Digestive System of Pila: Pila is herbivorous and it lives primarily on aquatic vegetation. They range from large predatory squids and octopus, some of which show a high degree of intelligence, to grazing forms with elaborately-sculpted and colored shells. For those grazing, two feeding strategies are predominant. It is estimated that 23 percent of all known marine species are mollusks; there are around 85,000 described species, making them the second most diverse phylum of animals. Some molluscs use internal fertilization and/or are hermaphrodites, functioning as both sexes; both of these methods require more complex reproductive systems. One such pest, the giant African snail Achatina fulica, has been introduced to many parts of Asia, as well as to many islands in the Indian Ocean and Pacific Ocean. 4) Foot is long and conical, protrudes through the anterior opening of the shell and is used in burrowing. The acephalic molluscs (i.e., bivalves) also have this ring but it is less obvious and less important. Within Europe, France remained the industry leader. [16] Molluscs never use phosphate to construct their hard parts,[29] with the questionable exception of Cobcrephora. It houses the organs of digestion, circulation, reproduction, and excretion. Foot. The simplest type of molluscan reproductive system relies on external fertilization, but more complex variations occur. [22], Molluscs have more varied forms than any other animal phylum. Nearly all produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults. The three most universal features defining modern molluscs are 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. The majority of species still live in the oceans, from the seashores to the abyssal zone, but some form a significant part of the freshwater fauna and the terrestrial ecosystems. (1955). Molluscs, especially bivalves such as clams and mussels, have been an important food source since at least the advent of anatomically modern humans, and this has often resulted in overfishing. 31 (3–4): 144..), Opinions vary about the number of classes of molluscs; for example, the table below shows seven living classes,[19] and two extinct ones. [76] The diagram on the right summarizes a phylogeny presented in 2007 without the annelid worms. [11], Molluscs have developed such a varied range of body structures, finding synapomorphies (defining characteristics) to apply to all modern groups is difficult. In abalones there are holes in the shell used for respiration and 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 depiction is visually rather similar to modern monoplacophorans. Budd, G. E. & Jensen, S. A critical reappraisal of the fossil record of the bilaterian phyla. Filter feeders are molluscs that feed by straining suspended matter and food particles from water, typically by passing the water over their gills. The mollusk body can generally be divided into three regions: the head, the foot, and a cluster of internal organs called the visceral mass. Click here to let us know! Body possesses a through gut with mouth and anus. About 42% of recorded extinctions since the year 1500 are of molluscs, consisting almost entirely of nonmarine species. Many taxa remain poorly studied.[5]. It enfolds most of the body. Numerous molluscs also live in freshwater and terrestrial habitats. The mantle cavity develops independently of the coelomic cavity. The ganglia above the gut are the cerebral, the pleural, and the visceral, which are located above the esophagus (gullet). Molluscs are the largest marine phylum, comprising about 23% of all the named marine organisms. The mantle cavity develops independently of the coelomic cavity. [94] When used for commercial transactions, they functioned as commodity money, as a tradable commodity whose value differed from place to place, often as a result of difficulties in transport, and which was vulnerable to incurable inflation if more efficient transport or "goldrush" behavior appeared.[95]. In molluscs, there is a sheath of skin extending from the visceral mass and hanging down each side of the body, protecting the soft parts of the organism. [72], Monoplacophorans("limpet-like", "living fossils"), Gastropods(snails, slugs, limpets, sea hares), Cephalopods(nautiloids, ammonites, octopus, squid, etc. In… The bite of the blue-ringed octopus is often fatal, and that of Octopus apollyon causes inflammation that can last over a month. In shelled mollusks, this foot is usually the same size as the opening of the shell. This would have been impermeable and thus forced the development of more sophisticated respiratory apparatus in the form of gills. The parasite itself is not a mollusc, but all the species have freshwater snails as intermediate hosts. visceral mass: the soft, non-muscular metabolic region of the mollusc that contains the body organs mantle: the body wall of a mollusc, from which the shell is secreted radula: the rasping tongue of snails and most other mollusks [1] The total number of described species is difficult to estimate because of unresolved synonymy. The body of the molluscs consists head, muscular foot and visceral mass. The visceral mass includes many of the organs