69-531: Aplacophora / æ p l ə ˈ k ɒ f ər ə / is a possibly paraphyletic taxon. This is a class of small, deep-water, exclusively benthic , marine molluscs found in all oceans of the world. All known modern forms are shell-less: only some extinct primitive forms possessed valves . The group comprises the two clades Solenogastres (Neomeniomorpha) and Caudofoveata (Chaetodermomorpha), which between them contain 28 families and about 320 species . The aplacophorans are traditionally considered ancestral to
138-430: A sphincter , with the latter part being highly coiled and functioning to compact the waste matter into faecal pellets . The anus opens just behind the foot. Chitons lack a clearly demarcated head; their nervous system resembles a dispersed ladder. No true ganglia are present, as in other molluscs, although a ring of dense neural tissue occurs around the oesophagus. From this ring, nerves branch forwards to innervate
207-479: A "single common ancestor" organism. Paraphyly is common in speciation , whereby a mother species (a paraspecies ) gives rise to a daughter species without itself becoming extinct. Research indicates as many as 20 percent of all animal species and between 20 and 50 percent of plant species are paraphyletic. Accounting for these facts, some taxonomists argue that paraphyly is a trait of nature that should be acknowledged at higher taxonomic levels. Cladists advocate
276-592: A cell nucleus, a plesiomorphy ) from its excluded descendants. Also, some systematists recognize paraphyletic groups as being involved in evolutionary transitions, the development of the first tetrapods from their ancestors for example. Any name given to these hypothetical ancestors to distinguish them from tetrapods—"fish", for example—necessarily picks out a paraphyletic group, because the descendant tetrapods are not included. Other systematists consider reification of paraphyletic groups to obscure inferred patterns of evolutionary history. The term " evolutionary grade "
345-427: A chiton are sometimes known as butterfly shells due to their shape. The girdle may be ornamented with scales or spicules which, like the shell plates, are mineralized with aragonite — although a different mineralization process operates in the spicules to that in the teeth or shells (implying an independent evolutionary innovation). This process seems quite simple in comparison to other shell tissue; in some taxa,
414-419: A group of dinosaurs (part of Diapsida ), both of which are "reptiles". Osteichthyes , bony fish, are paraphyletic when circumscribed to include only Actinopterygii (ray-finned fish) and Sarcopterygii (lungfish, etc.), and to exclude tetrapods ; more recently, Osteichthyes is treated as a clade, including the tetrapods. The " wasps " are paraphyletic, consisting of the narrow-waisted Apocrita without
483-413: A hard ferric/ferrous oxide mineral. The radula is used to scrape microscopic algae off the substratum. The mouth cavity itself is lined with chitin and is associated with a pair of salivary glands . Two sacs open from the back of the mouth, one containing the radula, and the other containing a protrusible sensory subradular organ that is pressed against the substratum to taste for food. Cilia pull
552-439: A kind of lizard). Put another way, viviparity is a synapomorphy for Theria within mammals, and an autapomorphy for Eulamprus tympanum (or perhaps a synapomorphy, if other Eulamprus species are also viviparous). Groupings based on independently-developed traits such as these examples of viviparity represent examples of polyphyly , not paraphyly. The following list recapitulates a number of paraphyletic groups proposed in
621-408: A larval eye. However, chitons lack a cerebral ganglion. Similar to many species of saltwater limpets , several species of chiton are known to exhibit homing behaviours, journeying to feed and then returning to the exact spot they previously inhabited. The method they use to perform such behaviors has been investigated to some extent, but remains unknown. One theory has the chitons remembering
690-448: A monophyletic clade. The affinities of Aplacophorans have long been uncertain. Molecular and fossil evidence seemed to put Aplacophorans in the clade Aculifera , as a sister group to Polyplacophora . The discovery of Kulindroplax in 2012, a fossil aplacophoran with a polyplacophoran-like armour, strongly supports this hypothesis, and shows that aplacophorans evolved from progenitors that bore valves. Paraphyletic Paraphyly
