What Percent Of The Animal Kingdom Do Vertebrates Makeup Mollusca

Introduction to Phylum Mollusca

Fig. 3.52. Celebrity-of-the-ocean snail (Conus gloriamaris) is prized by collectors for its ornate shell.

The phylum Mollusca is the second-largest animal phylum, with over 100,000 species. The molluscs include many familiar animals, including clams, snails, slugs, and squid, as well every bit some less familiar animals, similar tusk shells and chitons (Fig. 3.51 A). Molluscs are found in nearly all freshwater and marine environments, and some are plant also on land. The marine molluscs are probably the best-known and hands recognized members of the phylum. Many of their shells are highly valued past collectors. Shells from the glory-of-the-seas cone snail (Conus gloriamaris), for example, accept gained prices as high equally $10,000 (Fig. three.52). We have learned much of what nosotros know about this group from seashell collections.

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

Fig. iii.51. (C) Crinkle shells (course Bivalvia) Fig. three.51. (D) Bigfin reef squid (Sepioteuthis lessoniana; form Cephalopoda)

There are four major groups within the phylum Mollusca:

Form Polyplacophora consists of chitons, snail-like molluscs with eight-part overlapping scale shells (Fig. 3.51 A).
Course Gastropoda are true snails and slugs (Fig. 3.51 B). They represent the near various grade inside phylum Mollusca with threescore,000 to 80,000 extant species in marine, freshwater, and terrestrial habitats.
Class Bivalvia are molluscs with hinged 2-role shells (Fig. 3.51 C). Examples include clams, oysters, mussels, and scallops.
Class Cephalopoda are molluscs with big heads, large eyes, and grasping tentacles (Fig. iii.51 D). Examples include octopus, squid, cuttlefish, and nautiloids.

A slug, a snail, a clam, and a squid do not look alike, simply they are all molluscs. Although in that location is no unmarried feature that all molluscs possess, three features are so mutual in molluscs that they are used to distinguish them from organisms in other phyla:

All molluscs have a specialized human foot used in digging, grasping, or creeping. The pes is a muscular organ modified into unlike forms in different molluscan classes (Fig. 3.53).
Molluscs have a mantle or mass of soft mankind that covers the soft trunk and encloses the internal organs. In many species, the mantle produces a difficult shell. Not all molluscs produce a shell.
Many molluscs take a radula, which, in most 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 nada like information technology is found in any other group of organisms. Bivalve molluscs lack a radula.

Fig. 3.53. (A) Red-flecked mopalia chiton (Mopalia spectabilis; class Polyplacophora)

Fig. 3.53. (C) Razor clam (Ensis sp.; class Bivalvia)

The foot is a muscular organ establish in all molluscs. Polyplacophorans (chitons; Fig. 3.53 A) and gastropods (Fig. 3.53 B) have a unmarried flat foot used for crawling. Some bivalves, such every bit clams, accept a paddle-shaped pes adjusted for excavation into soft sediments (Fig. 3.53 C).

Considering a sea slug'south stomach is in its foot, it is named Gastropoda, "breadbasket-foot" (from the Greek root words gastro pregnant tum and pod pregnant foot; Fig. 3.53 B). The foot in octopus and squid is modified into many tentacles that are attached to the animal'due south head (Fig. 3.53 D). That feature gave the course its proper name Cephalopoda (from the Greek root word cephal- meaning head), or the "head-pes" molluscs. Octopus and squid employ their tentacles for moving and for grasping and holding the prey they capture for nutrient.

