Nematodes are the most numerous multicellular animals on earth.
A handful of soil will contain thousands of the microscopic worms, many of them parasites of insects, plants or animals. Free-living species are abundant, including nematodes that feed on bacteria, fungi, and other
nematodes, yet the vast majority of species encountered are poorly understood biologically. There are nearly 20,000 described species classified in the phylum Nemata .
Nematodes are structurally simple organisms. Adult nematodes are comprised of approximately 1,000 somatic cells, and potentially hundreds of cells associated with the reproductive system . Nematodes have been characterized as a tube within a tube ; referring to the alimentary canal which extends from the mouth on the anterior end, to the anus located near the tail. Nematodes possess digestive , nervous, excretory, and reproductive systems, but lack a discrete circulatory or respiratory system.
In size they range from 0.3 mm to over 8 meters.
Slugs & Snails
..Size: X 0.25
Any mollusk of the class Gastropoda (Gr. gaster, "stomach"; pous, "foot") are generally characterized by a single shell and an asymmetric body. They form the second largest class in the animal kingdom, outnumbered only by insects. The most recent estimate of the number of known species is 37,500, a revision downward from an earlier estimate of about 80,000.
Gastropods vary considerably in structure and way of life. The smallest species are barely visible, whereas the largest, a sea slug, weighs up to 13 kg (29 lb). Evolutionarily the animals are successful, being common in most marine and freshwater habitats and the only mollusks to flourish on land. The three gastropod subclasses are the Prosobranchia, the Opisthobranchia, and the Pulmonata, described below.
The ancestors of gastropods had bilateral symmetry; that is, they had right and left sides. The animals evolved, however, so as to become asymmetric. This happened through two processes, the first of which was torsion, a twisting of the body. Originally the gills and anus of a gastropod were at the hind end of the body. The left half of the body began to increase in relative size, however, and the upper part of the body (including the shell) rotated like a turret so that the gills and anus were now above the head. The change may have taken place to protect the head or make it easier for the body to balance the shell.
The second process leading to asymmetry was the development of a coiled shell, which usually spirals to one side. Not all shells followed this pattern; sometimes the shell is cap-shaped, with little or no coiling involved. The latter kind of snail, called a limpet, can cling to rocks or shells with its broad foot. When the shell is deep, however, it is usually coiled, and the snail can then crawl about freely. If it needs protection, it can withdraw into the shell and close the opening with a doorlike structure, called an operculum, on the foot. A further development is seen in slugs, which have lost or reduced the shell. Young slugs usually have well-developed shells, but these are either shed or kept as a small remnant in the adult. Both snails and slugs crawl slowly, mainly using waves of muscular contraction of the single foot; some, however, can swim.
In ancestral gastropods a space called a mantle cavity, with two gills and various body openings, existed above the head. In land snails and some other species the gills have become lost or reduced, and the mantle cavity has been transformed into a lung. In most gastropods the head usually bears eyes and tentacles. The animals can see and smell fairly well, although their behavior is not complicated. The mouth is usually equipped with a rasplike tongue called a radula; this is also found in other mollusks. Generally used in scraping up food, the radula may be considerably modified. In cone shells the teeth on the tongue become dartlike, and some species can inflict a dangerous wound. In oyster drills, the radula can bore through a shell.
The gut of a gastropod is a coiled tube with various glands and sometimes a gizzard. The nervous, circulatory, and reproductive systems are well developed.
Lower gastropods have separate sexes and reproduce by spawning eggs
into the water, where they are fertilized with sperm and develop. The young
larvae swim about, settle, and mature. In advanced gastropods fertilization
takes place internally, and coverings are produced that protect the eggs
and young, which are sometimes also guarded by the female. At times the
whole development process is internal.
In the more modified subclasses Opisthobranchia and Pulmonata, the animals are almost always hermaphroditic (containing both male and female reproductive organs). This allows them to mate with any mature animal of the same species. In some Pulmonata an ability to self-fertilize is common; some snails can reproduce without fertilization of the eggs.