mentioned in the previous concepts such as the stomach, the heart , the nephridia, and the gonads. Stings from a few species of large tropical cone shells can also kill, but their sophisticated, though easily produced, venoms have become important tools in neurological research. Apparently, only the ruling classes were allowed to wear these chlamys. visceral mass (gut, blood, gonads) mantle shell mantle cavity ctenidia (gills) trochophore larva spiral cleavage coelomic heart cavity hemocyanin blood pigments Molluscs and Humans 1. The Early Cambrian fossils Fordilla and Pojetaia are regarded as bivalves. The visceral mass inside the mollusk includes digestive, nervous, excretory, reproductive, and respiratory systems. [31], 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. [71] Even so, bivalves remain abundant and diverse. The depiction is visually rather similar to modern monoplacophorans. The "seed" method uses grains of ground shell from freshwater mussels, and overharvesting for this purpose has endangered several freshwater mussel species in the southeastern United States. 3) Body consist of head, foot, mantle and visceral mass. The mouths of most mollusks, except bivalves (e.g., clams) contain a specialized feeding organ called a radula , an abrasive tonguelike structure. A mantle (with an epidermis secreting a calcium carbonate shell) covering the visceral mass and projecting on each side of the body to cover a … [1] Haszprunar in 2001 estimated about 93,000 named species,[19] which include 23% of all named marine organisms. Carnivorous molluscs usually have simpler digestive systems. We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. Start studying Phylum Mollusca. Most mollusks possess a radula, which is similar to a tongue with teeth-like projections, serving to shred or scrape food. [12] The most general characteristic of molluscs is they are unsegmented and bilaterally symmetrical. All molluscs have a similar body plan with three main parts: A muscular foot A visceral mass – containing digestive, circulatory, respiratory and reproductive organs. [89] The discovery of large numbers of Murex shells on Crete suggests the Minoans may have pioneered the extraction of "imperial purple" during the Middle Minoan period in the 20th–18th centuries BC, centuries before the Tyrians. In mollusks, the visceral mass consists of digestive, circulatory, excretory, reproductive & other organs. Most pairs of corresponding ganglia on both sides of the body are linked by commissures (relatively large bundles of nerves). [81] Current molecular data are insufficient to constrain the molluscan phylogeny, and since the methods used to determine the confidence in clades are prone to overestimation, it is risky to place too much emphasis even on the areas of which different studies agree. Molluscs (also known as molluscs) are smooth-bodied, bilaterally symmetrical, stratified, coelomatous animals; usually shelled with a mantle, ventral foot, anterior head, and visceral dorsal mass. & Urgorri, V. (2018). [77] For example, it is uncertain whether the ancestral mollusc was metameric (composed of repeating units)—if it was, that would suggest an origin from an annelid-like worm. These hemocoelic spaces act as an efficient hydrostatic skeleton. For comparison, the great majority of mollusc species are marine, but only 41 of these appeared on the 2004 Red List. Visceral mass in any animal means the soft organs lying in thorax & abdomen ----accordingly called as thorax viscera & abdominal viscera. Body is divisible into three regions – head, a visceral mass, and ventral foot. Debate occurs about whether some Ediacaran and Early Cambrian fossils really are molluscs. 7. The mantle (also known by the Latin word pallium meaning mantle, robe or cloak, adjective pallial) is a significant part of the anatomy of molluscs: it is the dorsal body wall which covers the visceral mass and usually protrudes in the form of flaps well beyond the visceral mass itself. [80], However, an analysis in 2009 using both morphological and molecular phylogenetics comparisons concluded the molluscs are not monophyletic; in particular, Scaphopoda and Bivalvia are both separate, monophyletic lineages unrelated to the remaining molluscan classes; the traditional phylum Mollusca is polyphyletic, and it can only be made monophyletic if scaphopods and bivalves are excluded. [13] The blood of these molluscs contains the respiratory pigment hemocyanin as an oxygen-carrier. Cuttlefishes (Figure 16-37) also have a Cuttlefishes (Figure 16-37) also have a small, curved shell, but it is entirely enclosed by the mantle. The visceral mass in the shelled species displays torsion around the perpendicular axis on the center of the foot, which is the key characteristic of this group, along with a foot that is modified for crawling. [13], The mantle cavity, a fold in the mantle, encloses a significant amount of space. [16] 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. Body has more than two cell layers, tissues and organs. [13], In some forms the shell contains openings. Mollusk species that are exclusively aquatic have gills for respiration, whereas some terrestrial species