The ciliary activity of the young (prenervous) sea urchin larva, reflected by its swimming intensity, is measured according to a simple method. The changes in ciliary activity from hatching throughout gastrulation commonly show two main peaks coinciding with the two main phases of gastrulation.

The effects of cholinergic and monoaminergic agents of varying receptor specificities interfere with the ciliary activity in a stimulatory or inhibitory way suggesting that acetylcholine and some monoamine(s) are involved in a physiological ciliary control system.

The ciliary effects of sea water with altered calcium concentration or with tire addition of cAMP or cGMP (dibutyryl derivatives) together with the developmental changes in the larval content of cyclic nucleotides indicate that calcium ions and cyclic nucleotides are involved in a pharmacological control system shared by the ciliary activity and the morphogenetic cell movements.

The developmental changes in ciliary activity, including the minimum between its two main phases, may reflect a developmental change in the occurence of acetylcholine and monoamine(s) during larval development as well as some feedback mechanisms related to the calcium influx through the cell membrane and changes in the cyclic nucleotide metabolism.