To Which Class Of Mollusca Does This Animal Belong?

Introduction to Phylum Mollusca

Fig. 3.52. Glory-of-the-bounding main snail (Conus gloriamaris) is prized by collectors for its ornate shell.

The phylum Mollusca is the 2d-largest animate being phylum, with over 100,000 species. The molluscs include many familiar animals, including clams, snails, slugs, and squid, too as some less familiar animals, like tusk shells and chitons (Fig. 3.51 A). Molluscs are found in nearly all freshwater and marine environments, and some are found also on state. The marine molluscs are probably the best-known and hands recognized members of the phylum. Many of their shells are highly valued by collectors. Shells from the glory-of-the-seas cone snail (Conus gloriamaris), for instance, take gained prices as high as $10,000 (Fig. 3.52). We have learned much of what we know about this grouping from seashell collections.

Fig. 3.51. (A) Loki'south lined chiton (Tonicella lokii; grade Polyplacophora)

Fig. 3.51. (C) Cockle shells (form Bivalvia) Fig. three.51. (D) Bigfin reef squid (Sepioteuthis lessoniana; form Cephalopoda)

There are four major groups inside the phylum Mollusca:

Class Polyplacophora consists of chitons, snail-like molluscs with eight-part overlapping calibration shells (Fig. 3.51 A).
Course Gastropoda are true snails and slugs (Fig. 3.51 B). They represent the nearly diverse grade within phylum Mollusca with 60,000 to 80,000 extant species in marine, freshwater, and terrestrial habitats.
Class Bivalvia are molluscs with hinged two-part shells (Fig. three.51 C). Examples include clams, oysters, mussels, and scallops.
Grade Cephalopoda are molluscs with large heads, large eyes, and grasping tentacles (Fig. 3.51 D). Examples include octopus, squid, cuttlefish, and nautiloids.

A slug, a snail, a clam, and a squid do not look alike, but they are all molluscs. Although at that place is no single feature that all molluscs possess, three features are and then common in molluscs that they are used to distinguish them from organisms in other phyla:

All molluscs accept a specialized foot used in earthworks, grasping, or creeping. The foot is a muscular organ modified into different forms in unlike molluscan classes (Fig. 3.53).
Molluscs take a drapery or mass of soft flesh that covers the soft body and encloses the internal organs. In many species, the mantle produces a hard shell. Not all molluscs produce a shell.
Many molluscs take a radula, which, in well-nigh species, is a rasp-like scraping organ used in feeding (Fig. 3.54). The word derives from the Latin root prefix radul- meaning scraper. Not all molluscs have a radula, but cipher like information technology is found in any other group of organisms. Bivalve molluscs lack a radula.

Fig. iii.53. (A) Red-flecked mopalia chiton (Mopalia spectabilis; grade Polyplacophora) Fig. 3.53. (B) Lettuce bounding main slug (Elysia crispata; class Gastropoda)

Fig. 3.53. (C) Razor mollusk (Ensis sp.; class Bivalvia) Fig. iii.53. (D) Bigfin reef squid (Sepioteuthis lessoniana; form Cephalopoda)

The human foot is a muscular organ found in all molluscs. Polyplacophorans (chitons; Fig. 3.53 A) and gastropods (Fig. 3.53 B) have a single apartment foot used for crawling. Some bivalves, such as clams, take a paddle-shaped human foot adapted for excavation into soft sediments (Fig. 3.53 C).

Because a sea slug's stomach is in its pes, it is named Gastropoda, "stomach-foot" (from the Greek root words gastro pregnant stomach and pod meaning pes; Fig. iii.53 B). The pes in octopus and squid is modified into many tentacles that are attached to the creature'southward head (Fig. 3.53 D). That feature gave the grade its name Cephalopoda (from the Greek root word cephal- significant caput), or the "head-pes" molluscs. Octopus and squid use their tentacles for moving and for grasping and belongings the prey they capture for food.

In most molluscs, the pall produces a hard protective shell. The mantle likewise creates patterns of color on a shell. The shell is an exoskeleton, even though it is completely surrounded past soft tissue in some molluscs. The shell is continually produced and grows with the fauna. Chitons are in the grade Polyplacophora (poly meaning many; placo meaning plate or shell; phora significant begetting). A chiton's mantle produces eight trounce-like plates that cover the body. Joints between the plates let the chiton to curl up in a ball and to move flexibly (Figs. three.51 A and three.53 A). The class of molluscs chosen Bivalvia (from Latin root words bi- meaning two and -valv meaning folding door) includes clams, oysters, mussels, and scallops. Bivalves produce two shells that are hinged at the top (Fig. 3.49 B). The drapery of snails (gastropods) produces a single shell in a screw shape (Fig. 3.49 C). The mantle itself cannot be seen considering it is on the inner surface of the shell. In some gastropods, such as the cowries, the drape extends over the shell, keeping the trounce shiny and new in advent. In other gastropods, like the sea hares, and in some cephalopods, like the squid and the octopus, the shell is very small-scale and the mantle covers the beat out completely (Fig. three.49 D). The nudibranchs, or sea slugs (nudi- meaning naked, -co-operative meaning gill), are gastropods that don't produce a shell, so these animals are all soft-bodied (Fig. 3.49 Due east). The chambered nautilus is one cephalopod that secretes an external shell. Squid and cuttlefish produce internal shells that are independent within the drapery, and octopus do not produce shells at all.

