Cephalocarids

Cephalocarida are a class of very small marine crustaceans that comprises a single order, Cephalocarida. They are considered among the most primitive living crustaceans and are found in sandy or muddy sediments in shallow coastal waters worldwide.

Adults are typically only a few millimeters long and have a simple body plan that consists of a broad head (cephalon) and a segmented trunk, without a protective carapace. The head bears a pair of antennae and, in some species, stalked eyes. The mouthparts are primitive, including mandibles and maxillipeds. The trunk carries numerous paired appendages along its length; these limbs are generally biramous and are used for locomotion and feeding. The overall body organization lacks the pronounced specialization seen in many other crustaceans, and the abdomen is not clearly distinct from the thorax.

Cephalocarids are benthic and inhabit fine sediments where they lead slow, life-long lives. They are detritivores or filter-feeders, using their thoracic appendages to move water and collect organic particles.

In terms of development, cephalocarids exhibit direct development, lacking a free-swimming nauplius larva; juveniles resemble adults when they hatch. The group is of particular interest to evolutionary biology because its relatively simple morphology preserves features considered plesiomorphic for crustaceans.

Taxonomically, the class includes two living genera, Hutchinsoniella and Lightiella, with several described species such as Hutchinsoniella macracantha. Cephalocarida are studied to understand early crustacean evolution and the diversity of life in marine sediments.