759-549: A more inclusive clade, it often makes sense to study the paraphyletic group that remains without considering the larger clade. For example, the Neogene evolution of the Artiodactyla (even-toed ungulates, like deer, cows, pigs and hippopotamuses - Cervidae , Bovidae , Suidae and Hippopotamidae , the families that contain these various artiodactyls, are all monophyletic groups) has taken place in environments so different from that of
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#1732855980859828-424: A phylogenetic species concept that does not consider species to exhibit the properties of monophyly or paraphyly, concepts under that perspective which apply only to groups of species. They consider Zander's extension of the "paraphyletic species" argument to higher taxa to represent a category error When the appearance of significant traits has led a subclade on an evolutionary path very divergent from that of
897-400: A protective ball when dislodged and to cling tightly to irregular surfaces. In some species the valves are reduced or covered by the girdle tissue. The valves are variously colored, patterned, smooth, or sculptured. The most anterior plate is crescent-shaped, and is known as the cephalic plate (sometimes called a head plate, despite the absence of a complete head). The most posterior plate
966-571: A relatively good fossil record, stretching back to the Cambrian, with the genus Preacanthochiton , known from fossils found in Late Cambrian deposits in Missouri , being classified as the earliest known polyplacophoran. However, the exact phylogenetic position of supposed Cambrian chitons is highly controversial, and some authors have instead argued that the earliest confirmed polyplacophorans date back to
1035-464: A segmented shell gland forms on one side of the larva, and a foot forms on the opposite side. When the larva is ready to become an adult, the body elongates, and the shell gland secretes the plates of the shell. Unlike the fully grown adult, the larva has a pair of simple eyes, although these may remain for some time in the immature adult. Animals which prey on chitons include humans, seagulls , sea stars , crabs , lobsters and fish . Chitons have
1104-427: A shell composed of eight separate shell plates or valves . These plates overlap slightly at the front and back edges, and yet articulate well with one another. Because of this, the shell provides protection at the same time as permitting the chiton to flex upward when needed for locomotion over uneven surfaces, and even allows the animal to curl up into a ball when dislodged from rocks. The shell plates are encircled by
1173-517: A skirt known as a girdle . Chitons live worldwide, from cold waters through to the tropics. They live on hard surfaces, such as on or under rocks, or in rock crevices. Some species live quite high in the intertidal zone and are exposed to the air and light for long periods. Most species inhabit intertidal or subtidal zones, and do not extend beyond the photic zone , but a few species live in deep water, as deep as 6,000 m (20,000 ft). Chitons are exclusively and fully marine, in contrast to
1242-414: A tough spiny coat, and usually hatches to release a free-swimming trochophore larva, typical of many other mollusc groups. In a few cases, the trochophore remains within the egg (and is then called lecithotrophic – deriving nutrition from yolk), which hatches to produce a miniature adult. Unlike most other molluscs, there is no intermediate stage, or veliger , between the trochophore and the adult. Instead,
1311-419: Is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic with respect to the excluded subgroups. In contrast, a monophyletic grouping (a clade ) includes a common ancestor and all of its descendants. The terms are commonly used in phylogenetics (a subfield of biology ) and in
1380-439: Is a monophyletic group from which one or more subsidiary clades (monophyletic groups) are excluded to form a separate group. Philosopher of science Marc Ereshefsky has argued that paraphyletic taxa are the result of anagenesis in the excluded group or groups. A cladistic approach normally does not grant paraphyletic assemblages the status of "groups", nor does it reify them with explanations, as in cladistics they are not seen as
1449-433: Is allowed as a synonym of Magnoliopsida. Phylogenetic analysis indicates that the monocots are a development from a dicot ancestor. Excluding monocots from the dicots makes the latter a paraphyletic group. Among animals, several familiar groups are not, in fact, clades. The order Artiodactyla ( even-toed ungulates ) as traditionally defined is paraphyletic because it excludes Cetaceans (whales, dolphins, etc.). Under
