The horseshoe crab, though not stated in its name, is more closely related to arachnids, such as the spider and scorpion, rather than crabs. They are most commonly found near the northern Atlantic coast of the United States and the Gulf of Mexico, and have a lifespan of about 25 years.
The horseshoe crab's structure is composed of a tough outer layer covering its body that is known as its exoskeleton. This covering provides the animal with protection from predators such as the Atlantic Loggerhead Turtle.
Another way the horseshoe crab protects itself is by use of the two sets of eyes found on its prosoma (head). These two sets include one pair of "median eyes" - or "light receptors" - which can be found on the topmost part the head, and a single pair of compound eyes located toward the side of the head. 
Horseshoe crabs breathe with five sets of "book lungs" that are located on the ventral side of its body. Book lungs act as gills when the arthropod is underwater, but they also allow it to breathe while it's on land for small increments of time, as long as the lungs remain moist.
The mouth of the crab is located at the centermost part of its body which makes for easy food capture. It does not have a jaw line, however, but has mandibles - the kind of jaw found in many invertebrates. 
Five pairs of legs which are attached to the underside of it's body near the abdomen are used for both transportation and aiding in eating. Located on the tip of the thin legs are pointy pincers. Male horseshoe crabs have a large claw attached to a single leg which helps hold the female during reproduction, when eggs the female lays are then fertilized by the male then buried by the female.
A long spine-like tail is attached to the last section on its body. Though it looks as if it would be used for defense, it is, in fact, most useful when the animal is turned upside down on the beach. When such things happen to the crab, it will use its tail to turn its body upside right so its ventral side is back on the ground. 
Like all other arthropods, this "crab" will often times molt - meaning to shed its outer layer. It does this only when the shell holding its body together on the outside becomes too small to contain its inner structure. Horseshoe crabs can grow up to about 20 inches from head to tail. The male is often times smaller in size than the female.
Horseshoe crabs, like most invertebrates, reproduce sexually. The reproduction of these animals usually takes place in the spring when they meet each other on the beach. The male grabs the female with its large claw as it waits for eggs to emerge from her body.  The female will generally produce up to 60,000 tiny green eggs which she buries in the sand on the beach after the male fertilizes the them. Soon, small horseshoe crabs will find their way out of their eggs in the sand and head for the ocean. The larvae which is produced from these eggs begin to molt for the first time after several days of swimming and settling onto the ocean floor. The process of abandoning its outer layer continues as the crab grows and heads further out into the ocean. It does not reach sexual maturity, however, until about eleven years after it is first excreted from the egg. 
Horseshoe crabs are most commonly found near the northern Atlantic coast of the United States and the Gulf of Mexico. They typically feed upon mollusks, annelid worms, and other benthic invertebrates. They locate their food by crawling along on the ocean floor at night, making them predominantly nocturnal animals. This varies though, as the horseshoe crab may also be found active during the day time.
Predators of horseshoe crabs include large sea turtles such as the Atlantic Loggerhead Turtle.
It has been recently established that horseshoe crabs have the ability to regenerate. Regeneration is the ability to grow back lost limbs. When attacked, limbs can be discarded to escape a predators grasp, which will then grow back after a short period of time. 
Horseshoe crab's blood, which is seen as a blue color when mixed with air, is often used in the medical field. The protein in the blood, known as Limulus Amebocyte Lysate (LAL), helps to test the bacteria that can often times be the main cause of fevers and death.
Doctors also use the blood to test the "endotoxins" (bacteria) that cause the rapid growth of cancer cells. They hope to find a cure for leukemia by using the blood which reacts with the white and red blood cells in the human body.
A substance that has recently been discovered in the blood reacts with Vitamin B12, which is a main cause of several mental diseases, damage to the intestine, and pernicious anemia. Doctors hope to use the horseshoe crab's blood to find cures to these deficiencies. 
Horseshoe Crab and Evolution
Like many plants and animals, the horseshoe crab is a living fossil. Fossilized specimens have been found that are believed to be 300 million years old, and yet are virtually identical to modern varieties. These findings are consistent with the Bible, which says that God created the horseshoe crab and all the other types of sea creatures on the fifth day of creation only a few thousand years ago.
Their existence puzzles evolutionists. In National Geographic, Biologist Sue Schaller says "You've got an animal that predates dinosaurs by 200 million years, and it hasn't changed much at all. It hasn't had to evolve."
- Horseshoe Crab Wikipedia
- Horseshoe Crabs "A Living Fossil" Maryland Department of Natural Resources
- Horseshoe Crab Enchanted Learning
- BLB (KJV) Gen 1 Blue Letter Bible