The abundant and diverse gastropods are an important part of the food
web, whether as herbivores, carnivores, or omnivores. Some are parasites,
and many specialize by feeding on unusual, hard to eat, or poorly digestible
materials. They are also an important source of food. The edible gastropods
include the abalone and other marine snails, such as the conch and the
periwinkle, and land snails of various types. The abalone is taken commercially
and as sport.
A few gastropods are harmful. Some snails and slugs damage crops and garden plants, and others are pests in oyster beds. In some parts of the world, freshwater snails harbor blood flukes, worms that cause a serious disease in humans.
The first gastropods appeared in the early Cambrian period, about 600
million years ago. The most primitive living gastropods are of the subclass
Prosobranchia and are mostly marine, with a few freshwater and terrestrial
species. The three suborders are Archaeogastropoda (archaic forms such
as abalone and limpets), Mesogastropoda, and Neogastropoda (advanced forms
such as oyster drills and cone shells).
The subclass Opisthobranchia is almost wholly marine. The shell tends to be reduced, and the gill migrates toward the rear of the body. Eight orders exist, including the less modified tectibranches (bubble shells, sea hares, and allies), the shell-less nudibranches, and two groups of pteropods that swim in the plankton.
In the subclass Pulmonata the mantle cavity has become a lung, and the operculum is lost. The group has a few marine forms. Most terrestrial snails and slugs belong to the order Stylommatophora, and most freshwater snails belong to Basommatophora.
Common name for minute insects constituting the order Thysanoptera. Thrips are found worldwide and feed on a wide range of plants, including important crops and ornamentals. Most species are no more than 1.5 mm (0.06 in) long as adults; some are wingless, but others have two pairs of short wings fringed with hairs. Both adults and nymphs have sucking, piercing mouthparts. Although thrips pollinate some plants and also eat some insect pests, they are mainly considered pests and are controlled with insecticides. Species such as onion thrips, Thrips tabaci, also transmit plant diseases.
(Latin for "Poison")
Any of a number of organic entities consisting simply of genetic material surrounded by a protective coat. The term virus was first used in the 1890s to describe agents that caused diseases but were smaller than bacteria. By itself a virus is a lifeless form, but within living cells it can replicate many times and harm its host in the process. The hundreds of viruses that are known cause a wide range of diseases in humans, lower animals, insects, and plants.
The existence of viruses was established in 1892, with the discovery of tobacco mosaic virus. A few years later, viruses were found growing in bacteria. Then, in 1935, the American biochemist Wendell Meredith Stanley (1904-71) crystallized tobacco mosaic virus and showed that it is composed only of the genetic material called ribonucleic acid, or RNA and a protein covering. In the 1940s development of the electron microscope made visualization of viruses possible for the first time. This was followed by development of high-speed centrifuges used to concentrate and purify viruses. The study of animal viruses reached a major turning point in the 1950s with the development of methods to culture cells that could support virus replication in test tubes. Numerous viruses were subsequently discovered, and in the 1960s and '70s most were analyzed to determine their physical and chemical characteristics.
Viruses are submicroscopic intracellular parasites that consist of either
RNA or deoxyribonucleic acid (DNA)-never both-plus a protective coat of
protein or of protein combined with lipid or carbohydrate components. The
nucleic acid is usually a single molecule, either singly or doubly stranded.
Some viruses, however, may have nucleic acid that is segmented into two
or more pieces. The protein shell is termed the capsid, and the protein
subunits of the capsid are called capsomeres. Together these form the nucleocapsid.
Other viruses have an additional envelope that is usually acquired as the
nucleocapsid buds from the host cell. The complete virus particle is called
the virion. Viruses are the smallest entities that can replicate, but they
are obligate intracellular parasites; that is, their replication can take
place only in actively metabolizing cells. Outside of living cells, viruses
exist as inert macromolecules.
