Sentence:  Mygalomorph spiders tend to be relatively long lived. The majority of these spiders live in burrows in the soil. Many of the burrowing forms have a patch of short stiff spines on their chelicerae called a <WSD>rastellum</WSD> that they use for digging. 
Meaning of the word marked with <WSD> tags:  In certain mygalomorph spiders, a row of stiff spines on the chelicera.