have lungs for respiration. While most mollusc shells are composed mainly of aragonite, those gastropods that lay eggs with a hard shell use calcite (sometimes with traces of aragonite) to construct the eggshells. ; García-Álvarez, O.L. ANSWER: C. 8. Tentaculita may also be in Mollusca (see Tentaculites). They divide the mantle cavity so water enters near the bottom and exits near the top. Some shell "currencies" were not used for commercial transactions, but mainly as social status displays at important occasions, such as weddings. ", "Phylogenomics reveals deep molluscan relationships", "New data on Kimberella, the Vendian mollusc-like organism (White Sea region, Russia): palaeoecological and evolutionary implications", "Deep molluscan phylogeny: synthesis of palaeontological and neontological data", 10.1666/0022-3360(2002)076<0287:LECSSF>2.0.CO;2, "Shell microstructures in Early Cambrian molluscs", "A Broad Brush History of the Cephalopoda", 10.1666/0022-3360(2001)075<1119:BSDTC>2.0.CO;2, "Phylogenetic analysis of 73 060 taxa corroborates major eukaryotic groups", "The cell lineage of the polyplacophoran, Chaetopleura apiculata: variation in the spiralian program and implications for molluscan evolution", "Investigation of molluscan phylogeny on the basis of 18S rRNA sequences", "Phylogenetic support values are not necessarily informative: the case of the Serialia hypothesis (a mollusk phylogeny)", "A molecular palaeobiological hypothesis for the origin of aplacophoran molluscs and their derivation from chiton-like ancestors", "China catches almost 11 m tonnes of molluscs in 2005", "Importing fishery products or bivalve molluscs", 10.2983/0730-8000(2006)25[233:DOPOAO]2.0.CO;2, "Doctor snail—Lethal to fish and sometimes even humans, cone snail venom contains a pharmacopoeia of precision drugs", "Cone Shell Mollusc Poisoning, with Report of a Fatal Case", "Venomous mollusks: The risks of human accidents by conus snails (gastropoda: Conidae) in Brazil", "The Carter Center Schistosomiasis Control Program", Researchers complete mollusk evolutionary tree; 26 October 2011, Hardy's Internet Guide to Marine Gastropods, Archaeal Richmond Mine acidophilic nanoorganisms, https://en.wikipedia.org/w/index.php?title=Mollusca&oldid=1018863085, Articles containing Ancient Greek (to 1453)-language text, Wikipedia articles incorporating a citation from the 1911 Encyclopaedia Britannica with Wikisource reference, Articles with dead external links from July 2020, Articles with permanently dead external links, CS1 maint: bot: original URL status unknown, Short description is different from Wikidata, Wikipedia articles with multiple identifiers, Creative Commons Attribution-ShareAlike License, Snails, yes; slugs, mostly yes (internal vestigial), Octopuses, no; cuttlefish, nautilus, squid, yes, ancient lineage of molluscs with cap-like shells, seabed 1,800–7,000 metres (5,900–23,000 ft); one species 200 metres (660 ft). [37], Cephalopods are primarily predatory, and the radula takes a secondary role to the jaws and tentacles in food acquisition. 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. The Hyolitha are a class of extinct animals with a shell and operculum that may be molluscs. Their shells have also been used as money in some preindustrial societies. Each gill has an incoming blood vessel connected to the hemocoel and an outgoing one to the heart.[16]. 8. [63] All cephalopods with external shells except the nautiloids became extinct by the end of the Cretaceous period 65 million years ago. [52], There is an even sharper debate about whether Wiwaxia, from about 505 million years ago, was a mollusc, and much of this centers on whether its feeding apparatus was a type of radula or more similar to that of some polychaete worms. None of these. Although they are unlikely to form a clade, some older works combine the Caudofoveata and Solenogasters into one class, the Aplacophora. MolluscS. A significant part of the visceral hump consists of the mantle, or pallial, cavity. Natural pearls form when a small foreign object gets stuck between the mantle and shell. In the 1990s, this species reached the West Indies. A dorsal visceral mass containing internal organs. In addition to the debates about whether Kimberella and any of the "halwaxiids" were molluscs or closely related to molluscs,[49][50][53][54] debates arise about the relationships between the classes of living molluscs. Beating cilia (tiny "hairs") drive the mucus towards the stomach, so the mucus forms a long string called a "food string". [43] (An extensive list of molluscs with unusual feeding habits is available in the appendix of .mw-parser-output cite.citation{font-style:inherit}.mw-parser-output .citation q{quotes:"\"""\"""'""'"}.mw-parser-output .id-lock-free a,.mw-parser-output .citation .cs1-lock-free a{background:linear-gradient(transparent,transparent),url("//upload.wikimedia.org/wikipedia/commons/6/65/Lock-green.