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 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.
It must not indeed be assumed that those of the first pair were in all cases antenniform.
It will be sufficient here to define them as Arthropoda for the most part of aquatic habits, having typically two pairs of antenniform appendages in front of the mouth and at least three pairs of post-oral limbs acting as jaws.
The appendages of the second prosthomere are the well-known chelicerae of the Arach nids, rarely, if ever, antenniform, but modified as " retroverts" or clasp-knife fangs in spiders.