Classification of Living Things: Kingdom to Subphylum
(animals)
sponges, worms, insects, fish, amphibians,     reptiles, birds, and mammals
Most macroscopic creatures are either plants or animals.  Of   course, humans are animals.  The distinction between the plant and   animal kingdoms is based primarily on the sources of   nutrition and the capability of locomotion or movement.  Plants produce new cell    matter   out of inorganic material by photosynthesis .   They do not have the ability to move around their environment except by growing or   being transported by wind, water, or other external forces.
 
Kingdom Plantae
 
In contrast,   animals do not produce their own food but must eat other organisms to obtain it.  Animals are generally more complex structurally.  Unlike plants, they have nerves   and muscles that aid in rapid, controlled movement around their environment.  Animal cells usually do not have rigid walls like those of plants.  This accounts for the fact that your skin and flesh are flexible and the   trunk of a tree is not.
This simple dichotomy   between plants and animals is not adequate to encompass all life forms.  Some organisms have characteristics which do not qualify them to fit neatly   into either kingdom.  For instance, funguses and most bacteria do not   photosynthesize and most of them lack a means of controlled locomotion.  Some organisms have   attributes of both plants and animals.  For instance, there is a group   of common single-cell species living in fresh water ponds called Euglena that photosynthesize and have their own means of locomotion   (whip-like tail structures called flagella).  Because of these   and other exceptions, new kingdoms of living things had to be created.
  More information about the kingdoms      of living things   
Research done over the   last half century has shown us that there are even stranger single-celled organisms known as archaeobacteria that live in extremely harsh anaerobic environments such as hot   springs, deep ocean volcanic vents, sewage treatment plants, and swamp   sediments.  Unlike other life forms, they usually   get their energy from geological sources rather than from the sun.   There are also microscopic things that are not quite alive by definition but   have some characteristics that are similar to living things.  These are the viruses and prions .  It is easy to overlook the importance of these extremely small things   because they cannot be seen with the naked eye.  However, there are   very likely around ten times as many viruses as all living things put   together.  There are about 50 million viruses in 1 cm� of ocean   water.  It has been estimated that these viruses are responsible for   the death of 20% of all oceanic bacteria every day, thereby keeping the   phenomenal reproductive capability of bacteria under control.  There   are also complex interactions between bacteria, viruses, and other microbial   life forms within our own bodies.  Most of the time, there are about 10   times as many microbial cells within us as there are body cells.
Phylum
Immediately below kingdom   is the phylum
level of classification.  At   this level, animals are grouped together based on similarities in basic body plan or   organization.  For instance, species in the phylum Arthropoda
have external   skeletons as well as jointed bodies and limbs.  Insects, spiders, centipedes, lobsters, and crabs are all arthropods.
 
Phylum Mollusca
In contrast,   members of the phylum Mollusca
have   soft, unsegmented bodies that are usually, but not always, enclosed in hard shells.  They also usually have at least one strong foot that helps them move.  Octopi, squids, cuttlefish, snails, slugs, clams, and other shellfish are mollusks.
 
Bilateral symmetry
(phylum Chordata)
There are at least 33 phyla (plural   of phylum) of animals.  Humans are   members of the phylum Chordata
.   All of the chordates have elongated bilaterally symmetrical
bodies.  That is to say, the left and right sides are essentially mirror images of each other.  If there are two functionally similar body   parts, they are usually found roughly equidistant from the center line, parallel to each   other.  Note the location of the woman's eyes, nostrils, and cheeks relative to the   center line of her body.
 
Gill slits
(phylum Chordata)
At some time   in their life cycle, chordates have a pair of lateral gill slits or pouches used to obtain oxygen in a liquid environment.  In the case   of humans, other mammals, birds, and reptiles, lungs replace    rudimentary gill slits after the embryonic stage of development.  Frogs   replace them with lungs in the transition from tadpoles to adults.  Fish retain their   gill slits all of their lives.
Chordates also have a notochord
at some phase in their life cycle.  This is a rudimentary   internal skeleton made of stiff cartilage that runs lengthwise under the dorsal surface of the body.  Generally, there   is a single hollow nerve chord on top of the notochord.  Among humans and the other vertebrates , the notochord is replaced by a more   complex skeleton following the embryonic stage of development.
Members of   the phylum Chordata also often have a head, a tail, and a digestive system with   an opening at both ends of the body.  In other words,   the body organization is essentially that of a tube in which food enters one   end and waste matter passes out of the other.   The chordates include mammals, birds,   reptiles, amphibians, fish, as well as the primitive lancelets (or amphioxus) and tunicates (or sea squirts).
 
              Subphylum
The chordates are divided into three subphyla .  Humans are members of the subphylum Vertebrata
.  Among the vertebrates ,   the simple hollow dorsal nerve tube is replaced by a more complex tubular bundle of nerves called a spinal cord.  A segmented vertebral   (or spinal) column of cartilage and/or bone develops around the spinal cord of   vertebrates to protect it from injury.  At one end of the spinal cord is a head with   a brain and paired sense organs that function together to coordinate movement and   sensation.
Vertebrata is   the most advanced and numerous subphylum of chordates.  It includes all of the fish,   amphibians, reptiles, birds, and mammals.  Collectively, there are about 43,000   living vertebrate species in comparison to just over 1500 species in the other two invertebrate subphyla of chordates.
NOTE:  Because science is constantly expanding our   knowledge of living things, the precise details of how organisms are   classified in the Linnaean system are frequently in flux.  This is not due to confusion but rather to the   evolution of our understanding brought about by new discoveries.  For instance, as a result of the discovery of a   dramatically new form of life known as archaeobacteria, a growing number of researchers now use a classification level above kingdoms referred to as a domain.  They define 3 domains of living things: Archaeo (simple bacteria-like organisms that live in extremely harsh   anaerobic environments--these are the archaeobacteria), Bacteria (all other bacteria, blue-green algae, and spirochetes), and Eukarya (organisms with distinct nuclei in their cells--protozoans,   fungi, plants, and animals).