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#17328559808591518-413: Is apparent from some features of the digestive system; aplacophorans possess both a radula and a style . A variety of radular forms and functions exist. Solenogasters are hermaphroditic and assumed to have internal fertilization , in contrast to caudofoveates which have two sexes, and reproduce by external fertilization . During development, the mantle cavity of the larva curls up and closes, creating
1587-833: Is chitinous, and has an irregular texture. Spines bear an organic matrix. Sclerites can be hollow or solid; the void within the sclerites of some species fills during growth. Sclerites generally form within the cuticle, protruding through when they are fully grown. They probably start life as amorphous calcium carbonate, which the organic matrix coaxes into an aragonitic habit as the spines mature. Sclerites can be shaped as simple spines, straight, curved, keeled, striated or hooked; or as cupped blades; more complex arrangements are common in copulatory spicules. In several species of solenogaster, sclerites change morphology during growth; young specimens might bear flat, solid, scale-like sclerites, to be replaced with longer hollow spine-like sclerites in adults. The relationship with other molluscs, however,
1656-485: Is found in most polyplacophora (but not basal chitons, such as Hanleya ) but is unusual in aplacophora. Developmentally, sclerite-secreting cells arise from pretrochal and postrochal cells: the 1a, 1d, 2a, 2c, 3c and 3d cells. The shell plates arise primarily from the 2d micromere, although 2a, 2b, 2c and sometimes 3c cells also participate in its secretion. The girdle is often ornamented with spicules, bristles, hairy tufts, spikes, or snake-like scales. The majority of
1725-446: Is known as the anal plate (sometimes called the tail plate, although chitons do not have tails.) The inner layer of each of the six intermediate plates is produced anteriorly as an articulating flange, called the articulamentum. This inner layer may also be produced laterally in the form of notched insertion plates. These function as an attachment of the valve plates to the soft body. A similar series of insertion plates may be attached to
1794-442: Is paraphyletic with respect to birds . Reptilia contains the last common ancestor of reptiles and all descendants of that ancestor except for birds. Other commonly recognized paraphyletic groups include fish , monkeys , and lizards . The term paraphyly , or paraphyletic , derives from the two Ancient Greek words παρά ( pará ), meaning "beside, near", and φῦλον ( phûlon ), meaning "genus, species", and refers to
1863-482: Is sometimes used for paraphyletic groups. Moreover, the concepts of monophyly , paraphyly, and polyphyly have been used in deducing key genes for barcoding of diverse group of species. Current phylogenetic hypotheses of tetrapod relationships imply that viviparity , the production of offspring without the external laying of a fertilized egg, developed independently in the lineages that led to humans ( Homo sapiens ) and southern water skinks ( Eulampus tympanum ,
1932-503: The Cetacea (whales, dolphins, and porpoises) that the Artiodactyla are often studied in isolation even though the cetaceans are a descendant group. The prokaryote group is another example; it is paraphyletic because it is composed of two Domains (Eubacteria and Archaea) and excludes (the eukaryotes ). It is very useful because it has a clearly defined and significant distinction (absence of
2001-522: The Early Ordovician . Kimberella and Wiwaxia of the Precambrian and Cambrian may be related to ancestral polyplacophorans. Matthevia is a Late Cambrian polyplacophoran preserved as individual pointed valves, and sometimes considered to be a chiton, although at the closest, it can only be a stem-group member of the group. Based on this and co-occurring fossils, one plausible hypothesis for
2070-569: The ICN ) abandoned consideration of bacterial nomenclature in 1975; currently, prokaryotic nomenclature is regulated under the ICNB with a starting date of 1 January 1980 (in contrast to a 1753 start date under the ICBN/ICN). Among plants, dicotyledons (in the traditional sense) are paraphyletic because the group excludes monocotyledons . "Dicotyledon" has not been used as a botanic classification for decades, but