In most molluscs, the mantle produces a hard protective shell. The mantle as well creates patterns of color on a shell. The shell is an exoskeleton, even though it is completely surrounded by soft tissue in some molluscs. The beat is continually produced and grows with the animal. Chitons are in the course Polyplacophora (poly meaning many; placo meaning plate or shell; phora meaning bearing). A chiton's mantle produces eight shell-similar plates that embrace the body. Joints betwixt the plates allow the chiton to scroll upwardly in a ball and to move flexibly (Figs. 3.51 A and 3.53 A). The course of molluscs called Bivalvia (from Latin root words bi- pregnant two and -valv meaning folding door) includes clams, oysters, mussels, and scallops. Bivalves produce two shells that are hinged at the height (Fig. iii.49 B). The mantle of snails (gastropods) produces a unmarried shell in a spiral shape (Fig. 3.49 C). The mantle itself cannot be seen because information technology is on the inner surface of the shell. In some gastropods, such as the cowries, the mantle extends over the beat out, keeping the shell shiny and new in appearance. In other gastropods, like the ocean hares, and in some cephalopods, like the squid and the octopus, the beat out is very small and the pall covers the shell completely (Fig. three.49 D). The nudibranchs, or sea slugs (nudi- significant naked, -branch meaning gill), are gastropods that don't produce a shell, so these animals are all soft-bodied (Fig. iii.49 Eastward). 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.

Fig. 3.54. Variety of radula forms in gastropod molluscs Fig. 3.55. Herbivorous gastropod radula scraping food Fig. 3.56. Carnivorous gastropod radulas are specialized harpoon structures

The mouth structures of many molluscs include a especially adjusted 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 v to seven circuitous teeth. At that place is a corking diversity of radula forms in the mollusca (Fig. 3.54). The snail uses its radula similar a file, rasping it back and along over the substrate to scrape off small bits of food (Fig. 3.55). Equally radular teeth wearable down or break off, new teeth are formed to supercede 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 i at a time into their prey like harpoons (Fig. 3.56). A spinous radular molar fires through the proboscis, which is an extension of the mouth. It pierces the casualty, 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 fifty-fifty fish. Some cones produce a poison stiff plenty to impale humans who handle them carelessly. Their poison is a neurotoxin that attacks and destroys nerves.

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

Molluscs exhale with gills chosen ctenidia that sit in a cavity between the drape and body mass (Fig. 3.57). In some molluscs, nearly notably bivalves like oysters and mussels, the ctenidia are besides used equally filter feeding appliance to strain particulate food from the water. Molluscs have a complete digestive tract surrounded by a modest coelom. The molluscan circulatory system is composed of a series of blood sinuses or cavities, rather than closed, detached vessels. This is referred to as an open up circulatory arrangement. Molluscs brandish a large multifariousness 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 nearly intelligent of invertebrates.

Class Polyplacophora

Fig. three.58. (A) Lined chiton (Tonicella lineata)

Fig. 3.58. (C) Woods chiton (Mopalia lignosa)

Chitons (Polyplacophora) are basal relative to other extant molluscs (Fig. three.58). Their soft bodies are covered with a series of eight beat out plates. The joints between these shell plates enable to chitons to gyre upward 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 take poorly developed ganglia. Chitons are plant only marine environments. They are most commonly found in tide pools and rocky intertidal zones. Chitons tin tolerate the harsh atmospheric condition of these habitats where ocean and state meet.

Class Gastropoda

Fig. 3.59. (A) Common limpets (Patella vulgata)

Fig. 3.59. (C) An aeolid nudibranch

Gastropods are the most various group of molluscs (Fig. 3.59). The ones we ordinarily call back of are snails and slugs. Well-nigh gastropods have a calcareous beat protecting the soft-bodied animal inside. Some gastropods, such every bit bounding main slugs, body of water hares, and garden slugs, lack a shell or accept a reduced shell buried in the folds of their mantle. Most pitter-patter near on a flattened foot, just 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. 3.55) or to pierce establish 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 comprise many cannibal specialists. Many nudibranchs feed on merely 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.

Fig. iii.60. (A) Giant Due east African country snail (Achatina fulica) Fig. 3.60. (B) Terrestrial cannibal snail (Euglandina rosea)

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

In the Hawaiian Islands, the terrestrial carnivorous snail (Euglandina rosea; Fig. three.60 A) was introduced to control the giant E African country snail (Achatina fulica; Fig. three.sixty B). The giant East African country 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. 3.41 E) that can cause the brain affliction meningitis. Unfortunately, the carnivorous snails also predated native state snails, virtually driving them to extinction.

