Phylum arthroPoda
• Phylum Arthropoda (Gk. arthros = joint + Podos = foot) is the
largest phylum of Kingdom Animalia.
• They are important members of marine, freshwater and land
ecosystems and are one of the two major groups that have
adapted to life in dry environments. The other group is amniotes
(reptiles, birds and mammals).
general CharaCters
• The body is segmented externally and the segments are
grouped into two regions called tagmata (singular, tagma).
There may be two tagmata, head or cephalothorax and trunk
or abdomen or three tagmata, head, thorax and abdomen. The
segments are not separated internally by septa.
• The body is of varying shapes having bilateral symmetry.
• Arthropods are triploblastic with three germ layers i.e.
ectoderm, endoderm and mesoderm.
• Arthropods have organ-system level of organisation with a
tube-within-a-tube body plan. Tube-within-a-tube body
plan corresponds to presence of a complete digestive tract i.e.,
with mouth and anus inside the body cavity.
• The body walls consists of a thick, chitinous cuticle and a
single layered epidermis.
• Cuticle, a characteristic feature of arthropods, is thick,
non-living and horny or chitinous layer. It forms
the exoskeleton. It is secreted by hypodermis or
epidermis. The hard cuticle restricts growth and is
periodically shed during growth by a process known as
ecdysis. Cuticle is one of those features of arthropods
which are primarily responsible for their success on land.
Some specific features of cuticle are :- It provides support, an essential requirement of
terrestrial animals. - It provides rigidity. In insects, the flight depends on
the possession of rigid wings, which is provided by
cuticle. - It provides protection. Arthropods have hardened,
heavily sclerotized cuticle which makes them difficult
for predators to catch or parasites to parasitise. - Wax layer of cuticle restricts the water loss.
- Cuticle is also modified to form sense organs like
sensilla on antennae of various insects in the form
of hair (Bombyx), pegs (locusts), flat plates (bees
and aphids).
Cuticle
• The body cavity around the viscera contains blood and is called
the haemocoel. True body cavity or coelom is reduced to
cavities in the reproductive and excretory organs in adults.
PhysiologiCal CharaCters
digestive system
• It is complete and consists of a digestive tract divided into
3 distinct regions : stomodaeum (foregut), mesenteron
(midgut) and proctodaeum (hindgut).
• Digestion is intercellular or extracellular.
• Mouth consists of movable appendages, the mouth
parts are adapted for chewing, sucking, sponging etc. To
understand the mouth parts found in arthropods, insects
are taken as model.Circulatory system
• Circulatory system is of open type, or lacunar type i.e.,
blood does not flow in definite vessels.
• Blood flows in haemocoel. Haemocoel is made up of
irregular spaces known as lacunae or sinuses.
• Blood is colourless and contains WBCs only. It is also
known as haemolymph.
• Dorsal, often many-chambered, pulsatile heart is present.respiratory system
• Respiration takes place either by general body surface
or by special structures such as gills, book gills, book
lungs and tracheal system. Different types of respiratory
organs are found in different groups of Arthropoda.excretory system
• Excretory organs are green glands and Malpighian
tubules.
• Green glands are also called as antennal or antennary
glands. They are found in pair in the third segment
of crustaceans. They open to the exterior at the base
of second antenna and serve as osmoregulatory and
excretory organ.
• Malpighian tubules are small tubular structures that
are involved in excretion of nitrogenous wastes and are
found in insects, arachnids and centipedes.
• In insects, they open into the intestine. They selectively
extract uric acid from blood which along with water and
salts is deposited into the hindgut and excreted in faeces.
Some reabsorption may occur in tubules but most of it
occurs in hindgut.
• In some forms coxal glands are excretory organs. They
are paired ducts that lead from coelom to exteriors
and are common in arachnids where they are found in
cephalothorax opening at bases (coxae) of legs, thus
named so.
• Coxal glands and green glands open to exterior while
Malpighian tubules open inside the body cavity in some
organ, mostly intestine.