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This lecture is about
Phylum : Arthropods - Animals with Jointed Legs
General Characteristics

The phylum contains more species than any other phylum. They are commonly called Arthropods
(arthron = joint + pods = feet). Insects (cockroaches, grasshoppers, butterlies, mosquitoes) are
most commoh arthropods on the earth.
The body is segmented. Each segment is attached to its neighbour by means of a modiied portion
of cuticle which is thin and lexible. They possess jointed appendages. These appendages have
been modiied for specialized functions.
These are believed to have common origin with annelids because both have some common
characteristics such as segmented body, appendages and cuticle.
Arthropods have exploited every type of habitat on land and in water. The aquatic species include
both freshwater and marine. Many of these can ly, therefore visit air periodically.
Arthropods are variable structurally. Some are worm-like centipedes while the others are lying

insects with the body divided into distinct regions, the head, thorax and abdomen. The body is

covered with waterproof chitinous cuticle secreted by the epidermis.
The coelom is not present as the main body cavity. Instead a haemocoel has developed. It is reduced
coelom and communicates with blood vascular system.
The digestive system is in the form of alimentary canal with two openings, the mouth and anus. It is
divided into diferent parts each performing a speciic function. The food comprises of small plants
and animals.
A well developed excretory system comprising of Malpighian tubules is present in arthropods. The
nitrogenous wastes are excreted in the form of solid uric acid.
A highly deyeloped nervous system is present. It consists of paired ganglia (simple brain) connected
to a ventral double nerve cord. A ganglion is present in each segment. Nerves arise from these
ganglia. The sensory organs are usually a pair of compound eyes and antennae etc.
Most arthropods possess an extensive tracheal system formed of air tubes called tracheae for
the exchange of gases. Main tubes open to the exterior through paired openings, called spiracles.
Aquatic art hropods respire through gills and book lungs.
The blood circulatory system in arthropods is unique. It is open circulatory system. The blood lows
in the body cavity bathing the tissues of the body. However, there is a primitive heart and a main
blood vessel situated dorsaliy. Blood is colourless as it is without haemoglobin.
The skeleton is external, i.e., exoskeleton. It is in the form of an outer covering, the cuticle which
is light in weight; and is formed chiely of chitin. It provides surface for the attachment of muscles
which help in locomotion.
The arthropods exhibit active and swift movements. They swim, crawl or ly depending upon the
habitat they occupy. The organs of locomotion are paired appendages and in some cases paired
wings also.Reproduction and Life History
The sexes are separate. The testes and ovaries, produce sperm’s, and eggs respectively.
Metamorphosis
Life history of insects is characterized by metamorphosis (meta = change + morphe = form). This is
an abrupt change of form or structure during the life cycle. There are three morphologically distinct
stages in the life cycle, the egg “inally“ develops into larva which is converted into motionless pupa
that inally develops into an adult. In some primitive insects the metamorphosis is incomplete. The
larva resembles adult and is called nymph or instar. It lives in the same habitat as adult.

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