Fig. 3.54. Diversity of radula forms in gastropod molluscs Fig. 3.55. Herbivorous gastropod radula scraping nutrient Fig. three.56. Carnivorous gastropod radulas are specialized harpoon structures

The mouth structures of many molluscs include a specially adapted rasp-like tongue called a radula. The radula is a hard ribbon-shaped construction covered in rows of teeth. Herbivorous snails have a mouth with a radula of ordinarily five to seven circuitous teeth. There is a neat multifariousness of radula forms in the mollusca (Fig. iii.54). The snail uses its radula like a file, rasping it back and forth over the substrate to scrape off small bits of food (Fig. 3.55). Every bit radular teeth wear down or intermission off, new teeth are formed to supersede 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 chosen cone snails are cannibal (meat-eating) hunters that produce venom in glands virtually the mouth. Their radulas are shaped into long, hollow teeth, which they thrust one at a time into their casualty like harpoons (Fig. 3.56). A barbed radular molar fires through the proboscis, which is an extension of the mouth. It pierces the prey, paralyzing it with venom and preventing its escape. The cone snail "swallows" the prey by engulfing it with its proboscis. In this way cones stem 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 fretfulness.

Fig. 3.57. Molluscs breathe using gills called ctenidia every bit shown in the sea slug, Pleurobranchaea meckelii.

Molluscs exhale with gills called ctenidia that sit in a cavity between the pall and torso mass (Fig. 3.57). In some molluscs, most notably bivalves like oysters and mussels, the ctenidia are also used as filter feeding appliance to strain particulate food from the water. Molluscs have a complete digestive tract surrounded by a small-scale coelom. The molluscan circulatory system is composed of a serial of claret 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 organisation of the brainless bivalves to the complex systems of the cephalopods, who have well-developed brains and are considered the nearly intelligent of invertebrates.

Class Polyplacophora

Fig. iii.58. (A) Lined chiton (Tonicella lineata) Fig. 3.58. (B) Butterfly chiton (Cryptoconchus porosus)

Fig. three.58. (C) Forest chiton (Mopalia lignosa)

Chitons (Polyplacophora) are basal relative to other extant molluscs (Fig. 3.58). Their soft bodies are covered with a serial of eight shell plates. The joints between these shell plates enable to chitons to roll upward for protection. Chitons are mobile and contract their muscular human foot in waves to motility about. The primarily herbivorous chitons have a well-developed radula. Their nervous system is a series of ladder-like fretfulness and only a few species have poorly adult ganglia. Chitons are plant simply marine environments. They are about commonly establish in tide pools and rocky intertidal zones. Chitons can tolerate the harsh conditions of these habitats where ocean and land run across.

Class Gastropoda

Fig. three.59. (A) Common limpets (Patella vulgata) Fig. 3.59. (B) Tiger cowrie (Cypraea tigris) with soft mantle extended over shell

Fig. 3.59. (C) An aeolid nudibranch Fig. three.59. (D) Cherry-red-lined bubble snail (Bullina lineata)

Gastropods are the most diverse group of molluscs (Fig. 3.59). The ones we usually think of are snails and slugs. Nearly gastropods have a calcareous beat protecting the soft-bodied creature within. Some gastropods, such equally ocean slugs, body of water hares, and garden slugs, lack a beat or have a reduced shell cached in the folds of their pall. About creep almost on a flattened foot, but some swim, using extended folds of their mantle equally fins. Almost snails and terrestrial slugs are herbivorous. They use their radula to scrape algae from surfaces (Fig. 3.55) or to pierce found parts. For this reason, gardeners consider snails and slugs to be pests. Some gastropods are carnivores, stalking other snails, worms, and fish for food (Fig. 3.56). The colorful and striking nudibranchs incorporate many cannibal specialists. Many nudibranchs feed on merely one type of sponge; their body coloration and their eggs are patterned to alloy in with their prey. Other gastropods use their radula and acidic secretions to bore holes in shells and prey on other molluscs.

