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.



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.



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.

Thenautilus 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. Thecuttlefishcan 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.

Phylum Arthropoda


The largest phylum in the animal kingdom, arthropods are segmented animals with paired joined legs, a hard joined exoskeleton, a complete digestive tract, reduced coelom, no nephridia, a dorsal brain, and a ventral nerve cord with paired ganglia in each segment.

There are almost 1,000,000 known species of them on the Earth. There are five main groups of arthropods:insect, millipede, centipede, crustacean, arachnid.

Arthropods do not have bones inside their bodies. Instead, arthropods have a hard outer covering – an exoskeleton – on their bodies. The exoskeleton gives protection, provides a place where muscles attach, and allows movement. It is made up of nonliving material. This material cannot grow as the rest of the animal grows. When an arthropod’s skeleton becomes too small for the animal inside, the exoskeleton must be shed. The shedding process is called molting. A new exoskeleton has already begun to grow underneath. The new exoskeleton is soft and moist and provides little protection. Many arthropods hide after they molt until their new exoskeleton becomes hard.


5.8.1. Class Crustacea

Crustaceans are mostly aquatic, with two pairs of antennae, one pair of mandibles, chewing jaws and typically two pairs of maxillae. The thoracic segments have appendages, and the abdominal segments are with or without appendages. About 25,000 species.

The crustaceans include crabs, crayfish, lobsters, barnacles, shrimps, Daphnia (water fleas), and a number of smaller forms found mostly in fresh water and marine plankton, as well as some terrestrial forms. Crustaceans are mandibulates. Their body is divided into 2 main parts – a cephalothorax and an abdomen.

Unlike other arthropod groups, almost all the crustaceans are aquatic, but some, crabs in particular, are amphibious or land dwellers. The crustaceans can live in oceans and along the seashores (many of these crustaceans are highly prized as food); other crustaceans, such as crayfish and water fleas, live in lakes and ponds. A few crustaceans live in moist soil on land.

Amphibious crabs continue to breathe with gills, in their thoracic cavities which keep the gills wet and aerating the water through holes in their exoskeletons. The true land crab has lost the gill structures and instead has an area of highly vascularized epithelial tissue through which oxygen is exchanged.

The crayfish is a typical crustacean. It has the upper body region called a cephalothorax. The lower body region is its abdomen. Jointed legs are attached to the cephalothorax. The front pair of legs with the large claws are known as chelipeds. They are used for protection and catching the food and not for walking. The jaws the crayfish uses to chew its food are mandibles. Leglike maxillipeds hold the food in place. The crayfish uses its antennae to taste, smell and touch its surroundings. The small feathery structures under the crayfish’s abdomen are the swimmerets. They are used for forcing water over the gills and for reproducing. The tail of the crayfish is the flipper. When the flipper is moved down and forward, the crayfish lurches backward. The lurching motion usually stirs up a protective muddy cover that hides the crayfish from its enemies.



The crayfish moves oxygen from the water by means of its gills. The featherlike gills contain a high concentration of blood vessels. The gills are attached to the walking legs, between the thorax and the carapace. As the animal walks, water flows over the gills.


Crayfish, like spiders, have an open circulatory system. Blood enters the heart through three pairs of pores called ostia. Valves seal off the ostia, the heart contracts, and blood is forced into seven large arteries. These arteries then discharge blood into the spaces surrounding the organs. The blood drains out of these spaces and collects in a cavity called the sternal sinus. From there blood travels through other vessels to the gills. Blood returning from the gills enters the pericardial sinus and then returns to the heart through the ostia.

Digestion and Excretion

Crayfish eats living and dead plants, worms, larvae and tadpoles. Food moves from the mouth down the esophagus to the stomach, where it is ground by teeth of chitin. Undigested wastes move through the intestine and leave the body through the anus. Excretory organs called green glands are located on the base of the antennae. The green glands remove liquid wastes from the blood.

