Mystacocarids are tiny pigmentless crustaceans, less than long, that live in the spaces between sand grains on
intertidal beaches. They have a cylindrical body, with five
thoracic and five
abdominal segments. The appendages of the head—two pairs of long
antennae, a pair of limb-like
mandibles and two pairs of
maxillae—are much longer than those on the thorax and have a number of fine hairs that the animal uses to strain detritus from the water to feed on. Also a
labrum is present. The head is relatively large and divided into two by a stricture, so that the larger posterior part gives the appearance of being the thorax. The actual thorax has five pairs of thoracic limbs. The pair on the first segment has been modified into
maxillipeds, which collaborates with the maxillae in the feeding process, but is not fused with the head. These three pairs of limbs looks similar, but the maxillipeds can be distinguished from the maxillae in the form of a vestigial exopod. The remaining four thoracic pairs of limbs has been reduced to small, unsegmented lobes, but the last pair is slightly modified in males. Their limbless abdomen ends in a supra-anal plate, a
telson and a pair of large, pincer-like
furca which on the ventral surface bears two sets of combs with
setae. Because of their small size, there is no circulatory or respiratory system. A nauplius eye is completely absent. After mating, mystacocarids lay tiny eggs which hatch into a
nauplius or
metanauplius larva. ==References==