Such chelate rami or limbbranchesare independently developed in Crustacea and inArachnida, and are carried by somites of the body which do not correspond in position in the two groups.
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 first tribe, called Chelifera, from the usually chelate or claw-bearing first limbs, may be regarded as Isopoda anomala, of whieh some authors would form a separate order, Tanaidea.
The first pair of limbs is often chelate or prehensile, rarely antenniform; whilst the second, third and fourth may also be chelate, or may be simple palps or walking legs.
This chelate condition may be assumed by almost any of the appendages, and sometimes it appears in different appendages in closely related forms, so that no very great phylogenetic importC ance can in most cases be attached to it.
Appendages of 2nd pair, with their basal segments uniting in the middle line below the mouth, weakly chelate at apex.
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.