Fig. three.60. (A) Giant East African land snail (Achatina fulica) Fig. 3.60. (B) Terrestrial cannibal snail (Euglandina rosea)

Fig. 3.41. (E) Rat lungworm (Angiostrongylus cantonensis), a nematode parasite that can cause meningitis

In the Hawaiian Islands, the terrestrial cannibal snail (Euglandina rosea; Fig. 3.sixty A) was introduced to command the behemothic East African land snail (Achatina fulica; Fig. three.60 B). The giant East African country snail is considered to be an agronomical pest and is too known to be a carrier of the parasitic nematode rat lungworm (Angiostrongylus cantonensis; Fig. 3.41 Eastward) that can crusade the encephalon affliction meningitis. Unfortunately, the cannibal snails also predated native country 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 drape cavity. In the nudibranchs (nudi- meaning naked, -branch meaning gill), these ctenidia are exposed on the outside of the animate being'due south body. This distinctive trait makes nudibranchs an easily identifiable grouping of molluscs. Terrestrial slugs and snails, by dissimilarity are primarily in a subgroup known as the pulmonates that actually accept a drapery cavity that has become connected to the circulatory system (vascularized) to function every bit a lung.

Gastropods motility by contracting their muscular foot in a series of waves to creep forrad. Many gastropods secrete mucus (so called "snail trails") to help facilitate movement. These trails also provide chemical communication among gastropods. The cannibal snail, for example, tracks its casualty by following the mucus trail left behind. The gastropod nervous system includes bodily fretfulness and inductive ganglia with relatively sophisticated sensory systems, including light receptors and well adult chemosensory abilities.

Form Bivalvia

Fig. 3.61. (A) Hard-crush clams (Mercenaria mercenaria)

Fig. 3.61. (C) Atlantic bay scallop (Argopecten irradians)

The bivalve molluscs become their name from the two door-like valves or shells that make up their exoskeleton (Fig. iii.61). Foot size varies amidst marine bivalves. Clams have a muscular hatchet-shaped human foot for moving near and for burrowing in mud or sand (Fig. three.62). Past contrast, an oyster'due south or a mussel's foot is pocket-size because these animals attach themselves to hard objects early in life and do not movement around. Scallops don't utilise their minor pes to motility around either. They swim in curt bursts past jet propulsion, clapping their shells together and forcing water out the rim.

Fig. three.62. A mollusk using its foot to motion

Fig. iii.63. Diagram of the internal anatomy of a mollusk

Bivalves are more than enclosed by their shells than other molluscs. Water enters and leaves a bivalve past way of two tubes chosen siphons. One siphon takes in water while the other expels h2o and waste. The water taken in contains oxygen and food particles. Most bivalve species acquire energy and nutrients through filter feeding. Filter feeding or suspension feeding is the process of ingesting water and filtering out food particles. Invertebrate examples of filter feeders include sponges, corals, and bivalve molluscs. As water is taken into the torso, it flows across the gills. Oxygen (O₂) and carbon dioxide (CO₂) are exchanged between the circulatory system and the water. Mucus on the gills traps microscopic food particles, and tiny hairlike cilia move the food-laden fungus toward the mouth. Liplike structures chosen palps help sort the nutrient and straight information technology into the mouth. Bivalves do not have a radula (Fig. 3.63). The nutrient suspended in fungus moves through the digestive organs, which break it down and absorb it.

Bivalves such every bit clams, oysters, and scallops are valuable every bit food. They brand 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 sure times of twelvemonth, 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 tin can be fatal to humans.

Fig. three.64. Layers of nacre inside a black-lip pearl oyster (Pinctada margaritifera) beat out with all living tissue removed. A pearl is pocket-sized particle lodged between the drape and shell that becomes covered in a nacreous layer.

In some bivalves, such equally oysters, pall tissue secretes nacre (pronounced "NAY ker"), a pearly substance that coats any irritating foreign particles that lodge between the mantle and the shell (Fig. three.64). A pearl forms equally coats of nacre build upwardly around the foreign particle. Cultured pearls used in jewelry are produced when subcontract-raised oysters are intentionally seeded with foreign particles to stimulate the production of nacre. The cost of pearls varies with size, color, and luster. Earlier plastic came into utilise, the shells of bivalves were commonly used to make buttons. The cloth known equally mother-of-pearl is harvested from the nacre of mollusc shells.

Class Cephalopoda

The cephalopods are molluscs with big heads and tentacles. Examples of cephalopod molluscs include squid, octopus, cuttlefish, and nautilus (Fig. iii.65). Nigh cephalopods are relatively small. Only the giant octopus (Enteroctopus sp.), which lives along the west coast of the U.s.a., can grow to 1.5 m or more. The giant squid, the largest invertebrate, reaches lengths of 15 one thousand.