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-limited a,.mw-parser-output .id-lock-registration a,.mw-parser-output .citation .cs1-lock-limited a,.mw-parser-output .citation .cs1-lock-registration a{background:linear-gradient(transparent,transparent),url("//upload.wikimedia.org/wikipedia/commons/d/d6/Lock-gray-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-subscription a,.mw-parser-output .citation .cs1-lock-subscription a{background:linear-gradient(transparent,transparent),url("//upload.wikimedia.org/wikipedia/commons/a/aa/Lock-red-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-ws-icon a{background:linear-gradient(transparent,transparent),url("//upload.wikimedia.org/wikipedia/commons/4/4c/Wikisource-logo.svg")right 0.1em center/12px no-repeat}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:none;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{font-size:100%}.mw-parser-output .cs1-maint{display:none;color:#33aa33;margin-left:0.3em}.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em}.mw-parser-output .citation .mw-selflink{font-weight:inherit}GRAHAM, A. [10], The most universal features of the body structure of molluscs are a mantle with a significant cavity used for breathing and excretion, and the organization of the nervous system. Although molluscs are coelomates, their coeloms are reduced to fairly small spaces enclosing the heart and gonads. The members are known as molluscs or mollusks[a] (/ˈmɒləsk/). A phylogenetic study suggests the Polyplacophora form a clade with a monophyletic Aplacophora. A mollusk’s muscular foot is used for locomotion and anchorage, varies in shape and function, and can both extend and retract. The head-foot is the more active area, containing the feeding, cephalic sensory, and locomotor organs. The mantle is the dorsal epidermis in mollusks; in some mollusks it secretes a chitinous and hard calcareous shell. [59] However, other scientists are not convinced these Early Cambrian fossils show clear signs of the torsion that identifies modern gastropods twists the internal organs so the anus lies above the head. The visceral mass is the large, coiled mass of tissue sitting on the dorsal surface of the foot, posterior to the head. The monoplacophoran Neopilina uses its radula in the usual fashion, but its diet includes protists such as the xenophyophore Stannophyllum. [96] Potamopyrgus antipodarum is used by some water treatment plants to test for estrogen-mimicking pollutants from industrial agriculture. [75] A 2010 analysis recovered the traditional conchiferan and aculiferan groups, and showed molluscs were monophyletic, demonstrating that available data for solenogastres was contaminated. Reduced to its simplest dimensions, the mollusc body plan may be said to consist of a headfoot portion and a visceral mass portion (Figure 16-2). mass called the visceral mass rather than being located in a true cavity Animals: Phylum Mollusca; Ziser Lecture Notes, 2015.10 87 the body cavity of molluscs is a … The main body cavity is a hemocoel through which blood circulates; as such, their circulatory systems are mainly open. [45] Although most helcionellid fossils are only a few millimeters long, specimens a few centimeters long have also been found, most with more limpet-like shapes. [25], Freshwater and terrestrial molluscs appear exceptionally vulnerable to extinction. c) … B. [3] The number of fossil species is estimated between 60,000 and 100,000 additional species. Molluscus was itself an adaptation of Aristotle's τὰ μαλάκια ta malákia (the soft ones; < μαλακός malakós "soft"), which he applied inter alia to cuttlefish. It is the ventral-most organ, whereas the mantle is the limiting dorsal organ. Molluscs have, for centuries, also been the source of important luxury goods, notably pearls, mother of pearl, Tyrian purple dye, and sea silk. Another tropical species, Octopus apollyon, causes severe inflammation that can last for over a month even if treated correctly,[100] and the bite of Octopus rubescens can cause necrosis that lasts longer than one month if untreated, and headaches and weakness persisting for up to a week even if treated. CS1 maint: bot: original URL status unknown (, "Phylogeny and taxonomy of the Kermia–Pseudodaphnella (Mollusca: Gastropoda: Raphitomidae) genus complex: A remarkable radiation via diversification of larval development", "Evidence for a clade composed of molluscs with serially repeated structures: monoplacophorans are related to chitons", "First-ever observations of a live giant squid in the wild", "The global decline of nonmarine mollusks", 10.1641/0006-3568(2004)054[0321:TGDONM]2.0.CO;2, "A comparative study of the ultrastructure and mineralogy of calcified land snail eggs (Pulmonata: Stylommatophora)", "How the Pacific Oyster Responds to Ocean Acidification: Development and Application of a Meta-Analysis Based Adverse Outcome Pathway", "Xenophyophores (Protozoa, Sarcodina) in the diet of, "Coexistence of hydroid-eating Nudibranchs: Do feeding biology and habitat use matter? Two gonads sit next to the coelom, a small cavity that surrounds the heart, into which they shed ova or sperm. 