Marine and freshwater gastropods exhale using ctenidia or gills. In many of these gastropods the ctenidia are protected inside the drapery crenel. In the nudibranchs (nudi- significant naked, -branch meaning gill), these ctenidia are exposed on the outside of the animate being'due south torso. This distinctive trait makes nudibranchs an hands identifiable group of molluscs. Terrestrial slugs and snails, past contrast are primarily in a subgroup known as the pulmonates that really have a mantle cavity that has become connected to the circulatory organisation (vascularized) to function equally a lung.

Gastropods motility by contracting their muscular foot in a series of waves to creep frontward. Many gastropods secrete mucus (so chosen "snail trails") to assistance facilitate move. These trails also provide chemical advice amid gastropods. The cannibal snail, for example, tracks its prey by following the mucus trail left backside. The gastropod nervous system includes bodily nerves and anterior ganglia with relatively sophisticated sensory systems, including lite receptors and well developed chemosensory abilities.

Form Bivalvia

Fig. iii.61. (A) Hard-shell clams (Mercenaria mercenaria) Fig. 3.61. (B) Pacific oysters (Crassostrea gigas) prepared for raw consumption with one vanquish removed

Fig. 3.61. (C) Atlantic bay scallop (Argopecten irradians) Fig. iii.61. (D) California mussels (Mytilus californianus) with different species of barnacles and gastropod snails

The bivalve molluscs get their name from the two door-like valves or shells that make up their exoskeleton (Fig. iii.61). Pes size varies among marine bivalves. Clams have a muscular hatchet-shaped pes for moving almost and for burrowing in mud or sand (Fig. 3.62). By contrast, an oyster'due south or a mussel's foot is small because these animals attach themselves to hard objects early on in life and practise not move effectually. Scallops don't utilize their small pes to move around either. They swim in short bursts by jet propulsion, clapping their shells together and forcing h2o out the rim.

Fig. 3.62. A clam using its human foot to move

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

Bivalves are more enclosed by their shells than other molluscs. Water enters and leaves a bivalve past way of 2 tubes called siphons. Ane siphon takes in water while the other expels water and waste. The water taken in contains oxygen and food particles. Almost bivalve species learn 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. Equally h2o is taken into the body, it flows across the gills. Oxygen (O₂) and carbon dioxide (CO_two) 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 called palps assistance sort the food and direct information technology into the rima oris. Bivalves do not have a radula (Fig. three.63). The nutrient suspended in mucus moves through the digestive organs, which break it down and absorb it.

Bivalves such as clams, oysters, and scallops are valuable every bit food. They make up a major share of the marine invertebrate seafood industry. Bivalves should not exist eaten when the water in which they abound becomes polluted with chemicals or disease organisms. At certain times of yr, microscopic organisms called dinoflagellates multiply quickly in nearshore waters. Toxic substances produced past dinoflagellates can concentrate in the clams and oysters that apply them as nutrient. Although the bivalves are not harmed, the toxin tin can attack the nervous organization of humans who eat the tainted shellfish. Toxic shellfish poisoning can exist fatal to humans.

Fig. 3.64. Layers of nacre inside a black-lip pearl oyster (Pinctada margaritifera) shell with all living tissue removed. A pearl is small particle lodged between the mantle and shell that becomes covered in a nacreous layer.

In some bivalves, such every bit oysters, mantle tissue secretes nacre (pronounced "NAY ker"), a pearly substance that coats any irritating foreign particles that lodge between the drapery and the shell (Fig. three.64). A pearl forms as coats of nacre build up effectually the strange 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. Before plastic came into use, the shells of bivalves were commonly used to brand buttons. The material known as 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. 3.65). Virtually cephalopods are relatively small. But the giant octopus (Enteroctopus sp.), which lives along the west declension of the United states of america, tin can grow to one.5 m or more. The behemothic squid, the largest invertebrate, reaches lengths of 15 m.

