Author: Jan Scholten
Zoology: invertebrate; exoskeleton, an external skeleton; segmented body; jointed appendages; cuticles, which are mainly made of alpha-chitin; biomineralized with calcium carbonate; 1 million species, more than 80% of all described living animal species; some very successful in dry environments.
Name: jointed leg
3. Insects, arachnids.
An arthropod is an invertebrate that has an exoskeleton (external skeleton), a segmented body, and jointed attachments 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 characterized 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 periodically 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 meters long.
Arthropods' main internal cavity is a hemocoel, which accommodates their internal organs and through which their blood circulates, they have open circulatory systems. Like their exteriors, the internal organs of arthropods 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 esophagus. 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: in most species the ocelli can only detect the direction from which light is coming, and the compound eyes are the main source of information, but the main eyes of spiders are ocelli that can form images and, in a few cases, can swivel to track prey. 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.
Arthropods' methods of reproduction and development are diverse, all terrestrial species use internal fertilization, but this is often by indirect transfer of the sperm via an appendage or the ground, rather by direct injection. Aquatic species use either internal or external fertilization. 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 and work out their evolutionary ancestry, which dates back to the Cambrian period. From the late 1950s to late 1970s, it was thought that arthropods were polyphyletic, that is, there was no single arthropod ancestor. 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"), while annelids belong to another superphylum, Lophotrochozoa. The relationships between various arthropod groups are still actively debated.
Although arthropods contribute to human food supply both directly as food and more importantly as pollinators of crops, they also spread some of the most severe diseases and do considerable damage to livestock and crops.
1.5 Internal organs
2 Reproduction and development
3.1 Last common ancestor
3.2 Fossil record
3.3 Evolutionary family tree
4 Classification of arthropods
5 Interaction with humans
7 External links
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.