At Whitstable in Kent, and in many other places where the distribution of Nerine cirratulus has been examined in detail, it shows a maximum density of population in a marrow zone just below mean high water mark of neap tides. It is therefore a suitable animal to use in investigating some of the factors affecting the maintenance of a population by an intertidal animal in a definite zone on the shore.
The reproductive cycle of this polychaete was studied in an attempt to obtain larvae for experiments on larval settlement. In the course of this work records were obtained of the mean size of oöcytes extracted from adult females and of the quantity of coelomic gametes present in adult individuals of both sexes. These observations were made by sampling the adult population at Whitstable at regular intervals throughout the year. Additional information on the reproduction and larval life of N. cirratulus was obtained by conducting artificial fertilization experiments and by examining plankton samples for the pelagic stages. Recently settled worms were collected from the cirratulus zone at Whitstable by sieving substratum samples in seawater.
The observations on the seasonal variation in the size of coelomic oocytes and the quantity of coelomic gametes present in both sexes, indicate that the reproductive period begins early in the year and that it is of long duration at Whitstable. The similarity between the early planktonic stages of Nerine cirratulus, Nerine foliosa (Audouin & M. Edwards) and Scolelepis ciliata (Keferstein) is stressed and the identification of the oöcytes and prototrochophores of these spionids is discussed. It is shown that the oöcytes and prototrochophores of N. cirratulus are larger than those of S. ciliata and smaller than those of N. foliosa. The pelagic stages of N. cirratulus are found in the Whitstable plankton between March and October. The development of the larva of N. cirratulus to the two-setiger stage is described from an artificial fertilization experiment. An outline of the reproductive cycle is given. It is suggested that the length of pelagic life in N. cirratulus is rather more than thirty-four days and that larval settlement can be postponed if conditions are unsuitable for settlement and metamorphosis to take place. Nectosomas between the seventeen and twenty two-setiger stages of development were found in the Whitstable plankton. The smallest recently settled worm obtained from the cirratulus zone at Whitstable was a thirty-four-setiger individual and seventy-five-setiger worms of both sexes were found to contain coelomic gametes.