Radula

A typical radula comprises a number of bilaterally-symmetrical self-similar rows of teeth rooted in a radular membrane in the floor of their mouth cavity. Some species have teeth that bend with the membrane as it moves over the odontophore, whereas in other species, the teeth are firmly rooted in place, and the entire radular structure moves as one entity. Radular membrane The elastic, delicate radular membrane may be a single tongue, or may split into two (bipartite). Hyaline shield See Hyaline shield for more details. Odontophore The odontophore is the eversible, fleshy tongue underlying the radular membrane. It controls the organ's protrusion and return. It can be likened to a pulley wheel over which the radular 'string' is pulled. The teeth often tesselate with their neighbours, and this interlocking serves to make it more difficult for them to be removed from the radular ribbon. ==Radula formulae==

, Selenochlamys ysbryda The number, shape, and specialized arrangement of molluscan teeth in each transverse row is consistent on a radula, and the different patterns can be used as a diagnostic characteristic to identify the species in many cases. Each row of radular teeth consists of • One central or median tooth (or rachidian tooth, rachis tooth) • On each side: one or more lateral teeth • And then beyond that: one or more marginal teeth. This arrangement is expressed in a radular tooth formula, with the following abbreviations : • R : designates the central tooth or the rachis tooth (in case of lack of central tooth : the zero sign 0) • the lateral teeth on each side are expressed by a specific number or D, in case the outer lateral tooth is dominant. • the marginal teeth are designated by a specific number or, in case they are in very large numbers, the infinity symbol ∞ This can be expressed in a typical formula such as: 3 + D + 2 + R + 2 + D + 3 This formula means: Across the radula there are 3 marginal teeth, 1 dominant lateral tooth, 2 lateral teeth, and one central tooth. Another formula for describing radulae omits the use of letters and simply gives a sequence of numbers in the order marginal-lateral-rachidian-lateral-marginal, thus: 1-1-1-1-1 This particular formula, which is common to the scaphopods, means one marginal tooth, one lateral tooth, one rachidian tooth, one lateral tooth, and one marginal tooth across the ribbon. ==Morphology==

The morphology of the radula is related to diet. However, it is not fixed per species; some mollusks can adapt the form of their radular teeth according to which food sources are abundant. Pointed teeth are best suited to grazing on algal tissue, whereas blunt teeth are preferable if feeding habits entail scraping epiphytes from surfaces. ==Use==

The radula is used in two main ways: either as a rake, generally to comb up microscopic, filamentous algae from a surface; or as a rasp, to feed directly on a plant. The rhipidoglossan (see below) and, to a lesser extent, the taenigloissan radular types are suited to less strenuous modes of feeding, brushing up smaller algae or feeding on soft forms; mollusks with such radulae are rarely able to feed on leathery or coralline algae. On the other hand, the docoglossan gastropod radula allows a very similar diet to the polyplacophora, feeding primarily on these resistant algae, although microalgae are also consumed by species with these radular types. ==Early mollusks==

The first bona fide radula dates to the Early Cambrian, although trace fossils from the earlier Ediacaran have been suggested to have been made by the radula of the organism Kimberella. A so-called radula from the early Cambrian was discovered in 1974, this one preserved with fragments of the mineral ilmenite suspended in a quartz matrix, and showing similarities to the radula of the modern cephalopod Sepia. However, this was since re-interpreted as Salterella. [/Volborthella?] Based on the bipartite nature of the radular dentition pattern in solenogasters, larval gastropods and larval polyplacophora, it has been postulated that the ancestral mollusk bore a bipartite radula (although the radular membrane may not have been bipartite). ==In chitons==

Each row of the polyplacophoran radula has two mineralized teeth used to abrade the substrate, and two longer teeth that sweep up any debris. The other 13 teeth on each row do not appear to be involved in feeding. == In gastropods ==

'' freshwater snail with the radula visible. Anatomy and method of functioning The mouth of the gastropods is located below the anterior part of the mollusk. It opens into a pocket-like buccal cavity, containing the radular sac, an evaginated pocket in the posterior wall of this cavity. The radula apparatus consists of two parts : • the cartilaginous base (the odontophore), with the odontophore protractor muscle, the radula protractor muscle and the radula retractor muscle. • the radula itself, with its longitudinal rows of chitinous and recurved teeth, the cuticula. The odontophore is movable and protractible, and the radula itself is movable over the odontophore. Through this action the radular teeth are being erected. The tip of the odontophore then scrapes the surface, while the teeth cut and scoop up the food and convey the particles through the esophagus to the digestive tract. In a flexoglossate radula (the primitive condition), the teeth flex outwards to the sides as they round the tip of the odontophore, before flexing back inwards. In the derived stereoglossate condition, the teeth do not flex. Some marine gastropods lack a radula. For example, all species of sea slugs in the family Tethydidae have no radula, and a clade of dorids (the Porostomata) as well as all species of the genus Clathromangelia (family Clathurellidae) likewise lack the organ. The radula has been lost a number of times in the Opisthobranchia. == In cephalopods ==

'' Most cephalopods possess a radula as well as a horny chitinous beak, although the radula is reduced in octopuses and absent in Spirula. The cephalopod radula rarely fossilizes: it has been found in around one in five ammonite genera, and is rarer still in non-ammonoid forms. Indeed, it is known from only three non-ammonoid taxa in the Palaeozoic era: Michelinoceras, Paleocadmus, and an unnamed species from the Soom Shale. == In solenogasters ==

The solenogaster radula is akin to that of other mollusks, with regularly spaced rows of teeth produced at one end and shed at the other. The teeth within each row are similar in shape, and get larger in size towards the outer extreme. A number of teeth occur on each row; this number is usually constant but prone to small variations from row to row; indeed, it increases over time, with teeth being added to the middle of rows by addition or by the division of existing teeth. A number of radular formulae are exhibited by this class: 1:0:1 is most common, followed by 0:1:0 and n:0:n. ==In caudofoveates==

The radula of the caudofoveate Falcidens is unlike the conchiferan radula. It has a reduced form, comprising just a single row of teeth. On each side of the apparatus, two teeth appear at the front; behind these, the third teeth fuse to form a mineralized axial plate. Bars occur posterior to this, behind which a sheath encircles the apparatus. The rear of the apparatus consists of a large plate, the 'radular cone'. The unusual form of the radula is accompanied by an unusual purpose: rather than rasping substrates, Falcidens uses its teeth as pincers to grasp prey items. ==See also==