PHYLUM-ARTHROPODA
Phylum—Arthropoda (Arthron jointed; podos foot)
includes the first and simplest segmented animals. These are
commonly called as jointed legged animals. It is the largest
group of animal that includes about 1000000 insect species,
102248 spiders and scorpion species, 103248 arachnoid
species and 47000 crustacean species representing about
80% of total known animal species.
General Features
Some important general features of phylum—Arthropoda
are discussed below
(i) Habitat and Habit They may be aquatic or
terrestrial. They may occur as free-living or parasitic
forms e.g., bed bugs, ticks, mosquitoes, etc.
(ii) Body Parts Body is segmented externally. It has distinct
thorax and abdomen. Head bears many fused
segments and sense organs.
(iii) Symmetry and Body Organisation Arthropods are
bilaterally symmetrical, They are triploblastic with
organ system level of organisation.
(iv) Appendages They have jointed, paired appendages,
which are present in some or all somites or segments.
These perform various functions like walking
clinging, jumping, feeding, etc.
v) Exoskeleton The exoskeleton is formed of thick,
tough and non-living chitinous cuticle.
Exoskeleton protects animal from mechanical and
chemical injuries, prevents desiccation and also helps
in attachment.
i) Locomotion Jointed appendages help in
locomotion. Cilia are found to be absent in
arthropod.
(vii) Digestive System Alimentary canal is complete and
well-developed, i.e., divided into foregut, midgut
and hindgut.
(viii) Respiration It occurs through general body surface’
gills (e.g., prawn), book lungs (e.g., scorpion an
spider), trachea (e.g., cockroach) and book gills (e•g.,
king crab).
(ix) Circulation The circulatory system is open type,
i.e., blood flows in the haemocoel instead of blood
vessels.
(x) Excretion The excretory organs are Malpighian
tubules (opening into duct) or green glands opening
directly to the exterior.
(xi) Nervous System The nervous system consists of a
nerve ring and a double ganglionated ventral nerve cord.
(xii) Sense Organs Arthropods have simple eyes called
ocelli and compound eyes.
Aquatic forms have statocysts for balancing.
(xiii) Reproduction Arthropods reproduce by sexual
methods. Fertilisation is usually internal type. They
are generally oviparous but few are viviparous, i.e.,
scorpion. Development may be direct or indirect
involving metamorphosis.
(xiv) Classification The phylum—Arthropoda is divided
into five classes
Class—Crustacea, e.g., Palaemon (prawn), crab, etc.
Class—Myriapoda, e.g., Scolopendra (centipede),
Julus (millipede), etc.
Insecta, Musca (homefly), Apis (honeybee),
Pest-Locusta (locust), Vector-Mosquitoes
(Anopheles, Culex, Aedes), etc. e.g., spider, ticks,
mites, Limulus (king crab),
Economically important insects are
Apis – Honeybee
Bombyx – Silkworm
Laccifer – Lac insect
Limulus(king crab) is living fossil from Arthropoda
phylum.