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. [70] One bivalve group, the rudists, became major reef-builders in the Cretaceous, but became extinct in the Cretaceous–Paleogene extinction event. Characteristics of Mollusca: Bilaterally symmetrical. Budd, G. E., and S. Jensen. The nephridia extract the gametes from the coelom and emit them into the mantle cavity. Mollusks are predominantly a marine group of animals; however, they are known to inhabit freshwater as well as terrestrial habitats. It is called the Type your answer here and it is responsible for secreting and lining the shell of some molluscs. [13], The cephalic molluscs have two pairs of main nerve cords organized around a number of paired ganglia, the visceral cords serving the internal organs and the pedal ones serving the foot. For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org. [ "article:topic", "authorname:boundless", "showtoc:no" ], https://bio.libretexts.org/@app/auth/3/login?returnto=https%3A%2F%2Fbio.libretexts.org%2FBookshelves%2FIntroductory_and_General_Biology%2FBook%253A_General_Biology_(Boundless)%2F28%253A_Invertebrates%2F28.3%253A_Superphylum_Lophotrochozoa%2F28.3E%253A_Phylum_Mollusca, information contact us at info@libretexts.org, status page at https://status.libretexts.org, Describe the unique anatomical and morphological features of molluscs. 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. Most molluscs have muscular mouths with radulae, "tongues", bearing many rows of chitinous teeth, which are replaced from the rear as they wear out. Because molluscs are very diverse and come in many different shapes and sizes it … [16], At the tapered rear end of 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. [66][67][68][69] "Modern-looking" bivalves appeared in the Ordovician period, 488 to 443 million years ago. [15](pp300–343, 367–403) The best natural pearls are produced by marine pearl oysters, Pinctada margaritifera and Pinctada mertensi, which live in the tropical and subtropical waters of the Pacific Ocean. [98], All octopuses are venomous,[99] but only a few species pose a significant threat to humans. Schistosomiasis (also known as bilharzia, bilharziosis, or snail fever) is transmitted to humans by water snail hosts, and affects about 200 million people. There … [108], "Mollusk" redirects here. The foot is a retractable as well as an extendable organ. [106], Some species of molluscs, particularly certain snails and slugs, can be serious crop pests,[107] and when introduced into new environments, can unbalance local ecosystems. Despite their amazing diversity, all molluscs … Body without cavity. Their shells have also been used as a form of currency in some preindustrial societies. Members of the mollusc family use intracellular digestion to function. [4] The proportion of undescribed species is very high. Shell. [26], Because of the great range of anatomical diversity among molluscs, many textbooks start 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. [64] However, the shell-less Coleoidea (squid, octopus, cuttlefish) are abundant today.[65]. Legal. [51] In fact, some groups traditionally classified as molluscs may have to be redefined as distinct but related.[75]. It is lined with epidermis, and is exposed, according to habitat, to sea, fresh water or air. The generalized mollusc has two paired nerve cords, or three in bivalves. A pair of nephridia ("little kidneys") to the rear of and connected to the coelom extracts any re-usable materials from the urine and dumps additional waste products into it, and then ejects it via tubes that discharge into the mantle cavity.[16]. Good evidence exists for the appearance of gastropods, cephalopods, and bivalves in the Cambrian period, 541–485.4 million years ago. This includes digestive, nervous, excretory, reproductive, and respiratory systems. A ventral foot, muscular, variously shaped (missing only in the Class Caudofoveata). However, Plectronoceras and other early cephalopods crept along the seafloor instead of swimming, as their shells contained a "ballast" of stony deposits on what is thought to be the underside, and had stripes and blotches on what is thought to be the upper surface. Most mollusks are dioecious animals where fertilization occurs externally, although this is not the case in terrestrial mollusks, such as snails and slugs, or in cephalopods. Have questions or comments? The name “mollusca” signifies a soft body; the earliest descriptions of mollusks came from observations of unshelled cuttlefish. However, these "currencies" generally differed in important ways from the standardized government-backed and -controlled money familiar to industrial societies. Molluscs have soft bodies that consist of three basic parts, a foot, a visceral mass, and a mantle. [30] [92] Procopius, writing on the Persian wars circa 550 CE, "stated that the five hereditary satraps (governors) of Armenia who received their insignia from the Roman Emperor were given chlamys (or cloaks) made from lana pinna. [12] About 80% of all known mollusc species are gastropods. The chambers are connected by a cord of living tissue called a siphuncle, which extends from the visceral mass. Good evidence exists for the appearance of gastropods (e.g., Aldanella), cephalopods (e.g., Plectronoceras, ?Nectocaris) and bivalves (Pojetaia, Fordilla) towards the middle of the Cambrian period, c. 500 million years ago, though arguably each of these may belong only to the stem lineage of their respective classes.