The bodies of these animals are divided into two
  segments: a cephalothorax and an abdomen.

Chelicerates (spiders and their relatives) share three

First, cheliceraes do not have sensory antennae,

Second, most have simple eyes, and

Third, all chelicerates have a pair of appendages
  called chelicerae
Chelicerae are the anterior-most appendages,
 modified into pincers or fangs for killing
 prey and in some species the chelicerae can
 inject poison.
Most live in terrestrial environments.

In order to successfully live on land, the
  organism had to evolve with ways to retain
  water in their bodies.

Terrestrial chelicerates have three structures
  that aid in water retention: Malpighian
  tubules, an exoskeleton, and book lungs
Malpighian tubules are excretory structures
 that remove metabolic wastes from blood
 and return water to the cells.

Book lungs provide efficient gas exchange
 without water loss.

They are called book lungs because they look
 like the inverted pages of a book.
Horseshoe crabs are marine animals that
 belong to the small class Merostomata.

There are only four living species of
 horseshoe crabs and three of these species
 live off the coast of Southeast Asia with one
 species of horseshoe crab that lives in the
 coastal regions of the western North
 Atlantic and the Gulf of Mexico.
The horseshoe crab is not really a crab.

Horseshoe crabs belong to an ancient class of marine
 arthropods that evolved about 400 million years

Scientists often refer to the four surviving species of
  horseshoe crabs as “living fossils” because they
  closely resemble their fossil ancestors.
Horseshoe crabs have both compound and simple
 eyes and do not have Malpighian tubules.
Horseshoe crabs exchange gases in five pairs of
 book gills which are similar to book lungs and
 may have evolved into book lungs found in
 terrestrial arachnids.

Horseshoe crabs are among the few chelicerates that
 eat solid food by feeding on clam worms, soft-
 shelled clams, and other animals.
Arachnids are terrestrial chelicerates that
 make up the much larger class Arachnida.
There are three familiar orders of arachnids:
 spiders, scorpions, and mites and ticks.
All arachnids have a cephalothorax with six
 pairs of appendages: one pair of chelicerae,
 one pair of specialized appendages called
 pedipalps, and four pairs of walking legs.
Pedipalps are used for holding food and sense
The first order of arachnids includes spiders.
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Spiders are divided into two groups based on
  how they gather food; individuals that hunt
  prey and individuals that weave webs to
  catch prey.

Hunting spiders usually have strong legs and
 excellent eyesight.

Web weavers generally have long, slender
 legs and relatively poor eyesight.
Although not all spiders weave webs, they all
  produce silk.
Some spiders use silk for movement-as
  droplines or parachutes.
Other spiders use silk for protection-as a
  covering for their burrows or a canopy
  under which they sleep.
Still other spiders use silk to wrap their food
  for storage.
Spiders paralyze or kill their prey with
  chelicerae modified into fangs.
While holding their prey with their pedipalps,
  spiders secrete enzymes that digest and
  liquefy the prey.
They then suck the liquefied food into their
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The second order of arachnids includes
Scorpions have a long segmented abdomen
  that ends in a poisonous stinger.
A scorpion’s pedipalps are modified into large
  claws for capturing and holding prey.
As it curls its abdomen over trapped prey, the
 scorpion stings and paralyzes it, then
 crushes it and sucks its internal liquids.

Despite their reputation, only a few species of
 scorpions are harmful to humans.

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Mites and ticks are classified in a third order
  of arachinds.
Unlike spiders, mites have only one body
Some mites use their piercing and sucking
  mouthparts to drain the contents of plant
Other mite species eat bacteria that live in
  animal hair follicles.
Some mites even feed on the dead skin cells
  of humans.
Millions of dust mites live in our houses and
  are usually harmless but can cause problems
  for people with allergies.
Most ticks are parasites that require meals of
  blood before they molt.
Ticks attach to their hosts with specially
  adapted mouthparts and their feeding
  process can take several days, during which
  they may grow to many times their original
Ticks differ from spiders in that they have
  only a single fused body section.
Ticks can be dangerous because they can act
  as hosts to parasites that cause diseases such
  as Rocky Mountain spotted fever.
A common dog tick is easy to see because it is
  as large as a sesame seed.
The deer tick, however, which causes Lyme
  disease is very small and difficult to see.
Strauss, E. & Lisowski, M. (2000). Biology: The Web of
   Life, Second Edition. Scott Foresman-Addison Wesley:

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