What do mollusks use their feet for

Liplike structures called palps help sort the food and direct it into the mouth. Bivalves do not have a radula Fig. The food suspended in mucus moves through the digestive organs, which break it down and absorb it.

Bivalves such as clams, oysters, and scallops are valuable as food. They make up a major share of the marine invertebrate seafood industry. 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.

In creatures such as squid, the shell became small, moved inside the body and all but vanished. Another adaptation lay in brainpower.

Octopus and cuttlefish think, learn and react to their environment in ways surpassed only by vertebrates. If you have a soft body without a shell in the middle of the ocean, being clever is certainly one worthwhile strategy for survival. It can grow to over 50 feet in length with eyes the size of basketballs. Most mollusks are dioecious animals and fertilization occurs externally, although this is not the case in terrestrial mollusks, such as snails and slugs, or in cephalopods.

In some mollusks, the zygote hatches and undergoes two larval stages— trochophore and veliger —before becoming a young adult; bivalves may exhibit a third larval stage, glochidia. Phylum Mollusca is a very diverse 85, species group of mostly marine species. Mollusks have a dramatic variety of form, ranging from large predatory squids and octopus, some of which show a high degree of intelligence, to grazing forms with elaborately sculpted and colored shells. This phylum can be segregated into seven classes: Aplacophora, Monoplacophora, Polyplacophora, Bivalvia, Gastropoda, Cephalopoda, and Scaphopoda.

Figure 2. This chiton from the class Polyplacaphora has the eight-plated shell that is indicative of its class. These animals lack a calcareous shell but possess aragonite spicules on their epidermis.

They have a rudimentary mantle cavity and lack eyes, tentacles, and nephridia excretory organs. The morphology of the shell and the underlying animal can vary from circular to ovate. A looped digestive system, multiple pairs of excretory organs, many gills, and a pair of gonads are present in these animals. The monoplacophorans were believed extinct and only known via fossil records until the discovery of Neopilina galathaea in Today, scientists have identified nearly two dozen extant species.

These animals have a broad, ventral foot that is adapted for suction to rocks and other substrates, and a mantle that extends beyond the shell in the form of a girdle. Calcareous spines may be present on the girdle to offer protection from predators. Respiration is facilitated by ctenidia gills that are present ventrally.

These animals possess a radula that is modified for scraping. Eyespots are absent in these animals. A single pair of nephridia for excretion is present. Figure 3. These mussels, found in the intertidal zone in Cornwall, England, are bivalves. Members of this class are found in marine as well as freshwater habitats. The overall morphology is laterally flattened, and the head region is poorly developed. Eyespots and statocysts may be absent in some species.

These animals are suspension feeders—they eat material, such as plankton, that is suspended in the water around them. Due to their diet, this class of mollusks lacks a radula. Respiration is facilitated by a pair of ctenidia, whereas excretion and osmoregulation are brought about by a pair of nephridia. Bivalves often possess a large mantle cavity. In some species, the posterior edges of the mantle may fuse to form two siphons that serve to take in and exude water.

One of the functions of the mantle is to secrete the shell. This property has been commercially exploited to produce pearls. Gastropoda includes shell-bearing species as well as species with a reduced shell.

Learning Objectives Describe the unique anatomical and morphological features of molluscs. The visceral mass inside the mollusk includes digestive, nervous, excretory, reproductive, and respiratory systems.

Most mollusks possess a radula, which is similar to a tongue with teeth-like projections, serving to shred or scrape food. The mantle is the dorsal epidermis in mollusks; in some mollusks it secretes a chitinous and hard calcareous shell. Key Terms visceral mass : the soft, non-muscular metabolic region of the mollusc that contains the body organs mantle : the body wall of a mollusc, from which the shell is secreted radula : the rasping tongue of snails and most other mollusks.

Phylum Mollusca Phylum Mollusca is the predominant phylum in marine environments.