A device, utilising the suction produced by a disposable syringe, was developed to restrain a range of very small organisms in flowing seawater. The technique was developed during an investigation of the swimming response of barnacle nauplii to changes in temperature. The combination of beat frequency and the proportion of time spent active was held constant for B. balanoides and B. hameri but steadily increased with temperature for C. montagui and B. amnhitrite. E. modestus showed an intermediate response. Methods for utilising video-tape recordings and a micro-impedance pneumograph for analysing limb beat activity, were developed during an investigation of the feeding behaviour of nauplii. The feeding mechanism was reappraised and an increase in the volume of water handled by the larvae noted in the presence of food organisms. This increase was shown by E. modestus nauplii in response to dissolved organic substances, indicating that the nauplii could determine when to feed, and what was edible, from the shell of dissolved material surrounding food particles. A quantitative study of the grazing of E. modestus nauplii showed that the ingestion rate of algal cells increased up to concentrations of 150 - 200 cells/ul, then remained steady. Larger algal cells were taken from algal mixtures, in preference to smaller ones. Restraint techniques and video-recordings of free-swimming cypris larvae showed them to be prodigeous swimmers, with considerable control over the direction and magnitude of the thrust produced. The close observation of larvae allowed by the restraint technique enabled mechanical stimulation of particular sense organs. The function of these organs had been interpreted from their structure by previous authors. In some instances, a mechano-receptive function was confirmed, but in others, doubt was cast. Restrained cyprids responded to complex sound fields by swimming less often. A dual mode sound chamber was used to differentiate between sound pressure and displacement, but negative results were obtained. The cyprids did, however, respond to substrate vibrations, at low frequencies, by swimming off the substrate. All the developed techniques were further employed to show that the classical copepod feeding swirls were artefacts. Temora produced'only a posteriorly flowing current for feeding and swimming, when restrained in larger volumes of seawater. The amount of water handled by Temora was also shown to increase when food algae were present.