Arachnids

Arachnids are a class (Arachnida) of joint-legged invertebrate animals in the subphylum Chelicerata. All arachnids have eight legs, although in some species the front pair may convert to a sensory function. Arachnids are chiefly terrestrial arachnids, but are also found in freshwater and, with the exception of the pelagic zone, in all marine environments. They comprise over 100,000 named species, including spiders, scorpions, harvestmen, ticks, and mites.It is commonly understood that arachnids have four pairs of legs, and that arachnids may be easily distinguished from insects by this fact (insects have six legs or three pairs). Arachnids are further distinguished by the fact they have no antennae and no wings. Their body is organized into two tagma called the prosoma, or cephalothorax, and the opisthosoma, or abdomen. The cephalothorax (prosoma) is derived from the fusion of the cephalon (head) and the thorax. The abdomen (opisthosoma) can be further divided into the preabdomen and postabdomen in many taxa, although in some orders such as the Acari the abdominal sections are fused.
Arachnids are mostly carnivorous, feeding on the pre-digested bodies of insects and other small animals. Only in the harvestmen and among mites, such as the house dust mite, is there ingestion of solid food particles and thus exposure to internal parasites , althougth it is not unusual for spiders to eat their own silk. Several groups are largely venomous — they secrete venom from specialized glands to kill prey or enemies. Several mites are parasites, some of which are carriers of disease. Arachnids usually lay eggs, which hatch into immatures that resemble adults, but scorpions bear live young.
(informention taken from wikipedia)