Nervous System

The nervous system of crayfish consists of a dorsal brain formed from a pair of ganglia. Branches from the brain run to the eyes, antennules and antennae. Other nerves encircle the esophagus and connect with a ventral nerve cord that runs the length of the body. Each of the crayfish’s segments has a pair of ganglia. The crayfish has two eyes located on movable stalks on the front of the body. Each eye has over 2500 lenses. Eyes that have more than one lens are called compound eyes. Compound eyes form images by combining the sensations from multiple lenses. Such eyes respond rapidly to light and readily detect motion. Arthropods are the only animals with compounds eyes.

Reproduction and Growth

Crayfish mate in the spring or fall. The male deposits sperm into the female, who stores it. About two weeks later, the female produces 200 to 300 eggs. They are then fertilized by the stored sperm and hatch in six weeks. The fertilized eggs are attached to the last three pairs of swimmerets of the female until they hatch. Crayfish molt twice a year. Molting begins when the outer layer of body cells digests the inner layer of the exoskeleton weakening it. The crayfish swells by absorbing water and taking in excess air. This causes the exoskeleton to crack and the crayfish backs out. The outer body cell layer secretes salts that harden the new, developing exoskeleton.


5.8.2. Class Arachnida

Arachnids include spiders, mites, scorpions, ticks, daddy long-legs. Most members are terrestrial, air-breathing; usually have 4 pairs of legs. The arachnids are chelicerates. The first pair of appendages are chelicerae, which are sharp and pointed and are used for capturing and then paralyzing prey by the injection of a poison. The second pair are pedipalps, which are used for handling and tearing food. There are about 30,000 species.

Most arachnids live on land. Many ticks and mites can be found on the skin of animals and people. Arachnids, like the crustaceans, have two body parts: a cephalothorax and an abdomen. Arachnids also have eight jointed walking legs.

Spiders, the most familiar arachnids, are among the word’s most fascinating animals. But very few of the 30.000 species are harmful to humans. Spiders are contribute to human welfare by eating insect pests.


Spiders, like other arachnids, live on a completely liquid diet. Spiders feed by injecting enzymes into their prey to dissolve the body substances. The spiders then suck out and swallow the liquefied remains. Malpighian tubules found near the base of the abdomen form the excretory system of the spider. The Malpighian tubules remove excess nitrogen, a by-product of metabolism, from the blood. Spiders also have waste-removing organs on the first and third pairs of legs.


Oxygen is carried through the spider’s body in two unusual ways. One way is through tubes called tracheae, which carry air directly to the cells. The tubes receive air through slits in the exoskeleton called spiracles. The opening and closing of the spiracles regulates air flow. In the second method of respiration, blood circulates through a structure called the book lung, so named because its sheets of tissue hang down like the pages of the book. The book lung is located near the front of the abdomen and absorbs oxygen through the spiracles. Some spiders have either tracheae or book lungs but most spiders have both. These structures are unique to arthropods.


Like most arthropods, spiders are either male or female. A male spider approaches a female with caution. Since spiders are habitually solitary animals, a female might mistake a male suitor for potential prey. For this reason many species engage in complex courtship rituals, such as taping on the web or stroking the female. Having captured the attention of the female, the male puts sperm on his pedipalps and puts it into her genital opening on the outside of the abdomen. Eggs, laid in special webs or cocoons, hatch in about two weeks.

The weaving of a web is not learned behavior, nor does it require any practice. A spider confined from egg to adult can weave a perfect web on its first try. A spider releases silk for its web through openings in its abdomen. On the posterior portion of the spider’s abdominal surface is a cluster of spinnerets, fingerlike organs from which a fluid protein exudes that hardens into silk as it comes into contact with the air. Spiders spend a great deal of energy rebuilding their webs every day. Some webs can be as large as a meter around. Webs trap insects and other small animals that the spiders use for food. Silk is used not only for the variety of webs made by the different species but for a number of other purposes as well, such as for a drop line, on which the spider goes sky diving, for a cocoon, for lining a burrow, for the shroud of a victim or for wrapping an edible offering presented to the female of certain species by the courting male. Most spiders can spin several kinds and thicknesses of silk.