Viruses vary considerably in size and shape. Three basic structural groups exist: isometric; rod shaped or elongated; and tadpolelike, with head and tail (some bacteriophages). The smallest viruses are icosahedrons (20-sided polygons) that measure about 18 to 20 nanometers wide (one-millionth of a millimeter = 1 nanometer). The largest viruses are rod shaped. Some rod-shaped viruses may measure several microns in length, but they are still usually less than 100 nanometers in width. Thus, even the largest viruses are below the limits of resolution of the light microscope, which is used to study bacteria and other large microorganisms.
Many of the viruses with helical internal structure have outer coverings, or envelopes, composed of lipoprotein or glycoprotein, or both. These viruses appear roughly spherical or in various other shapes, and they range from about 60 to more than 300 nanometers in diameter. Complex viruses, such as some bacteriophages, have heads and a tubular tail, which attaches to host bacteria. The pox viruses are brick shaped and have a complex protein composition. These are exceptions, however, and most viruses have a simple shape.
Viruses do not contain the enzymes and metabolic precursors necessary
for self-replication. They have to get these from the host cells that they
infect. Viral replication, therefore, is a process of separate synthesis
of viral components and assembly of these into new virus particles. Replication
begins when a virus enters the cell. The virus coat is removed by cellular
enzymes, and the virus RNA or DNA comes into contact with the appropriate
cell-replicative apparatus. There it directs the synthesis of proteins
specified by the viral nucleic acid. The nucleic acid then replicates itself,
and the protein subunits constituting the viral coat are synthesized. Thereafter
the two components are assembled into a new virus. One infecting virus
can give rise to thousands of progeny viruses. Some viruses are released
by destruction of the infected cell. Others are released by budding through
cell membranes and do not kill the cell. In some instances, infections
are "silent," in that viruses may replicate within the cell but cause no
obvious cell damage.
The RNA-containing viruses are unique among replicative systems in that the RNA can replicate itself independently of DNA. In some cases, the RNA can function directly as messenger RNA and replicate itself, using the cell's ribosomal and metabolic precursor systems. In other cases, RNA viruses carry within the coat an RNA-dependent enzyme that directs the synthesis of virus RNA. Some RNA viruses, which have come to be known as retroviruses, may produce an enzyme that can synthesize DNA from the RNA molecule. The DNA thus formed then acts as the viral genetic material.
Bacterial viruses and animal viruses differ somewhat in their interaction with the cell surface during infection. The "T even" bacteriophage that infects the bacterium Escherichia coli, for instance, first attaches to the surface and injects its DNA directly into the bacterium. No adsorption and uncoating take place. The basic events of virus replication, however, are the same after the nucleic acid enters the cell.
Viruses in Medicine.
Viruses represent the last major challenge to medical science in combating infectious diseases. Many cause diseases that are of major importance to humans and that are extraordinary in their diversity.
Included among virus diseases is the common cold, which affects millions of people every year. Others are important because they are frequently fatal. Among these are rabies, hemorrhagic fevers, encephalitic diseases, poliomyelitis, and yellow fever. Most viruses, however, cause diseases that usually only create acute discomfort unless the patient develops serious complications from the virus or from a bacterial infection. Some of these diseases are influenza, measles, mumps, fever blisters (herpes simplex), chicken pox, shingles (herpes zoster), respiratory diseases, acute diarrhea, warts, and hepatitis. Still others, such as rubella (German measles) virus and cytomegalovirus, may cause serious abnormalities or death in unborn infants. Acquired immune deficiency syndrome is caused by a retrovirus. Only two retroviruses are unequivocally linked with human cancers but some papilloma virus forms are suspected. Increasing evidence also indicates that other viruses may be involved in some types of cancer and in chronic diseases such as multiple sclerosis and other degenerative diseases. Some of the viruses take a long time to cause disease; kuru and Creutzfeldt-Jakob disease, both of which gradually destroy the brain, are slow virus diseases.
Viruses that cause important human disease are still being discovered. Most can be isolated and identified by laboratory methods, but these usually take several days to complete. One of the most recently discovered viruses is rotavirus, the causal agent of infant gastroenteritis.
