This study was undertaken to increase our very incomplete knowledge of the annual cycle inascidians.

Berrill (1950) states that ‘the age of ascidians, with one exception, is practically impossible to estimate, unless a certain inhabited area is followed closely through seasons and years’. In the present work the method has been to follow certain aspects of the ascidian population of a chosen area for a period of nearly two years. The area was the Old Dock in Ardrossan Harbour, Ayrshire, and the ascidian population contained four species: Diplosoma listerianum (Milne Edwards), Ciona intestinalis (Linnacus), Ascidiella aspersa (Müller) and Botryllus schlosseri (Pallas). Diplosoma belongs to the order Enterogona, suborder Aplousobranchiata; Ciona and Ascidiella to the order Enterogona, suborder Phlebobranchiata; and Botryllus to the order Pleurogona, suborder Stolidobranchiata. Thus each of the three main suborders is represented, and the four ascidians studied are amongst the commonest British species.