Bivalves should not be eaten when the water in which they grow becomes polluted with chemicals or disease organisms. At certain times of year, microscopic organisms called dinoflagellates multiply rapidly in nearshore waters.
Toxic substances produced by dinoflagellates can concentrate in the clams and oysters that use them as food. Although the bivalves are not harmed, the toxin can attack the nervous system of humans who eat the tainted shellfish. Toxic shellfish poisoning can be fatal to humans. A pearl forms as coats of nacre build up around the foreign particle. Cultured pearls used in jewelry are produced when farm-raised oysters are intentionally seeded with foreign particles to stimulate the production of nacre.
The cost of pearls varies with size, color, and luster. Before plastic came into use, the shells of bivalves were commonly used to make buttons.
The material known as mother-of-pearl is harvested from the nacre of mollusc shells. The cephalopods are molluscs with large heads and tentacles. Examples of cephalopod molluscs include squid, octopus, cuttlefish, and nautilus Fig.
Most cephalopods are relatively small. But the giant octopus Enteroctopus sp. The giant squid, the largest invertebrate, reaches lengths of 15 m. The foot in this group has specialized by dividing into arms that are attached to the head, thus the name cephalopod, meaning head-foot.
Like other molluscs, cephalopods have a mantle and mantle cavity that houses the respiratory ctenidia. The mantle cavity is also used to take in and rapidly expel water to facilitate the jet propulsion swimming mode of most cephalopods.
When the mantle closes forcefully, seawater ejected through the siphon propels the animal in short bursts. Both squid and octopus change course by redirecting their siphon.
They steer by pressing their arms together and can use their speed to elude an attacking predator. They can also squirt ink from the ink sac into the water, creating an ink cloud for camouflage and confusing the predator. Deep-water cephalopods can even produce luminescent ink. Cephalopods also have a small radula, but the radula is not used for food capture.
In the mouth of the squid is a beak shaped much like the beak of a parrot. Figure 3. The beak is not part of the shell but a separate tooth-like structure. When a squid catches prey, such as a fish, it bites off and swallows chunks of it. The octopus spends most of its time crawling around the bottom, capturing prey with its arms and the suction cups lining the inner surfaces of the arms.
After capturing its prey, the octopus bites it, injecting both a poison and digestive enzymes. The enzymes soften the food before the octopus sucks it into its stomach for further digestion.
The tiny blue-ringed octopus of the Indo-Pacific region has developed particularly strong venom that is used for defense as well. The venom of this octopus is potent and has been implicated in the deaths of several humans who unknowingly picked up the little octopus and received a defensive bite. Most cephalopods do not have external shells. The nautilus is the only living exception, having a complete, well-developed shell separated into geometrically precise chambers.
These chambers contain gas that the animal produces to regulate changes in buoyancy when it moves to shallower or deeper water. The amount of gas in the chamber changes, so that the nautilus rests, rises, or sinks. The squid has an internal remnant of a shell, called a pen , that looks like a sheet of thick plastic Fig. This long, thin shell helps support the body. The cuttlefish, a close relative of the squid, has a harder, more brittle plate, called a cuttlebone Fig.
The cuttlebone in cuttlefish helps to keep the body rigid. A cuttlebone is made of calcium carbonate secreted by the animal; in composition it is similar to the shells of other molluscs. Gas moving in and out of chambers in the cuttlebone lets the cuttlefish move up and down in the water.
The octopus has no shell at all. Its only hard body part is its beak, which, as in the squid, is not a remnant of the shell. Because an octopus has no hard skeleton, its soft body can squeeze through tiny openings in a reef and hide in crevices or between rocks Fig. Octopuses in aquariums are notorious for their ability to escape.
Cephalopods are also masters of camouflage, using pigments in their skin cells to quickly change their skin color to blend into their surroundings Fig. These skin cells, called chromatophores , contain an elastic sac filled with pigment. The cells are attached to a set of muscle cells.
When the muscle cells contract, they pull the chromatophore out flat, spreading the pigment over a larger area and making the skin darker. All of the snails of the genus Allogona are united by a few common characteristics. The shells of each of the species are umbilicate, have a reflected white peristome, and no more than a single, low blunt, tooth. They also have some similarities between their soft anatomies Leonard , This snail is also part of the family Polygyridae and is united to this family by common anatomical and physiological attributes.
Shells are of little value when classifying families, but all polygyrids have lense-shaped to globose shells with a reflected lip on the peristome Leonard , When snails multiply and damage crops or in some way affect the species of a region or the human being, they are considered pests. Certain species grow up to the size of an adult hand, and of course, their feeding needs also increase.
That is why it is important to pay attention to the recommendations about the handling and care of snails. In conclusion, snails are much more than a shell.
On this site, we talk about land snails. Therefore we implicitly exclude freshwater and sea snails and refer only to the terrestrial gastropod mollusks that have shells.
Cambrian period A period of the Paleozoic era, from to million years ago. Snails tend to feed on a variety of items found in their natural habitat. What they will actually consume depends on where they live and the species of snail that they are. Read More… Snails for Kids. Being a Gastropod The most striking physical feature of snails is their spiral shell that they load on the back. Top Facts about Snails The largest land snail recorded was 12 inches long and weighed near 2 pounds.
Garden snails helix apersa a top speed of 50 yards per hour, this is about 1. Read More…. Snail FAQs What do snails eat? Snails for Kids Snails are so diverse that there are land snails, sea snails and freshwater snails. There are thousands of species of snails. Arthropoda Mollusca Echinodermata Chordata. Know more about Biology and ace the concept of Zoology.
Electric charges and coulomb's law Basic. Start Now. The correct answer is Mollusca. Key Points Mollusca is the second-largest invertebrate animal phylum after Arthropoda. Snails and slugs belong to a broad taxonomic class of invertebrates within the phylum of Mollusca called the Gastropoda. They are a large part of the phylum Mollusca and the most widely diversified class in the phylum, with between 65, and 80, live snail and slug species.
The Gastropoda class comprises a vast majority of named animals, second only to the total number of insects. Get Started for Free Download App. More Zoology Questions Q1. The disease of scabies fever in animals is caused by.
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