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). It includes all the living and extinct genera of chitons. Before this, some organisms have been interpreted (tentatively) as stem-group polyplacophora, potentially stretching the record of polyplacophora back to the Ordovician. [1], The chitons evolved from multiplacophora during the Palaeozoic, with their relatively conserved modern-day body plan being fixed by the Mesozoic.[34]. The diet of chitons consists primarily of algae, phytoplankton, marine moss animals, and bacteria, although some species eat small crustaceans. [33], Kimberella and Wiwaxia of the Precambrian and Cambrian may be related to ancestral polyplacophora. [17], The primary sense organs of chitons are the subradular organ and a large number of unique organs called aesthetes. On the dorsal (upper) surface is a row of eight overlapping plates surrounded or covered by a tough girdle. [3] These plates overlap slightly at the front and back edges, and yet articulate well with one another. There are male and female chitons, and they reproduce by releasing … (Thorne. Barnawell, E. B. If they get knocked off their rock, they can usually find their way back. Duxbury Reef, Bolinas, CA, low tide, 1.0 ft. Saturday, July 19, 2014, 12:34pm. [16], 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 tremendous numbers of free-swimming young (trochophores) are an important element in the marine plankton. Chitons are very flexible and can fit snugly into rock crevices or curl into a ball when detached. Although known since the early Paleozoic, chitons do not show a marked increase in diversity until the Cretaceous. How do Chitons move? 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. 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. A few species of chitons are predatory, such as the small western Pacific species Placiphorella velata. External morphology of the Polyplacophora. The radula is used to scrape microscopic algae off the substratum. By The Marine Detective on September 30, 2012 I’m excited to share video with you of Gumboot Chitons spawning. The excretory system consists of two nephridia, which connect to the pericardial cavity around the heart, and remove excreta through a pore that opens near the rear of the mantle cavity. Do all flatworms live in water? These marine neighbours most often seem quite inanimate – having a life where they keep their undersides protected by sucking down hard on rocks but, when it’s time to mate . The 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 the Sumerian word gada or gida. The oesophagus, in turn, opens into a stomach, where enzymes from a digestive gland complete the breakdown of the food. Generally flattened body and broad foot; mantle covered with cuticle and spicules; 8 middorsal serial shell plates (valves) enclosing photoreceptive papillae (aesthetes); mantle cavity peripedal with multiple pairs of ctenidia; marine; mainly algae-scraping on hard bottoms in 0–7,000 m; 3 mm to 43 cm;…. Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree.... Save 50% off a Britannica Premium subscription and gain access to exclusive content. 5. 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 is known as the anal plate (sometimes called the "tail plate", although chitons do not have tails. About 5 cm (2 inches) is the maximum length of most chitons, but Cryptochiton stelleri, of the Pacific coast of North America, may grow to about 43 cm. 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. Unit 3 - Tides and the Rocky Shore Snails, Limpets and Chitons: Moving On FOR SEA—Institute of Marine Science ©2000 J. Because of this, the shell provides protection at the same time as permitting the chiton to flex upward when it needs to move. Gumboot Chitons Spawning! Updates? Some species live quite high in the intertidal zone and are exposed to the air and light for long periods. The intestine is divided in two by a sphincter, with the latter part being highly coiled and functioning to compact the waste matter into faecal pellets. They are a common food on the Pacific coast of South America and in the Galápagos. In most cases, fertilization takes place either in the surrounding water, or in the mantle cavity of the female. Chitons have separate sexes, and fertilization is usually external. [11], The sculpture of the valves is one of the taxonomic characteristics, along with the granulation or spinulation of the girdle.[11]. [16], Animals which prey on chitons include humans, seagulls, sea stars, crabs, lobsters and fish. 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. Chitons move sluggishly up and down rocks or reefs in the intertidal zone, clamping tightly to the surface when threatened by fish, birds, or other predators. Native Americans of the Pacific coasts of North America eat chitons. Chitons-how do they move?-where are their gills arranged?-what is an adaptation to avoid predators?-what is their diet?-creep along using broad flat foot -gills arranged around foot-grips the substrate making it difficult to dislodge by predators-grazing herbivores. [10], The wide form of girdle ornament suggests it serves a secondary role; chitons can survive perfectly well without them. [22], 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 the ocelli, whose precise function is unclear, likely the most recent eyes to evolve.[2].