Scorpions have two distinctive features. Their greatly enlarged pedipalps are held in a forward position, and they have an abdomen that ends in a tapered stinger. Scorpions are most common in tropical areas and deserts. The 800 known species range in length from 1.3 to 17.6 cm. Scorpions hide under rocks and in crevices by day and are active mainly at night. They feed on insects and spiders. A scorpion catches and holds its prey in its pedipalps until the stinging abdomen can curl over the top of its body and inject poison into the prey.

Ticks and Mites

Ticks and Mites are tiny animals, usually less then 1 mm long. They differ from other arachnids in that the cephalothorax and abdomen are fused to form a single body part. parasites. Ticks attach themselves to an animal’s skin and suck blood. Many ticks carry disease-causing organisms that are ransmitted to other animals through the tick’s bite. Most human beings have mites living in their hair follicles. The mites live off.

5.8.3. Class Insecta

Almost three-quarters of all animal species on Earth are insects. Although insects are not found in salt water, they live in almost every freshwater and land habitat. Insects range from the fairy fly, only 0.2 mm long, to the African Goliath beetle, over 10 cm long. Insects are the only invertebrates that can fly.

Insects include bees, ants, beetles, butterflies, fleas, lice etc. The insects constitute the largest class, by far, of the arthropods. In fact, there are more species of insects than of all other animals combined. About 800,000 are known, of which about 275,000 are beetles. Most insects are terrestrial and most breathe by means of trachea.

The evolutionary success of insects is due in part to the characteristic structure they share with other arthropods – exoskeletons, segmented bodies, and joint appendages. A key to their success is that most insects are small and have adapted to specific habits and needs. In many cases this allows multiple species of insects to exist without competing with one another for scarce resources. Their ability to fly has also added to their evolutionary success.

Insects are both harmful and beneficial to human society. Only about 1 percent of insect species are destructive to crops and property. Harmful insects include household pests, such as termites; crop and livestock pests, such as boll weevils; and hosts of disease-causing organisms, such as mosquitoes infected with parasitic protozoa.

Many insects, on the other hand, are beneficial to human society. Insects pollinate fruit trees, flowers, and many field crops. Bees produce honey and beeswax, silkworms form cocoons from which silk is spun, and lac insects provide the raw material for commercial shellac. Some kinds of insects are natural enemies of destructive insects. For example, the larvae of certain wasps feed on caterpillars that destroy plants.

The characteristic features of insects: three main divisions – the head, the thorax, and the abdomen; three pairs of legs; one pair of antennae and two kinds of eyes. The simple eyes detect light and darkness the compound eyes detect shapes. One more characteristic feature - a set of mouthparts similar to those of the lobster. In the more primitive insects, such a grass-hopper, the mouthparts are used for handling and masticating food but in the more highly specialized groups, the mouthparts are often molded into sucking, piercing, slicing or sponging organs, many of which are exquisitely adapted to the nectaries (nectar- holding organs) of special flowers.

On the head are antennae, compound eyes and mouthparts. An insect’s mouthparts are modified for different methods of feeding. Butterflies have mouthparts shaped like coiled tubes, which they uncoil and use to suck nectar from deep within flowers. A praying mantis tears its food apart with mandibles. Flies have no mandibles and therefore lap up food with a tonguelike lower lip.

Three pairs of legs and any wings the insect has are attached to the thorax. The legs are usually adapted for jumping, walking or running.

Most adult insects have two pairs of wings made up of light strong sheets of chitin; the veins in the wings are chitinous tubules that serve primarily as braces. The wings of the various orders of insects have evolved separately from one another. In some, such as the fleas and lice, they have been partially or totally lost returning the insect to the condition of its wingless ancestors.

The abdomen may have as many as eleven segments. The abdomen houses the heart, the respiratory and excretory organs, and the spiracles through which the insect breathes. In respiratory, tracheae carry air to body cells. All insects have an open circulatory system, through which blood flows into large open spaces surrounding the organs.


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