Phylum Molluska: Mollusks

The mollusks constitute one of the largest phyla of animals, both in numbers of species and in number of individuals. They are characterized by soft bodies within a hard, calcium-containing shell, although in some forms the shell has been lost in the course of evolution, as in slugs and octopuses, or greatly reduced in size and internalized, as in squids. There are three major classes of mollusks: (1) the gastropods, such as the snails, whose shells are generally in one piece; (2) the bivalves, including the claims, oysters, and mussels, which have two shells joined by a hinge ligament; and (3) the cephalopods, the most active and most intelligent of the mollusks, including the cuttlefish, squids and octopuses.

Although the mollusks diverse in size and shape, they all have the same fundamental body plan. Their body is basically bilaterally symmetrical, and they have a true coelom. There are three distinct body zones: a head-foot, which contains both the sensory and the motor organs; a visceral mass, which contains the organs of digestion excretion, and reproduction; and a mantle, which hangs over and enfolds the visceral mass and which secretes the shell. The mantle cavity, a space between the mantle and the visceral mass, houses the gills, and the digestive, excretory, and reproductive systems discharge into it. Water sweeps into the mantle cavity propelled by cilia on the gills, passing through the gills and aerating them. It then passes by the nephridia, gonophores and rectum, which are always downstream from the gills. Water leaving the mantle cavity carries excreta and, in season, gametes.

The digestive tract is far more convoluted and so provides more convolution and more working surface than that of the annelids. In all mollusks, the digestive tract is extremely ciliated, with many different working areas. Food particles are absorbed by the cells lining the stomach and the anterior intestine, and, from them, it is passed into the bloodstream.

Mollusks get food in different ways. A bivalve filters food particles from the water that flows under its mantle. Gastropods or univalves, eat with a scraper which is like a tongue with teeth. This toothed organ called radula - a characteristic organ of this phylum. It serves both to tear or scrape off algae and other food material and also to convey them backward to the digestive tract. Octopuses and squids use sucking discs on their powerful arms to seize food.

Octopuses and squids have a good way to escape from enemies in a hurry. They use their mantles to squirt water in a forward direction. This can propel the organism backwards. In the meantime, the octopus or squid can squirt out a dark ink that helps into hide and confuse their enemies.

Mollusks have gills. A gill is a structure with an increased amount of surface area, through which gases can diffuse, and enrich body supply for the transport of these gases.

Mollusks have three-chambered hearts; two of the chambers (atria) collect oxygenated blood from the gills and the third (the ventricle) pumps it to the oxygen-depleted tissue. Cephalopods, which are extremely fast and active animals, have accessory hearts that propel blood into the gills.

Many mollusks pass through a similar larval stage of development. The mollusk larva, called a trochophore, is pear-shaped with a band of cilia around its middle. Because a trochophore swims about freely, it helps disperse the species. This opportunity is especially important for mollusks, which are encumbered with heavy shells as adults. Some marine annelids also pass through a trochophore larval stage. This similarity has led scientists to conclude that annelids and mollusks are closely related groups.

Mollusks reproduce sexually. The man clam, for instance, releases sperm into the water. The sperm are swept to the eggs under the female’s mantle.

Mollusks are classified according to the kind of shell they have. Of the seven classes in the phylum Molluska, the three major classes are two-shaped mollusks, such as clams, scallops and oisters; one-shelled mollusks, such as snails; and head-footed mollusks, such as octopuses, squids, and cuttlefish.

Mollusks with Two Shells

Clams, oysters, and scallops belong to a class of mollusks that have two shells hinged together. These are called valves. The animals themselves are referred to as bivalves. A characteristic of bivalves is the shape of their muscular foot. For this reason, the class is called Pelecypoda which means ‘hatchet-foot’.

Clams

Most clams live in salt water, buried in the sand or mud at the sea bottom. Some clams, however, live in fresh water. Clams range in size from the tiny Condylocardia 0.1 mm across to South Pacific giants 1.2 m across.

