Author: Jan Scholten
Name: Arthropoda in Greek means "jointed feet; arthron is ‘joint", and podos is "foot".
Genera: ± 1 million species, more than 80% of all described living animal species.
Members: Insects, arachnids; Crustaceans.
Arthropoda are invertebrate animals with an exoskeleton, an external skeleton. They have a segmented body, each with a pair of appendages. They have jointed appendages, cuticles, which are mainly made of alpha-chitin and biomineralized with calcium carbonate. The rigid cuticle inhibits growth, so arthropods replace it periodically by molting. The body is so versatile that they have been compared to Swiss Army knives. They range in size from microscopic plankton up to forms a few meters long.
They are very versatile an successful, even in dry environments.
They have open circulatory systems. The internal organs are generally built of repeated segments. Their nervous system is "ladder-like", with paired ventral nerve cords running through all segments and forming paired ganglia in each segment. Their heads are formed by fusion of varying numbers of segments, and their brains are formed by fusion of the ganglia of these segments and encircle the oesophagus. The respiratory and excretory systems of arthropods vary, depending as much on their environment as on the subphylum to which they belong.
Their vision relies on various combinations of compound eyes and pigment-pit ocelli. Arthropods also have a wide range of chemical and mechanical sensors, mostly based on modifications of the many setae (bristles) that project through their cuticles.
Almost all arthropods lay eggs, except for scorpions, who give birth to live young after the eggs have hatched inside the mother. Arthropod hatchlings vary from miniature adults to grubs and caterpillars that lack jointed limbs and eventually undergo a total metamorphosis to produce the adult form. The level of maternal care for hatchlings varies from zero to the prolonged care provided by scorpions.
The versatility of the arthropod modular body plan has made it difficult for zoologists and paleontologists to classify them. Formerly they were regarded as polyphyletic, now they are generally regarded as monophyletic. Traditionally the closest evolutionary relatives of arthropods were considered to be annelid worms, as both groups have segmented bodies. It is now generally accepted that arthropods belong to the superphylum Ecdysozoa, animals that molt. Annelids belong to another superphylum, Lophotrochozoa.
Subphyla and Classes
Trilobita: trilobites (extinct)
Arachnida: spiders, scorpions, etc.
Xiphosura: horseshoe crabs, etc.
Pycnogonida: sea spiders
Eurypterida: sea scorpions (extinct)
Branchiopoda: brine shrimp etc.
Cephalocarida, horseshoe shrimp
Maxillopoda: barnacles, fish lice, etc.
Ostracoda: seed shrimp
Malacostraca: lobsters, crabs, shrimp, etc.