Chitons crawl along rocks and forage for food mostly algae using their radulae to scrape it off of the substrate. Magnetite is used for hardening the teeth of the radula in chitons, so they can scrape coraline algae off of rocks. These teeth are so hard that they can etch glass! These creatures are extremely slow moving. In a year, a chiton may not move more than ten feet.
They can detect the presence of light with primitive eyes embedded in the shell plates. The bivalves meaning "two-shells" are perhaps the most well known mollusks simply because of their history as a source of food. Clams, mussels, oysters and scallops are all bivalves. The bivalve may either burrow through the bottom, or attach itself to the substrate with glue-like strings called "byssal threads.
Most bivalves feed by filtering organic particles from water, and therefore do not have a radula. The gills are used in feeding by means of a mucous coating which traps food particles as water passes through them.
Some of the giant clams found in the Pacific have symbiotic algae in their mantle tissue, and can utilize the light of the sun to make food, although they still filter feed as well.
The shell is generated by the mantle from the inside. Pearls are made by clams, oysters and mussels when a grain of sand or other small irritant becomes painfully stuck in the mantle of the creature. The bivalve coats the irritant with the same material which is secreted to produce the inner lining of the shell. This makes the irritant smooth, and theoretically, less painful to the bivalve.
Although many people think of pearls as coming only from oysters, most bivalves can produce pearls, as well as some snails, like the conch. Although we usually think of Mollusks as benthic bottom dwelling , the cephalopods have taken to a nektonic swimming , rather than benthic, existence in the ocean. Squid, octopods, cuttlefish and nautiluses are all members of the class cephalopoda, meaning "head-footed.
The "feet" usually called arms or mistakenly called tentacles are attached to the part of the body containing the eyes the "head" while the rest of the body is out in front of the head.
Thus, the body does not connect directly to the arms. The cephalopods may not seem very closely related to the other mollusks, but physiologically, they are similar in internal construction. Perhaps the most obvious difference between most cephalopods and other mollusks is the apparent lack of a shell. The octopods do not have shells at all, and the squid have a small chitinized internal shell. Nautiluses are the only Cephalopods with an external shell.
Nautiluses are found in the South Pacific and Indian oceans. Cephalopods have the most well developed nervous systems of all mollusks, as well as the most well developed eye.
The cephalopod eye is one of the most notable examples of convergent evolution in all of the animal world, because this eye evolved from a completely different direction than the mammalian eye, yet it turned out to function in almost the exact same way.
Cephalopods, therefore, have extremely good eyesight. This eyesight is well suited for finding prey. The cephalopod then grasps the prey firmly with its arms and eats the prey with a mouth located between the arms.
The mantle of snails gastropods produces a single shell in a spiral shape Fig. The mantle itself cannot be seen because it is on the inner surface of the shell. In some gastropods, such as the cowries, the mantle extends over the shell, keeping the shell shiny and new in appearance.
In other gastropods, like the sea hares, and in some cephalopods, like the squid and the octopus, the shell is very small and the mantle covers the shell completely Fig. The chambered nautilus is one cephalopod that secretes an external shell. Squid and cuttlefish produce internal shells that are contained within the mantle, and octopus do not produce shells at all. The mouth structures of many molluscs include a specially adapted rasp-like tongue called a radula. The radula is a hard ribbon-shaped structure covered in rows of teeth.
Herbivorous snails have a mouth with a radula of usually five to seven complex teeth. There is a great diversity of radula forms in the mollusca Fig.
The snail uses its radula like a file, rasping it back and forth over the substrate to scrape off small bits of food Fig. As radular teeth wear down or break off, new teeth are formed to replace them.
The tooth patterns of snail radulas are distinctive to species, and scientists can identify snails by looking at their radulas. Some radulas are highly specialized.
A group of gastropods called cone snails are carnivorous meat-eating hunters that produce venom in glands near the mouth. Their radulas are shaped into long, hollow teeth, which they thrust one at a time into their prey like harpoons Fig.
A barbed radular tooth fires through the proboscis, which is an extension of the mouth. It pierces the prey, paralyzing it with venom and preventing its escape. In this way cones stalk and capture worms, molluscs, and even fish. Some cones produce a poison strong enough to kill humans who handle them carelessly. Their poison is a neurotoxin that attacks and destroys nerves.
