The single pair of palpiform appendages in front of the mouth has been found in one instance to be antenniform, whilst the numerous post-oral appendages in the same genus were bi-ramose.
The members of this group, whilst resembling the lower Crustacea (as all lower groups of a branching genealogical tree must do), differ from them essentially in that the head exhibits only one prosthomere (in addition to the eye-bearing prosthomere) with palpiform appendages (as in all Arachnida) instead of two.
(After Beecher.) ancestor of the grade) grouped in three regions of six - (a) the " prosoma " with palpiform appendages, (b) the " mesosoma " with plate-like appendages, and (c) the " metasoma " with suppressed FIG.
Limulus thus agrees with Scorpio and differs from the Crustacea, in which there are three prosthomeres - one ocular and two carrying palpiform appendages.