2139-584: The Narungga Nation Traditional Fishing Agreement. A chiton creeps along slowly on a muscular foot. It has considerable power of adhesion and can cling to rocks very powerfully, like a limpet . Chitons are generally herbivorous grazers, though some are omnivorous and some carnivorous. They eat algae , bryozoans , diatoms , barnacles , and sometimes bacteria by scraping the rocky substrate with their well-developed radulae . A few species of chitons are predatory , such as
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2208-753: The Philippines , where it is called kibet if raw and chiton if fried. Indigenous people of the Pacific coasts of North America eat chitons. They are a common food on the Pacific coast of South America and in the Galápagos . The foot of the chiton is prepared in a manner similar to abalone . Some islanders living in South Korea also eat chiton, slightly boiled and mixed with vegetables and hot sauce. Aboriginal people in Australia also eat chiton; for example they are recorded in
2277-522: The Sumerian word gada or gida . The Greek-derived name Polyplacophora comes from the words poly- (many), plako- (tablet), and -phoros (bearing), a reference to the chiton's eight shell plates. Most classification schemes in use today are based, at least in part, on Pilsbry's Manual of Conchology (1892–1894), extended and revised by Kaas and Van Belle (1985–1990). Since chitons were first described by Linnaeus (1758), extensive taxonomic studies at
2346-638: The ants and bees . The sawflies ( Symphyta ) are similarly paraphyletic, forming all of the Hymenoptera except for the Apocrita, a clade deep within the sawfly tree. Crustaceans are not a clade because the Hexapoda (insects) are excluded. The modern clade that spans all of them is the Tetraconata . One of the goals of modern taxonomy over the past fifty years has been to eliminate paraphyletic "groups", such as
2415-478: The bivalves , which were able to adapt to brackish water and fresh water, and the gastropods which were able to make successful transitions to freshwater and terrestrial environments. All chitons bear a protective dorsal shell that is divided into eight articulating aragonite valves embedded in the tough muscular girdle that surrounds the chiton's body. Compared with the single or two-piece shells of other molluscs, this arrangement allows chitons to roll into
2484-459: The pericardial cavity around the heart, and remove excreta through a pore that opens near the rear of the mantle cavity. The single gonad is located in front of the heart, and releases gametes through a pair of pores just in front of those used for excretion. The mouth is located on the underside of the animal, and contains a tongue-like structure called a radula , which has numerous rows of 17 teeth each. The teeth are coated with magnetite ,
2553-448: The tree model of historical linguistics . Paraphyletic groups are identified by a combination of synapomorphies and symplesiomorphies . If many subgroups are missing from the named group, it is said to be polyparaphyletic. The term received currency during the debates of the 1960s and 1970s accompanying the rise of cladistics , having been coined by zoologist Willi Hennig to apply to well-known taxa like Reptilia ( reptiles ), which
2622-516: The Mesozoic. The earliest fossil evidence of aesthetes in chitons comes from around 400 Ma, during the Early Devonian . Chitons were first studied by Carl Linnaeus in his 1758 10th edition of Systema Naturae . Since his description of the first four species, chitons have been variously classified. They were called Cyclobranchians (round arm) in the early 19th century, and then grouped with
2691-495: The actual products of evolutionary events. A group whose identifying features evolved convergently in two or more lineages is polyphyletic (Greek πολύς [ polys ], "many"). More broadly, any taxon that is not paraphyletic or monophyletic can be called polyphyletic. Empirically, the distinction between polyphyletic groups and paraphyletic groups is rather arbitrary, since the character states of common ancestors are inferences, not observations. These terms were developed during
2760-608: The aplacophorans in the subphylum Amphineura in 1876. The class Polyplacophora was named by de Blainville 1816. The name chiton is Neo-Latin derived from the Ancient Greek word khitōn , meaning tunic (which also is the source of the word chitin ). The Ancient Greek word khitōn can be traced to the Central Semitic word *kittan , which is from the Akkadian words kitû or kita'um , meaning flax or linen, and originally