A clam’s shell is usually off-white and consists of three layers: a tough horny outer layer, a smooth shiny inner layer – the pearly layer, and a middle prismatic layer. The two shells are held together by ligaments. Two adductor muscles open and close the shell.

Like all bivalves, clams are entirely encased in their shells. As a result, they have no real head and no radula. Sense organs are poorly developed, though sensory cells along the edge of the mantle do respond to light and touch. Two long pairs of nerve cords connect the sets of nerve cells, which are found above the mouth, in the digestive system, and in the foot.

Clams obtain both food and oxygen from the water that flows through their bodies. Water enters the clam through the incurrent siphon. Cilia move the water across the respiratory organs, called gills, in the mantle cavity. Gills have a large surface area and an abundant supply of blood to allow for the exchange of gases. Water drawn into the clam by the incurrent siphon has more oxygen than the blood supply of the gills has. For this reason oxygen diffuses from the water to the blood, and carbon dioxide diffuses from the blood to the water. The clam then expels the water through the excurrent siphon.

Clams are filter-feeders. They live on microscopic organic matter carried in the water that flows through their siphons. Mucus on the gills traps the food matter, and cilia push the food-laden mucus on the clam’s mouth. From there the food passes into the stomach. Undigested food particles leave the clam through the anus.

Clams have an open circulatory system. This means that the blood flows through large open spaces, or sinuses, rather than through a system of blood vessels. A three-chambered heart pumps the blood through the clam.

Most clam species have separate sexes. The sperm and eggs are shed into the water, where fertilization takes place. The fertilized egg becomes a trochophore larva that settles on the bottom and develops into an adult clam.

Other Bivalves

Scallops live in all oceans and range in diameter from 2.5 cm to 15 cm. They have a fan-shaped shell (smooth or sculptured). Scallops have a single large adductor muscle. The scallop’s eyes are located along the edge of the mantle. One scallop may have as many as 100 eyes. Although the eyes cannot focus, they can distinguish between light and dark and can sense passing shadows.

Unlike scallops, oysters cannot move about. Early in its life, an oyster permanently attaches its flat lower shell to a hard surface.

The outer shell is rough in texture, while the inner surface of the shell is smooth and white. If an irritant such as a grain of sand enters an oyster shell, the oyster protects itself by covering the foreign matter with several layers of calcium carbonate. This process forms a pearl.

Mollusks with One Shell

The largest class of mollusks is the Gastropoda, a name that means ‘belly-footed’. The 37,500 or more species include snails and slugs that live in water and on land. Most gastropods are univalves (they have only one shell). The snail’s protective shell is usually coiled, which allows the long pointed body of the snail to enclose itself in a compact form. Slugs have no outer shell. Some slugs have an internal shell, which is actually an external shell that has been reduced and covered over by the mantle.

A characteristic of gastropods is torsion, or a twisting of the body, which occurs during larval development. Before torsion, the body plan of a snail is bilaterally symmetrical. One half of the body grows faster than the other, causing the anus to curve the forward while the head and foot remain in place. The organs on one side of the visceral mass twist over to the other side. The mantle cavity, which originally faced backwards, now faces forward. This arrangement of body parts allows the snail to draw its head into the shell and then plug the hole with its foot.

The nervous system of gastropods is more developed than that of bivalves. In gastropods, six pairs of ganglia are interconnected with nerve cords. Gastropods can detect light and shadows by means of eyes located on tentacles that extend from their heads. Like bivalves, gastropods have an open circulatory system. Unlike bivalves, most gastropods reproduce by fertilizing eggs internally.

Snails

Most snails are less than 2.5 cm long. On land the largest is the giant African snail which grows a shell 20 cm in length.

