SUMMABY1The rearing ofBalanusnauplii in the laboratory and the stage at which hatching occurs are discussed.2The external morphology of the second stage nauplius ofBalanus per‐foratusis described. The setae on the limbs I divide into two types– swimming setae and feeding setae.3The limbs beat in a metachronal rhythm. During the back‐stroke particles are swept backwards and inwards, converging from each side just behind the body. From here they are sucked forward along the ventral surface during the fore‐stroke of the mandibles and antennae.4They are retained in the region behind the labrum by the feeding setae and their setules, by spinules on the ventral surface of the body, and by setules on the tip of the labrum.5As the limbs beat back the feeding setae push the particles towards the mouth. A groove under the labrum leads to the mouth itself, into which the particles are pushed by prongs on the antennal jaw processes.6Feeding is rapid and not selective. But particles over 6/x indiametercannot be swallowed, though diatoms 25fi longare freely taken. It appears that single cells may be torn from filamentous algae and then swallowed. But there is never any true mastication, and larger bodies do not form part of the food.7The first stage nauplius does not feed. The mouth is apparently open, but the feeding structures are very rudimentary, and particles can thus reach the mouth only by a