2829-402: The body is a snail -like foot, but no head or other soft parts beyond the girdle are visible from the dorsal side. The mantle cavity consists of a narrow channel on each side, lying between the body and the girdle. Water enters the cavity through openings in either side of the mouth, then flows along the channel to a second, exhalant, opening close to the anus . Multiple gills hang down into
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2898-448: The bulk of the matrix. The girdle spines often bear length-parallel striations. The wide form of girdle ornament suggests it serves a secondary role; chitons can survive perfectly well without them. Camouflage or defence are two likely functions. Certainly species such as some members of the genus Acanthochitona bear conspicuous paired tufts of spicules on the girdle. The spicules are sharp, and if carelessly handled, easily penetrate
2967-461: The convex anterior border of the cephalic plate or the convex posterior border of the anal plate. The sculpture of the valves is one of the taxonomic characteristics, along with the granulation or spinulation of the girdle. After a chiton dies, the individual valves which make up the eight-part shell come apart because the girdle is no longer holding them together, and then the plates sometimes wash up in beach drift. The individual shell plates from
3036-407: The crystal structure of the deposited minerals closely resembles the disordered nature of crystals that form inorganically, although more order is visible in other taxa. The protein component of the scales and sclerites is minuscule in comparison with other biomineralized structures, whereas the total proportion of matrix is 'higher' than in mollusc shells. This implies that polysaccharides make up
3105-490: The debates of the 1960s and 1970s accompanying the rise of cladistics . Paraphyletic groupings are considered problematic by many taxonomists, as it is not possible to talk precisely about their phylogenetic relationships, their characteristic traits and literal extinction. Related terms are stem group , chronospecies , budding cladogenesis, anagenesis, or 'grade' groupings. Paraphyletic groups are often relics from outdated hypotheses of phylogenic relationships from before
3174-471: The descendants of a unique common ancestor. By comparison, the term polyphyly , or polyphyletic , uses the Ancient Greek prefix πολύς ( polús ), meaning "many, a lot of", and refers to the fact that a polyphyletic group includes organisms arising from multiple ancestral sources. Groups that include all the descendants of a common ancestor are said to be monophyletic . A paraphyletic group
3243-450: The examples given here, from formal classifications. Species have a special status in systematics as being an observable feature of nature itself and as the basic unit of classification. Some articulations of the phylogenetic species concept require species to be monophyletic, but paraphyletic species are common in nature, to the extent that they do not have a single common ancestor. Indeed, for sexually reproducing taxa, no species has
3312-506: The extent of their visual acuity. It is known that they can differentiate between a predator's shadow and changes in light caused by clouds. An evolutionary trade-off has led to a compromise between the eyes and the shell; as the size and complexity of the eyes increase, the mechanical performance of their shells decrease, and vice versa. A relatively good fossil record of chiton shells exists, but ocelli are only present in those dating to 10 million years ago or younger; this would make
3381-417: The food through the mouth in a stream of mucus and through the oesophagus , where it is partially digested by enzymes from a pair of large pharyngeal glands. The oesophagus, in turn, opens into a stomach , where enzymes from a digestive gland complete the breakdown of the food. Nutrients are absorbed through the linings of the stomach and the first part of the intestine . The intestine is divided in two by
3450-459: The human skin, where they detach and remain as a painful irritant. Spicules are secreted by cells that do not express engrailed , but these cells are surrounded by engrailed-expressing cells. These neighbouring cells secrete an organic pellicle on the outside of the developing spicule, whose aragonite is deposited by the central cell; subsequent division of this central cell allows larger spines to be secreted in certain taxa. The organic pellicule
