INTRODUCTION

Balanus balanoides (L.) is a cross-fertilizing hermaphrodite. Insemination takes place in November in North Wales. Spermatozoa are deposited in the mantle cavity of a receptive individual by the extended penis of a neighbouring ‘acting male’. If a receptive individual – that is one that will accept insemination – is examined before oviposition takes place, the oviducal glands are found to be distended by a clear fluid (Walley, 1965). Oviposition follows insemination and fertilization is external, taking place within the confined space of the mantle cavity. The oocytes enter the mantle cavity, via the paired oviducal glands, and become enclosed in a pair of thin membranes. These membranes, enveloping the egg masses, are formed by distension of the elastic sacs secreted by the oviducal glands (Walley, 1965) and the spermatozoa have to pass through them in order to fertilize the eggs. If spermatozoa which have been deposited in the mantle cavity of a receptive individual are removed and examined before oviposition begins, they are immotile. If, however, they are examined several minutes later, during oviposition, they are found to be swimming vigorously (Barnes & Crisp, 1956): they have apparently been activated. The work described in this paper yields some information on the activation of the spermatozoa in B. balanoides, supplements recent work on the structure of cirripede spermatozoa (Brown, unpublished2; Turquier & Pochon-Masson, 1969; Bocquet-Vedrine & Pochon-Masson 1969; Munn & Barnes, 1970), and fixes the time within the sequence of maturation divisions of the oocyte at which fertilization takes place.