Crabs, prawns, lobsters, etc
are members of a large category of animals called Arthropoda:
All Arthropoda have joined legs and an outer skeleton whose
components include a horny substance called chitin.
Phylum Arthropoda includes
the following groups:
Class Chilopoda: Centipedes
Class Diplopoda: Millipedes
Class Insecta: Insects
Class Arachnida: Spiders and other Arachnida
Arachinda includes Arthropoda with four pairs of
legs, with neither wings nor antennae. The body is divided into
two parts: the cephalothorax in front and the abdomen, or the
two parts are fused into an unsegmented whole. The Class includes
smaller groups such as the following:
Order Scorpionida: Scorpions
Order Phalangida: Harvestmen
Order Acarina: Mites and ticks
Order Araneidae: Spiders
Golden Web Spider
St Andrew's Cross Spider
are different from other Arachnids in at least three ways:
abdomen is attached to the cephalothorax by a narrow waist.
In scorpions, harvestmen and mites, the cephalothorax and
abdomen are joined by a broad connection.
some primitive spiders have plates corresponding to segments
at the abdomen, the abdomen of spiders is not divided into
segments as in scorpions.
are the only Arachnids that are equipped with spinnerets
at the hind end of the abdomen for silk production.