The second has for its pair of appendages the small pair of limbs which in all living Arachnids is either chelate or retrovert (as in spiders), and is known as the chelicerae.
Head tritognathous and diprosthomerous - that is to say, with two prosthomeres, the first bearing typical eyes, the second a pair of appendages reduced to a single ramus, which is in more primitive forms antenniform, in higher forms chelate or retrovert.
The " retrovert " or bent-back first pair of appendages is provided with a poison gland opening on the fang or terminal segment.