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

Fig. 3.65. (C) Cuttlefish (Sepia sp.) Fig. iii.65. (D) Palau nautilus (Nautlius belauensis)

The human foot in this group has specialized by dividing into arms that are fastened to the head, thus the name cephalopod, meaning head-foot. Like other molluscs, cephalopods have a mantle and curtain cavity that houses the respiratory ctenidia. The mantle cavity is too used to have in and speedily expel water to facilitate the jet propulsion swimming manner of most cephalopods. When the mantle closes forcefully, seawater ejected through the siphon propels the brute in short bursts. Both squid and octopus modify form by redirecting their siphon. They steer by pressing their artillery together and can use their speed to elude an attacking predator. They tin can also squirt ink from the ink sac into the water, creating an ink cloud for cover-up and confusing the predator. Deep-h2o cephalopods can even produce luminescent ink.

Fig. 3.66. (A) Cartoon of beak parts

Cephalopods also have a small radula, but the radula is not used for food capture. In the mouth of the squid is a nib shaped much like the beak of a parrot. Figure three.66 shows the giant squid (Architeuthis sp.) beaks . The beak is non office of the crush but a split molar-like structure. When a squid catches prey, such every bit a fish, information technology bites off and swallows chunks of it. The octopus spends most of its time crawling around the bottom, capturing casualty 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 farther digestion. The tiny bluish-ringed octopus of the Indo-Pacific region has adult particularly potent venom that is used for defence equally well. The venom of this octopus is strong and has been implicated in the deaths of several humans who unknowingly picked up the little octopus and received a defensive seize with teeth.

Fig. 3.67. The nautilus (Nautilus spp.) has geometric chambers information technology uses for buoyancy, as shown in this shell that has been cut in half. Fig. 3.68. The pen in a squid acts to keep the trunk rigid.

Most cephalopods practice not accept external shells. The nautilus is the only living exception, having a complete, well-developed beat separated into geometrically precise chambers. (Figs. 3.65 D and iii.67). These chambers comprise gas that the animal produces to regulate changes in buoyancy when it moves to shallower or deeper water. The corporeality of gas in the sleeping room 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. three.68). This long, thin shell helps support the body. The cuttlefish, a shut relative of the squid, has a harder, more brittle plate, chosen a cuttlebone (Fig. 3.69). The cuttlebone in cuttlefish helps to proceed the trunk rigid. A cuttlebone is made of calcium carbonate secreted past 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 movement upward and downward in the h2o. The octopus has no crush at all. Its merely hard torso part is its beak, which, as in the squid, is not a remnant of the trounce. Because an octopus has no hard skeleton, its soft body can clasp through tiny openings in a reef and hide in crevices or between rocks (Fig. iii.70). Octopuses in aquariums are notorious for their ability 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 shell Fig. 3.seventy. (B) Big octopus hiding under a rock shelf

Cephalopods are also masters of camouflage, using pigments in their skin cells to quickly alter their skin color to blend into their surroundings (Fig. 3.71). These pare cells, called chromatophores, contain an rubberband sac filled with paint. The cells are attached to a set of muscle cells. When the musculus cells contract, they pull the chromatophore out flat, spreading the pigment over a larger area and making the peel darker. When the chromatophore is smaller, the skin appears lighter. Chromatophores can change shape very quickly, producing a pulsating pattern of complex color changes. Squid and cuttlefish too have chromatophores, which they use to communicate with swain squid. These cephalopods can also moving ridge paddle-shaped fins to move slowly forward or backward. The complex communication systems of cephalopods emphasize the highly adult nervous systems of these animals. Dissimilar other molluscs, cephalopods have fairly well-developed brains and have image-forming optics like to those of vertebrates (with the exception of the nautilus that has a more basic eye). Octopuses have been trained in captivity and even demonstrate rudimentary learning abilities.

Fig. three.71. (A) A reef cuttlefish (Sepia latimanus) changing from darker pink coloration to light pink-white coloration