3519-572: The island of Taiwan . Polyplacophora See text . Chitons ( / ˈ k aɪ t ən z , - t ɒ n z / ) are marine molluscs of varying size in the class Polyplacophora ( / ˌ p ɒ l i p l ə ˈ k ɒ f ər ə / POL -ee-plə- KOF -ər-ə ), formerly known as Amphineura . About 940 extant and 430 fossil species are recognized. They are also sometimes known as sea cradles or coat-of-mail shells or suck-rocks , or more formally as loricates , polyplacophorans , and occasionally as polyplacophores . Chitons have
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#17328559808593588-605: The literature, and provides the corresponding monophyletic taxa. The concept of paraphyly has also been applied to historical linguistics , where the methods of cladistics have found some utility in comparing languages. For instance, the Formosan languages form a paraphyletic group of the Austronesian languages because they consist of the nine branches of the Austronesian family that are not Malayo-Polynesian and are restricted to
3657-468: The mantle cavity along part or all of the lateral pallial groove, each consisting of a central axis with a number of flattened filaments through which oxygen can be absorbed. The three-chambered heart is located towards the animal's hind end. Each of the two auricles collects blood from the gills on one side, while the muscular ventricle pumps blood through the aorta and round the body. The excretory system consists of two nephridia , which connect to
3726-462: The mouth and subradula, while two pairs of main nerve cords run back through the body. One pair, the pedal cords, innervate the foot, while the palliovisceral cords innervate the mantle and remaining internal organs. Some species bear an array of tentacles in front of the head. The primary sense organs of chitons are the subradular organ and a large number of unique organs called aesthetes . The aesthetes consist of light-sensitive cells just below
3795-514: The ocelli, whose precise function is unclear, likely the most recent eyes to evolve. Although chitons lack osphradia , statocysts , and other sensory organs common to other molluscs, they do have numerous tactile nerve endings, especially on the girdle and within the mantle cavity. The order Lepidopleurida also have a pigmented sensory organ called the Schwabe organ. Its function remains largely unknown, and has been suggested to be related to that of
3864-533: The origin of polyplacophora has that they formed when an aberrant monoplacophoran was born with multiple centres of calcification, rather than the usual one. Selection quickly acted on the resultant conical shells to form them to overlap into protective armour; their original cones are homologous to the tips of the plates of modern chitons. The chitons evolved from multiplacophora during the Palaeozoic, with their relatively conserved modern-day body plan being fixed by
3933-993: The other mollusc classes. However, the relationship between the two aplacophoran groups and to the other molluscan classes and to each other is as yet unclear. Aplacophorans are cylindrical and worm-like in form, and most very small, being no longer than 5 centimetres (2 in); some species, however, can reach a length of 30 centimetres (12 in). Caudofoveates generally burrow into the substrate while solenogasters are usually epibenthic . Both taxa are most common in water regions deeper than 20 metres (66 ft) where some species may reach densities up to four or five specimens per square metre (three or four per square yard). Solenogasters are typically carnivores feeding on cnidarians or sometimes annelids or other taxa while caudofoveates are mostly detritovores or feed on foraminiferans . Aplacophorans are worm-like animals, with little resemblance to most other molluscs. They have no shell , although small calcified spicules are embedded in
4002-565: The polarity and the inclination of the Earth's magnetic field . Experimental work has suggested that chitons can detect and respond to magnetism. Chitons are eaten in several parts of the world. This includes islands in the Caribbean, such as Trinidad , Tobago , The Bahamas , St. Maarten, Aruba, Bonaire, Anguilla and Barbados , as well as in Bermuda. They are also traditionally eaten in certain parts of
4071-675: The ranks of the ICZN Code , the two taxa are separate orders. Molecular studies, however, have shown that the Cetacea descend from artiodactyl ancestors, although the precise phylogeny within the order remains uncertain. Without the Cetaceans the Artiodactyls are paraphyletic. The class Reptilia is paraphyletic because it excludes birds (class Aves ). Under a traditional classification, these two taxa are separate classes. However birds are sister taxon to
