 Subphylum
Chelicerata comprises a major group within Phylum
Arthropoda, including such animals  as
the arachnids (e.g., spiders and scopions), the extinct eurypterids,
and the extant horseshoe crabs that are considered to be living
fossils. These arthropods and their ancestral and extinct forms
were and are mainly predators. Chelicerates are now predominently
terrestrial animals, with most marine chelicerates, including
all of the eurypterids, now extinct.
Their
name comes from their chelicerae, pointed appendages that grasp
food, that differ from the  chewing
mandibles of most arthropods. Being unable to ingest solid food,
most Chelicerata either drink blood or spit or otherwise inject
digestive enzymes into their prey, and feed on the fluidized result.
Like all arthropods, chelicerates' bodies and appendages are covered
with a tough cuticle primarily composed of chitin and proteins
that chemically harden. Because this exoskeleton cannot stretch,
chelicerates have to molt in order to grow. Thus, they have to
molt the old, and await hardening of the new, during which time
they have reduced mobility and are otherwise more defenseless.
Subphylum
Chelicerata Classification
Subphylum
|
Class |
Orders |
| Chelicerata |
Class
Arachnida |
Acarina (ticks and mites) |
| Amblypygi
(whip scorpions) |
| Araneae
- Spiders (note 3) |
| Haptopoda |
| Opiliones
- Phalangids, harvestmen or daddy longlegs (note 4) |
| Palpigradi - microwhip scorpion |
| Phalangiotarbida |
| Pseudoscorpionida -
Pseudoscorpions (note 5) |
| Ricinulei - hooded tickspiders |
| Schizomida |
| Scorpiones
- Scorpions (note 6) |
| Solifugae - camel spiders |
| Thelyphonida - vinegaroons |
| Trigonotarbida
- Extinct (note 2) |
| Class
Merostomata |
Order
Xiphosura - Horseshoe crabs (note 8) |
| Order
Eurypterida (eurypterids) (note 9) |
| Class
Pycnogonida |
Sea
spiders |
- Arachnids,
which are named for the mythological figure Arachne, are
mainly terrestrial arthropods
comprising some 70,000 extant species including the spiders,
scorpions, harvestmen, ticks, and mites. The arachnids
are easily differentiated from insects by their eight
legs instead of six. The leg-like pedipalpi of some arachnids
are adapted for sensing, capturing prey or reproducing.
- The
Trigonotarbida were spider-like arthropods, and possibly
the oldest terrestrial arthropods. Despite having eight
legs, they are believed to have been an evolutionary offshoot
of the true spiders that arose about the same time in
the Devonian.
- note
3: Spiders are arthropods that produce silk, have eight
legs and no wings. The order is divided into two sub-orders:
the Opisthothelae (which include the infraorders Mygalomorphae
(trapdoor and tarantula spiders) and Araneomorphae (the
modern spiders), and the Mesothelae, which contains the
Family Liphistiidae, primitive burrowing spiders from
Asia.
- Note
4: Opiliones are commonly called harvestmen or daddy long
legs, have eight legs like spiders, except they are exceptionally
long. The order is mainly omnivorous.
- Note
5: Pseudoscorpions are tiny, scorpion-like arthropods
with flat, pear-shaped bodies and eight legs. They have
two very long pedipalps, or pincers, that resemble a scorpion's
claws, but unlike scorpion's have a short and rounded
abdomen without tail and stinger. The pincer contains
a venom gland and duct and the poison is used to capture
and immobilize their tiny prey.
- Note
6: Scorpion remains found in coal deposits from the Carboniferous
Period have no demonstrable difference and extant species,
suggesting no evolutionary need for change for hundreds
of millions of years. All species of scorpion possess
venom that in most species is not dangerous to humans.
- Mites
and ticks were among the first land arthropods; the oldest
known so far are Late Devonian. However, because of their
small size, the Acari are uncommon in the fossil record.
A few are known from the Carboniferous, and more from
Tertiary amber.
- Note
8: The horseshoe crab has a long fossil record from
the
lower ordovician or latest Cambrian, predating flying
insects and dinosaurs, during which time they have
undergone
little
change.
- Note
9: The eurypterids were among the largest and most fearsome
marine predators of the Paleozoic. Varying in size from
some 10 centimeters up to two meters in length, they were
also the largest arthropods that ever lived. They arose
in the Ordovician, and went extinct in the Permian. There
fossil records includes all the continents.
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