Class Arachnida of Phylum Arthropoda
Body divided into cephalothorax and abdomen. Cephalothorax along with four (4) pairs of legs; abdomen segmented or unsegmented, with or with no appendages.-Respiratory organs are, either tracheae, or book lungs. Excretory organs are Malpighian tubules or coxal glands. Brain bilobed associated to ventral ganglionic mass forming a ring. Eyes simple mostly oviparous, no metamorphosis. Arachnids, the largest of all the chelicerate classes, include some of the common and well-known but diverse forms, like spiders; ticks, mites, scorpions, pseudoscorpions, whip scorpions, harvestmen (daddy longlegs) etc. Arachnids are one of the oldest classes of arthropods, the fossil forms dating back to Silurian period. Most living arachnids are terrestrial. The result of it is that, the epicuticle has became waxy reducing water loss, the book gills became changed into booklungs for use in air and the appendages became better adapted for terrestrial locomotion. The body of arachnids displays three district regions: the prosoma that is unsegmented and covered by a carapace; the mesosoma or preabdomen and the metasoma or postabdomen. Except in scorpions, in other arachnids the two divisions of the abdomen are not conspicuous and the segments are fused. The appendages are generally confined to prosoma and consist of a pair of chelicerae, a pair of pedipalpi and four pairs of walking legs. There are no antennae and mandibles.