Molluscs breathe with gills called ctenidia that sit in a cavity between the mantle and body mass Fig. In some molluscs, most notably bivalves like oysters and mussels, the ctenidia are also used as filter feeding apparatus to strain particulate food from the water. Molluscs have a complete digestive tract surrounded by a small coelom.
The molluscan circulatory system is composed of a series of blood sinuses or cavities, rather than closed, discrete vessels. This is referred to as an open circulatory system. Molluscs display a large diversity of nervous systems, from the rudimentary nervous system of the brainless bivalves to the complex systems of the cephalopods, who have well-developed brains and are considered the most intelligent of invertebrates.
Chitons Polyplacophora are basal relative to other extant molluscs Fig. Their soft bodies are covered with a series of eight shell plates. The joints between these shell plates enable to chitons to roll up for protection. Chitons are mobile and contract their muscular foot in waves to move about.
The primarily herbivorous chitons have a well-developed radula. Their nervous system is a series of ladder-like nerves and only a few species have poorly developed ganglia. Chitons are found only marine environments. They are most commonly found in tide pools and rocky intertidal zones. Chitons can tolerate the harsh conditions of these habitats where ocean and land meet. Gastropods are the most diverse group of molluscs Fig. The ones we usually think of are snails and slugs.
Most gastropods have a calcareous shell protecting the soft-bodied animal inside. Some gastropods, such as sea slugs, sea hares, and garden slugs, lack a shell or have a reduced shell buried in the folds of their mantle. Most creep about on a flattened foot, but some swim, using extended folds of their mantle as fins.
Most snails and terrestrial slugs are herbivorous. They use their radula to scrape algae from surfaces Fig. For this reason, gardeners consider snails and slugs to be pests. Some gastropods are carnivores, stalking other snails, worms, and fish for food Fig. The colorful and striking nudibranchs contain many carnivorous specialists. Many nudibranchs feed on only one type of sponge; their body coloration and their eggs are patterned to blend in with their prey.
Other gastropods use their radula and acidic secretions to bore holes in shells and prey on other molluscs. In the Hawaiian Islands, the terrestrial cannibal snail Euglandina rosea ; Fig. The giant East African land snail is considered to be an agricultural pest and is also known to be a carrier of the parasitic nematode rat lungworm Angiostrongylus cantonensis ; Fig.
Unfortunately, the cannibal snails also predated native land snails, nearly driving them to extinction. Marine and freshwater gastropods breathe using ctenidia or gills. In many of these gastropods the ctenidia are protected within the mantle cavity. This distinctive trait makes nudibranchs an easily identifiable group of molluscs.
Terrestrial slugs and snails, by contrast are primarily in a subgroup known as the pulmonates that actually have a mantle cavity that has become connected to the circulatory system vascularized to function as a lung. Gastropods move by contracting their muscular foot in a series of waves to creep forward.
These trails also provide chemical communication among gastropods. The cannibal snail, for example, tracks its prey by following the mucus trail left behind. The gastropod nervous system includes bodily nerves and anterior ganglia with relatively sophisticated sensory systems, including light receptors and well developed chemosensory abilities. The bivalve molluscs get their name from the two door-like valves or shells that make up their exoskeleton Fig. Foot size varies among marine bivalves. Clams have a muscular hatchet-shaped foot for moving about and for burrowing in mud or sand Fig.
They swim in short bursts by jet propulsion, clapping their shells together and forcing water out the rim. Bivalves are more enclosed by their shells than other molluscs. Water enters and leaves a bivalve by way of two tubes called siphons. One siphon takes in water while the other expels water and waste.
The water taken in contains oxygen and food particles. Most bivalve species acquire energy and nutrients through filter feeding. English Language Learners Definition of mollusk. Kids Definition of mollusk. Medical Definition of mollusk. Get Word of the Day daily email! Test Your Vocabulary. Can you spell these 10 commonly misspelled words? Love words? Need even more definitions? Homophones, Homographs, and Homonyms The same, but different.
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