Fig. three.65. (A) Caribbean reef squid (Sepioteuthis sepioidea) Fig. 3.65. (B) The coconut octopus (Amphioctopus marginatus) has been observed carrying kokosnoot shells and mollusc shells while moving along the seafloor.

Fig. 3.65. (C) Cuttlefish (Sepia sp.)

The foot in this group has specialized by dividing into artillery that are attached to the head, thus the name cephalopod, meaning head-foot. Like other molluscs, cephalopods take a drapery and mantle crenel that houses the respiratory ctenidia. The curtain cavity is also used to take in and chop-chop expel water to facilitate the jet propulsion pond mode of well-nigh 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 artillery together and can utilize 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-h2o cephalopods can even produce luminescent ink.

Fig. 3.66. (A) Drawing of beak parts Fig. three.66. (B) Dissected beak shown with muscle attachments

Cephalopods also have a pocket-size radula, merely the radula is non used for food capture. In the oral cavity of the squid is a bill shaped much similar the beak of a parrot. Figure iii.66 shows the giant squid (Architeuthis sp.) beaks . The beak is not office of the shell but a separate tooth-like structure. When a squid catches prey, such equally a fish, information technology bites off and swallows chunks of it. The octopus spends about of its time itch around the bottom, capturing prey with its arms and the suction cups lining the inner surfaces of the arms. Later capturing its prey, the octopus bites it, injecting both a toxicant and digestive enzymes. The enzymes soften the food earlier the octopus sucks information technology into its stomach for further digestion. The tiny blue-ringed octopus of the Indo-Pacific region has developed particularly potent 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 upwards the little octopus and received a defensive bite.

Fig. three.67. The nautilus (Nautilus spp.) has geometric chambers information technology uses for buoyancy, equally shown in this shell that has been cut in half. Fig. 3.68. The pen in a squid acts to go on the body rigid.

Most cephalopods do not have external shells. The nautilus is the but living exception, having a complete, well-developed shell separated into geometrically precise chambers. (Figs. iii.65 D and 3.67). These chambers contain gas that the creature produces to regulate changes in buoyancy when information technology moves to shallower or deeper h2o. The amount of gas in the chamber changes, and so that the nautilus rests, rises, or sinks. The squid has an internal remnant of a beat out, called a pen, that looks like a sheet of thick plastic (Fig. 3.68). This long, thin shell helps back up the trunk. The cuttlefish, a shut relative of the squid, has a harder, more than breakable plate, chosen a cuttlebone (Fig. 3.69). The cuttlebone in cuttlefish helps to keep the body rigid. A cuttlebone is made of calcium carbonate secreted by the animal; in composition information technology 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 vanquish at all. Its just hard body part is its bill, which, equally in the squid, is not a remnant of the crush. Because an octopus has no hard skeleton, its soft body tin can squeeze through tiny openings in a reef and hide in crevices or between rocks (Fig. 3.70). Octopuses in aquariums are notorious for their power to escape.

Fig. three.69. (A) Dorsal view of a cuttlebone Fig. 3.69. (B) Ventral view of a cuttlebone

Fig. 3.70. (A) Octopus hiding in a mollusc vanquish

Cephalopods are also masters of camouflage, using pigments in their peel cells to chop-chop change their skin color to alloy into their environs (Fig. three.71). These skin cells, called chromatophores, contain an elastic sac filled with pigment. The cells are attached to a gear up of muscle cells. When the muscle cells contract, they pull the chromatophore out flat, spreading the paint over a larger expanse and making the skin darker. When the chromatophore is smaller, the skin appears lighter. Chromatophores can change shape very apace, producing a pulsating pattern of circuitous color changes. Squid and cuttlefish also have chromatophores, which they use to communicate with fellow squid. These cephalopods can also wave paddle-shaped fins to move slowly forward or backward. The complex communication systems of cephalopods emphasize the highly developed nervous systems of these animals. Unlike other molluscs, cephalopods have fairly well-developed brains and accept prototype-forming eyes similar to those of vertebrates (with the exception of the nautilus that has a more basic eye). Octopuses have been trained in captivity and fifty-fifty demonstrate rudimentary learning abilities.

Fig. 3.71. (A) A reef cuttlefish (Sepia latimanus) changing from darker pink coloration to light pink-white coloration

Fig. iii.71. (B) Chromophores visible on Caribbean area reef squid (Sepioteuthis sepioidea) Fig. 3.71. (C) 2 Caribbean reef squid (Sepioteuthis sepioidea) exhibiting chromatophore colour changes