4140-563: The rise of cladistics. The prokaryotes (single-celled life forms without cell nuclei) are a paraphyletic grouping, because they exclude the eukaryotes , a descendant group. Bacteria and Archaea are prokaryotes, but archaea and eukaryotes share a common ancestor that is not ancestral to the bacteria. The prokaryote/eukaryote distinction was proposed by Edouard Chatton in 1937 and was generally accepted after being adopted by Roger Stanier and C.B. van Niel in 1962. The botanical code (the ICBN, now
4209-407: The same way, but they may leave chemical cues along the rock surface and at the home scar which their olfactory senses can detect and home in on. Furthermore, older trails may also be detected, providing further stimulus for the chiton to find its home. The radular teeth of chitons are made of magnetite , and the iron crystals within these may be involved in magnetoreception , the ability to sense
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#17328559808594278-405: The situation in which one or several monophyletic subgroups of organisms (e.g., genera, species) are left apart from all other descendants of a unique common ancestor. Conversely, the term monophyly , or monophyletic , builds on the Ancient Greek prefix μόνος ( mónos ), meaning "alone, only, unique", and refers to the fact that a monophyletic group includes organisms consisting of all
4347-454: The skin; these spicules are occasionally coated with an organic pellicle that is presumably secreted by microvilli. Caudofoveates lack a foot while solenogasters have a narrow foot which lacks intrinsic musculature. The mantle cavity is reduced into a simple cloaca , into which the anus and excretory organs empty, and is located at the posterior of the animal. The head is rudimentary, and has no eyes or tentacles. The cuticle of both subclasses
4416-449: The small western Pacific species Placiphorella velata . These predatory chitons have enlarged anterior girdles. They catch other small invertebrates , such as shrimp and possibly even small fish, by holding the enlarged, hood-like front end of the girdle up off the surface, and then clamping down on unsuspecting, shelter-seeking prey. Chitons have separate sexes, and fertilization is usually external . The male releases sperm into
4485-453: The species level have been made. However, the taxonomic classification at higher levels in the group has remained somewhat unsettled. The most recent classification, by Sirenko (2006), is based not only on shell morphology, as usual, but also other important features, including aesthetes , girdle, radula, gills, glands, egg hull projections, and spermatozoids. It includes all the living and extinct genera of chitons. Further resolution within
4554-527: The surface of the shell, although they are not capable of true vision. In some cases, however, they are modified to form ocelli , with a cluster of individual photoreceptor cells lying beneath a small aragonite -based lens . Each lens can form clear images, and is composed of relatively large, highly crystallographically aligned grains to minimize light scattering. An individual chiton may have thousands of such ocelli. These aragonite -based eyes make them capable of true vision; though research continues as to
4623-426: The topographic profile of the region, thus being able to guide themselves back to their home scar by a physical knowledge of the rocks and visual input from their numerous primitive eyespots. The sea snail Nerita textilis (like all gastropods ) deposits a mucus trail as it moves, which a chemoreceptive organ is able to detect and guide the snail back to its home site. It is unclear if chiton homing functions in
4692-406: The water, while the female releases eggs either individually, or in a long string. In most cases, fertilization takes place either in the surrounding water, or in the mantle cavity of the female. Some species brood the eggs within the mantle cavity, and the species Callistochiton viviparus even retains them within the ovary and gives birth to live young, an example of ovoviviparity . The egg has
4761-578: The worm-like form of the adult. Caudofoveates also differ from Solenogasters in having a head shield and a body that is differentiated into three sections. This class was once classified as sea cucumbers in the echinoderms . In 1987, they were officially recognized as molluscs and given their own class. It consists of two clades : the Solenogastres and the Caudofoveata . It has been considered polyphyletic but more recent molecular evidence supports it as
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