To cause new cases of disease, viruses must be spread from person to person. Many viruses, such as those causing influenza and measles, are transmitted by the respiratory route when virus-containing droplets are put into the air by people coughing and sneezing. Other viruses, such as those that cause diarrhea, are spread by the fecal-oral route. Still others, such as yellow fever and viruses called arboviruses, are spread by biting insects. Virus diseases are either present most of the time (endemic), causing disease in susceptible persons, or they come in large waves (epidemic) and attack thousands of people. An example of the latter kind is the almost yearly, worldwide occurrence of influenza.
Currently, no completely satisfactory treatments exist for virus infections,
because most drugs that destroy viruses also damage the cell. The drug
alpha-Adamantanamine is used extensively in some countries for treatment
of respiratory infections caused by influenza A viruses, and IsatinBetathiosemicarbazone
is effective against smallpox. Certain analogues of nucleic-acid precursors
also appear useful in severe herpes virus infections.
One promising antiviral agent, interferon is produced by the cell itself. This nontoxic protein, which is produced by some animal cells infected with viruses, can protect other cells against such infection. The use of interferon for treating cancer is under intensive study. Until recently, study of the use of interferon has been restricted by its limited availability in pure form; but new techniques of molecular cloning of genetic material now make it possible for scientists to obtain the protein in larger quantities. Its relative value as an antiviral agent may be established within a few years.
The only effective way to prevent viral infection is by the use of vaccines. Vaccination for smallpox on a worldwide scale in the 1970s, for example, eradicated this disease. Many antiviral vaccines have been developed for humans and lower animals. Those for humans include vaccines for rubeola (measles), rubella, poliomyelitis, and influenza. Immunization with a virus vaccine stimulates the body's immune mechanism to produce a protein (called antibody) that will protect against infection with the immunizing virus. The viruses are always altered before they are used for immunization so that they cannot themselves produce disease.
Viruses cause a wide variety of diseases in plants and frequently cause serious damage to crops. Common plant-disease viruses are turnip yellow mosaic virus, potato X virus, and tobacco mosaic virus. Plants have rigid cell walls that plant viruses cannot penetrate; so the most important means of plant-virus spread is provided by animals that feed on plants. Often, healthy plants are inoculated by insects that carry on their mouthparts viruses acquired while feeding on infected plants. Nematodes (roundworms) may also transmit viruses while feeding on roots of healthy plants.
Plant viruses can accumulate in enormous quantities within infected cells. For instance, tobacco mosaic virus may represent as much as 10 percent of the dry weight of infected plants. Studies on the interaction of plant viruses with plant cells are limited, because plants often cannot be infected directly, but only by means such as an insect vector. Cell cultures in test tubes, which can be infected with plant viruses, are not generally available.
The study of viruses and their interaction with host cells has been
a major motivation for the host of fundamental biological studies at a
molecular level. For example, the existence of messenger RNA, which carries
the genetic code from DNA to define what proteins are made by a cell, was
discovered during studies of bacteriophages replicating in bacteria. Studies
of bacteriophages have also been instrumental in delineating the biochemical
factors that start and stop the utilization of genetic information. Knowledge
of how virus replication is controlled is fundamental to understanding
biochemical events in higher organisms.
The reason that viruses are so useful as model systems for studying events that control genetic information is that viruses are, in essence, small pieces of genetic information that is different from the genetic information of the cell. This allows scientists to study a smaller and simpler replicating system, but one that works on the same principle as that of the host cell. Much of the research on viruses is aimed at understanding their replicative mechanism in order to find ways to control their growth, so that viral diseases can be eliminated. Studies on virus diseases have also contributed greatly to understanding the body's immune response to infectious agents. Antibodies in blood serum, as well as secretions of the mucous membranes, all of which response to infectious agents. Antibodies in blood serum, as well as secretions of the mucous membranes, all of which help the body eliminate foreign elements such as viruses, have been more thoroughly characterized by studying their responses to viral infection. Intense scientific interest is now concentrated on studies designed to isolate certain viral genes. These genes can be used in molecular cloning systems to produce large amounts of particular virus proteins, which can in turn be used as vaccines.