What is the function and origin of silk and spinnerets in spiders?

What is the function and origin of silk glands and spinnerets in spiders?

The development of spinnerets and silk represents a major evolutionary shift that has defined the biological and ecological uniqueness of spiders within the arachnids. Silk glands produce the silk that the spider uses for a variety of purposes. The spinnerets are the special organs that the spider uses to extract and manipulate the silk as is it is produced from the silk glands.

Spiders evolved from ancestors that had limbs on the abdomen, as did arthropods like crustaceans such as crayfish. In fact, one of their few living marine relatives, Limulus, the so-called "king crabs", has retained abdominal limbs, which have been lost or greatly modified in terrestrial spiders and other arachnids. The spiders' spinnerets are almost certainly derived from these ancestral abdominal limbs. In the basal (lowest) segments of spiders' limbs are small excretory glands - the coxal gl