motion or contrast but also colors. The habitats they occupy would also need to display
12669-424: The result of social selection the environment of cephalopods' ancestors would have to fit a number of criteria. One, there would need to be some kind of mating ritual that involved signaling. Two, they would have to experience demonstrably high levels of sexual selection. And three, the ancestor would need to communicate using sexual signals that are visible to a conspecific receiver. For color change to have evolved as
12792-549: The same class. Octopuses are generally not seen as active swimmers; they are often found scavenging the sea floor instead of swimming long distances through the water. Squids, on the other hand, can be found to travel vast distances, with some moving as much as 2000 km in 2.5 months at an average pace of 0.9 body lengths per second. There is a major reason for the difference in movement type and efficiency: anatomy. Both octopuses and squids have mantles (referenced above) which function towards respiration and locomotion in
12915-399: The same depth. As such, the cost of transport of many squids are quite high. That being said, squid and other cephalopod that dwell in deep waters tend to be more neutrally buoyant which removes the need to regulate depth and increases their locomotory efficiency. The Macrotritopus defilippi , or the sand-dwelling octopus, was seen mimicking both the coloration and the swimming movements of
13038-452: The sand-dwelling flounder Bothus lunatus to avoid predators. The octopuses were able to flatten their bodies and put their arms back to appear the same as the flounders as well as move with the same speed and movements. Females of two species, Ocythoe tuberculata and Haliphron atlanticus , have evolved a true swim bladder . Two of the categories of cephalopods, octopus and squid, are vastly different in their movements despite being of
13161-446: The sandy sea floor. The color change of chromatophores works in concert with papillae, epithelial tissue which grows and deforms through hydrostatic motion to change skin texture. Chromatophores are able to perform two types of camouflage, mimicry and color matching. Mimicry is when an organism changes its appearance to appear like a different organism. The squid Sepioteuthis sepioide has been documented changing its appearance to appear as
13284-489: The seabed. Squids and cuttlefish can move short distances in any direction by rippling of a flap of muscle around the mantle. While most cephalopods float (i.e. are neutrally buoyant or nearly so; in fact most cephalopods are about 2–3% denser than seawater ), they achieve this in different ways. Some, such as Nautilus , allow gas to diffuse into the gap between the mantle and the shell; others allow purer water to ooze from their kidneys, forcing out denser salt water from
13407-613: The seafloor and metamorphoses into the adult form. While metamorphosis is the usual state in molluscs, the cephalopods differ in exhibiting direct development: the hatchling is a 'miniaturized' form of the adult. The development of molluscs is of particular interest in the field of ocean acidification as environmental stress is recognized to affect the settlement, metamorphosis, and survival of larvae. Most molluscs are herbivorous, grazing on algae or filter feeders. For those grazing, two feeding strategies are predominant. Some feed on microscopic, filamentous algae, often using their radula as
13530-415: The smaller particles, mainly minerals, to the prostyle so eventually they are excreted, while the larger ones, mainly food, are sent to the stomach's cecum (a pouch with no other exit) to be digested. The sorting process is by no means perfect. Periodically, circular muscles at the hindgut's entrance pinch off and excrete a piece of the prostyle, preventing the prostyle from growing too large. The anus, in
13653-608: 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 fresh water , 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 fresh water due to multiple biochemical constraints, and in their >400 million year existence have never ventured into fully freshwater habitats. Cephalopods occupy most of
13776-440: The species and for warning ) or active camouflage , as their chromatophores are expanded or contracted. Although color changes appear to rely primarily on vision input, there is evidence that skin cells, specifically chromatophores , can detect light and adjust to light conditions independently of the eyes. The octopus changes skin color and texture during quiet and active sleep cycles. Cephalopods can use chromatophores like