Snails that live in water breathe through gills. They have one pair of tentacles. Land snails breathe through a network of blood vessels in the mantle cavity. To allow for the exchange of gases by diffusion, the blood tissues in the mantle cavity must be kept moist. Consequently, snails are more active at night or early morning when the air is moist. In dry weather, snails seal themselves inside their shells with a mucus plug in order to retain moisture. Some snails have a flat plate on the side of their foot called an operculum, which can be used like a trap door to close off the shell from the outside. Land snails have two pairs of tentacles.

Snails move by contracting their foot in a wavelike motion from back to front. They glide over a trail of mucus laid down by the front of the foot.

Most land snails feed on plants. They scrape off bits of plant matter using the radula. Snails help break down decayed matter, but too many snails in a garden can cause serious damage to plants.

Slugs can survive without shells because they live in moist environments. Like land snails, slugs that live on land respire through blood vessels in the mantle cavity. Most sea slugs breathe through gills. However, some sea slugs, called nudibranches, lack gills, shells, and mantle cavities. These slugs have decorative plumes on their backs that may function as respiratory organs.

Head-Foot Mollusks

The class Cephalopoda includes the most evolutionary advanced of all mollusks – the squid, octopus, cuttlefish, and nautilus. Cephalopod means ‘head-foot’. All members of the class have a large well-developed head and a foot divided into many armlike tentacles. Only the nautilus has an outer shell. In the squid and the cuttlefish, the shell is reduced in size and overgrown by the mantle. The octopus has no shell at all.

Cephalopods have a closed circulatory system and a well-developed nervous system with many ganglia and a complex brain. The central mouth has jaws and a radula. It is surrounded by tentacles – up to 8 in the octopus, 10 in the squid, and 94 in the nautilus. Suckers on the tentacles help the animals to grasp their prey and to move along the ocean bottom.

All cephalopods are marine animals and live at all depths. Cephalopods are predators – that is, they kill and eat other animals, such as fishes, crabs and bivalves.

Octopuses

The octopus is highly specialized for its predatory way of life. Because the octopus is not unclosed by an outer shell, it has great freedom to move about in search of prey. The octopus moves rapidly by jet propulsion. By forcibly contracting the muscles of its mantle cavity, the octopus squirts out a jet of water through its siphon and speeds off in the opposite direction. The octopus also has special senses that help it locate prey. The suckers on its tentacles, more sensitive than human fingertips, contain special receptors that respond to chemicals in the water. The octopus uses its tentacles to reach into crevices for prey it cannot see.

The sexes are separate in the octopus, as they are in all cephalopods. The male octopus uses one of its tentacles, specialized for this function, to transfer sperm from its mantle cavity to the mantle cavity of the female. Later the female lays a mass of fertilized eggs encased in a gelatinous cover. The female broods the eggs – that is, she guards and cleans the eggs until they hatch.

Squid range in size from about 1.5 cm to the giant 20-meter species that weighs 3,360 kg. Squid do not use their tentacles for crawling. They move by getting out streams of water through siphons and using two finlike extensions of the mantle cavity for steering. Squid have been studied much less than octopuses due to the difficulty of maintaining them in captivity.

The nautilus lives in the outermost chamber of a many-chambered coiled shell. A tube that runs from its visceral mass secretes a gas into all but the outermost chamber. By adjusting the amount of gas in the chambers, the nautilus can control the depth at which it floats. The cuttlefish can adjust its buoyancy in a similar fashion by controlling the amount of gas in its porous inner shell.

There are three main classes of phylum Mollusca: the mollusks are represented by the bivalves, the gastropods (snails), and the cephalopods (octopus and clams). The body is always divided into a head-foot, visceral mass and mantle. In each of these classes the basic body plan is the same, but it has been modified in the course of adaptation to a particular environment. In most mollusks respiration is carried out by means of gills, thin – walled structure that is an extension of the epidermis. It is richly endowed with blood vessels that serve as an area of gaseous exchange (respiration). Mollusks are also characterized by an efficient three-chambered heart and a toothed tongue, the radula. Nervous systems and behavior vary among the species reaching a zenith of a complexity in the brainy octopus.

Date: 2014-12-22; view: 1028