Chelicerata

Taxonomy
The subphylum Chelicerata consists of three classes, but is best known by the Arachnids (spiders, scorpions, etc.). The Class Merostomata includes Xiphosura (horseshoe crabs), and Eurypterida (an extinct order of chelicerates). However, some argue that Xiphosura should be considered a class as its placement in Merostomata constitutes an acceptation of an unnatural (paraphyletic) group. Pycnogonida is the sea spiders.

Anatomy
The Chelicerate body is divided into two sections the Prosoma or Cephalothorax, which is the head region, and the Opisthosoma or Abdomen which is the rest of the body. Spiders have an exoskeleton that is made up of a tough material called cuticle. Cuticle is a composite material made up of a protein with microfibres of chitin embedded in it. The cuticle is divided into four layers the Epicuticle, the Exocuticle, the Mesocuticle, and the Endocuticle. Spiders get their color from pigment granules contained in the epithelial cells. It is not commonly known but spiders also have a small endoskeleton. This endoskeleton contains no chitin, but is made up of cartilage like material based on collagen fibres and a homogenous ground substance. Attached to the Prosoma are four pairs of walking legs, one pair of Chelicerae (fangs) and one pair of pedipalps. The pedipalps of mature male spiders are modified as copulatory organs. Each of the spiders legs has seven segments. Starting from the body and working outwards they are called Coxa, Trochanter, Femur, Patella, Tibia, Metatarsus and Tarsus. The femur, tibia and metatarsus are normally long where as coxa, trochanter and the patella are short. The tarsus is intermediate in length, but more slender than chunky. The pedipalps have only six segments, they are missing the metatarsus. The pedipalps are normally shorter than the legs, although the relationships between the separate sections is similar. The chelicerae consist of two segments. Nearest the body, and attached to it, is a stout section called the basal segment, beyond this is the fang. The fang is movable and normally serrated on its inner edge. Along the inner edge of the basal segment there is a groove, often with serrated edges that the fang folds into when it isn't being used. 

Spiders have an open circulatory system, meaning the organs of the body are bathed an the internal sea of respiratory fluid called haemolymph. The heart is a tube with several ostia. These ostia are small holes that allow the haemolymph into the heart. When the heart muscles contract these holes close and the blood is forced out into the two main arteries that run from each end of it. These arteries are called the anterior aorta (to the prosoma) and the posterior aorta (to the opisthosoma). The heart has valves at either end to ensure the blood always flows in the same direction. These arteries branch out through the whole animal, until they become very small. They have open ends which allows the haemolymph to reach the animals tissues. From here it passes through the book lungs where gaseous exchange occurs. From the book lungs veins take the blood into the pericardium, which is a cavity or space that contains the heart, from where it can enter the heart during diastole and start its journey again. Spiders respire through book lungs. The trachea lies posterior to the book lungs and open to the external world just anterior of the spinnerets, often through a single opening called a stigma. The development of the trachea is quite varied between different spider families. Usually there are two sets of tubes called the lateral and median tubes. Trachea can be very complicated and highly branched or simple and unbranched.

The nervous center of a spider is situated in the prosoma. The central nervous system is compacted with the brain to produce a single mass of nervous tissue. This single mass can be divided into the lower star-shaped subesophageal ganglion and the upper spherical supraesophageal ganglion. A number of nerves arise from these ganglia and spread out to the body, making up the peripheral nervous system. The supraesophageal ganglion can again be divided into the cheliceral ganglion and the brain. The cheliceral ganglion controls the musculature of the chelicerae, the pharynx and the poison glands. The brain is mainly concerned with association activities. It only receives information from the eyes, via the optic nerve. Motor nerves that control the the legs and the pedipalps originate laterally from the subesophageal ganglion. Posteriorly the subesophageal ganglion gives rise to a set of nerves called the 'cauda equina' which pass through the pedicel to control the opisthosoma. Spiders are hairy, and most of the hairs on a spiders body are connected to nerves and serve to give the spider information. The lightest touch can trigger a response. The spines on the legs are also connected to nerves. Spiders also use their hairs to groom themselves, as with all animals keeping clean is important to spiders. Thirdly, many spiders have special groups of hairs on their feet called the scopulae. This is a dense collection of short hairs each of which splits into 500 to 1,000 micro-hairs at its end. Slit sense organs are stress and strain receptors in the spider's exoskeleton. They can be found all over the spider's body but are most common on the legs. Even small spiders have thousands of them. They perform a number of roles including detecting certain sounds, detecting other vibrations, even through water, detecting gravity and stresses in the exoskeleton and thus helping the spider move effectively. Chemical senses are also important to spiders, both those for taste and those for smell. Little is known about some of the spider's sense of smell, though we know they have one. The tarsal organs (small pits on the dorsal side of each tarsus) are now believed to be moisture detectors. Spiders are also known to be sensitive to fine changes in temperature, both inside and outside their own bodies. The tips of the legs and the spinnerets are believed to be the most sensitive areas. 

Most spiders have eight eyes. Spiders have simple eyes, meaning there is just a single lens to each eye. They have primary and secondary eyes. In primary eyes the rhabdomeres (the light sensitive part of a visual cell in the retina) is toward the light. In secondary eyes the rhabdomeres face away from the light, (as they do in our eyes). Primary eyes have no tapetum (the reflective layer at the back of the eye that causes eye-shine in a car's headlights in cats and dogs), but secondary eyes do have a tapetum. 

Reproduction
Most of the organisms in Chelicerata reproduce sexually. There are a lot of organisms involved with the subphylum Chelicerata. First of all, in the class Arachnid, there are spiders, scorpions, and other similar creatures. Spiders all reproduce sexually. The female spider keeps the male sperm until she is ready to use it on the eggs. The next class is Xiphosura (Merostomata). The horseshoe crab is one of the main organisms in this class. The reproduction of the crab is as follows. The female goes onto the beach where she is grabbed by the male. Then, the male waits for the eggs to come out, he spermates them, and the female buries the eggs. The sea spider represents the class Pycnogonida which comes after the class Xiphosura (Merostomata). The sea spider reproduces externally with a female and then cares for the eggs and the young. Some of the sea spiders do not produce sexually.  These are examples of organisms in the classes that are in the subphylum Chelicerata.