The arachnids, are a class of joint-legged invertebrate animals in the phylum Arthropoda. They are chiefly terrestrial, comprising some 65,000 to 73,000 named species including spiders, scorpions, harvestmen, ticks, and mites. Arachnids may be easily distinguished from insects by the fact that arachnids have eight legs whereas insects have six. The small leg like pair of appendages called pedipalps of some species have been adapted for sensory, prey capture or reproductive functions. They have a two-segmented body, made up of a cephalothorax and an abdomen, the cephalothorax being derived from the fusion of the cephalon (head) and the thorax. Arachnids are mostly carnivorous, feeding on the pre-digested bodies of insects and other small animals. Many are venomous - they secrete poison from specialized glands to kill prey or enemies. Others are parasites, some of which are carriers of disease. Arachnids usually lay eggs, which hatch into immature adults.