Fossil range: Cambrian or earlier – Recent
Tarantula Brachypelma sp.
Subphyla and Classes
• Trilobita — trilobites (extinct)
• Arachnida — spiders, scorpions, etc.
• Xiphosura — horseshoe crabs, etc.
• Pycnogonida — sea spiders
• Eurypterida — sea scorpions (extinct)
• Chilopoda — centipedes
• Diplopoda — millipedes
Insecta — insects
• Branchiopoda – brine shrimp etc.
• Cephalocarida – horseshoe shrimp
• Maxillopoda — barnacles, fish lice, etc.
• Ostracoda – seed shrimp
• Malacostraca — lobsters, crabs, shrimp, etc.
An arthropod is an invertebrate that has an exoskeleton
(external skeleton), a segmented body, and jointed at-
tachments called appendages. Arthropods are animals
belonging to the Phylum Arthropoda (from Greek
ἄρθρον arthron, "joint", and ποδός podos "foot", which
together mean "jointed feet"), and include the insects,
arachnids, crustaceans, and others. Arthropods are char-
acterized by their jointed limbs and cuticles, which are
mainly made of α-chitin; the cuticles of crustaceans are
also biomineralized with calcium carbonate. The rigid
cuticle inhibits growth, so arthropods replace it period-
ically by molting. The arthropod body plan consists of
repeated segments, each with a pair of appendages. It is
so versatile that they have been compared to Swiss
Army knives, and it has enabled them to become the
most species-rich members of all ecological guilds in
most environments. They have over a million described
species, making up more than 80% of all described living
species, and are one of only two groups very successful
in dry environments – the other is amniotes. They range
in size from microscopic plankton up to forms a few
Arthropods’ main internal cavity is a hemocoel,
which accommodates their internal organs and through
which their blood circulates – they have o