13899-412: The stomach and projecting slightly into the hindgut is the prostyle, a backward-pointing cone of feces and mucus, which is rotated by further cilia so it acts as a bobbin, winding the mucus string onto itself. Before the mucus string reaches the prostyle, the acidity of the stomach makes the mucus less sticky and frees particles from it. The particles are sorted by yet another group of cilia, which send
14022-435: The subject of molluscan anatomy by describing what is called an archi-mollusc , hypothetical generalized mollusc , or hypothetical ancestral mollusc ( HAM ) to illustrate the most common features found within the phylum. The depiction is visually rather similar to modern monoplacophorans . The generalized mollusc is an unsegmented, bilaterally symmetrical animal and has a single, " limpet -like" shell on top. The shell
14145-459: The tail propulsion used by fish. The efficiency of a propeller -driven waterjet (i.e. Froude efficiency ) is greater than a rocket . The relative efficiency of jet propulsion decreases further as animal size increases; paralarvae are far more efficient than juvenile and adult individuals. Since the Paleozoic era , as competition with fish produced an environment where efficient motion
14268-499: The three functioning as the principal center of "thinking". Some such as the scallops have eyes around the edges of their shells which connect to a pair of looped nerves and which provide the ability to distinguish between light and shadow. The simplest molluscan reproductive system relies on external fertilization , but with more complex variations. All produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults. Two gonads sit next to
14391-423: The total number of mollusc species ever to have existed, whether or not preserved, must be many times greater than the number alive today. Molluscs have more varied forms than any other animal phylum . They include snails , slugs and other gastropods ; clams and other bivalves ; squids and other cephalopods ; and other lesser-known but similarly distinctive subgroups. The majority of species still live in
14514-402: The urine and dumps additional waste products into it, and then ejects it via tubes that discharge into the mantle cavity. Exceptions to the above are the molluscs Planorbidae or ram's horn snails, which are air-breathing snails that use iron-based hemoglobin instead of the copper-based hemocyanin to carry oxygen through their blood. Most molluscs have only one pair of gills, or even only
14637-520: The usual fashion, but its diet includes protists such as the xenophyophore Stannophyllum . Sacoglossan sea-slugs suck the sap from algae, using their one-row radula to pierce the cell walls, whereas dorid nudibranchs and some Vetigastropoda feed on sponges and others feed on hydroids . (An extensive list of molluscs with unusual feeding habits is available in the appendix of GRAHAM, A. (1955). "Molluscan diets" . Journal of Molluscan Studies . 31 (3–4): 144. .) Opinions vary about
14760-417: The water over their gills. Most bivalves are filter feeders, which can be measured through clearance rates. Research has demonstrated that environmental stress can affect the feeding of bivalves by altering the energy budget of organisms. Cephalopods are primarily predatory, and the radula takes a secondary role to the jaws and tentacles in food acquisition. The monoplacophoran Neopilina uses its radula in
14883-437: Was at the rear in the earliest molluscs, but its position now varies from group to group. The anus , a pair of osphradia (chemical sensors) in the incoming "lane", the hindmost pair of gills and the exit openings of the nephridia (kidneys) known as "Organs of bojanus" and gonads (reproductive organs) are in the mantle cavity. The whole soft body of bivalves lies within an enlarged mantle cavity. The mantle edge secretes
15006-463: Was crucial to survival, jet propulsion has taken a back role, with fins and tentacles used to maintain a steady velocity. Whilst jet propulsion is never the sole mode of locomotion, the stop-start motion provided by the jets continues to be useful for providing bursts of high speed – not least when capturing prey or avoiding predators . Indeed, it makes cephalopods the fastest marine invertebrates, and they can out-accelerate most fish. The jet
15129-401: Was published indicating that cephalopod chromatophores are photosensitive; reverse transcription polymerase chain reactions (RT-PCR) revealed transcripts encoding rhodopsin and retinochrome within the retinas and skin of the longfin inshore squid ( Doryteuthis pealeii ), and the common cuttlefish ( Sepia officinalis ) and broadclub cuttlefish ( Sepia latimanus ). The authors claim this
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