Balanus balanoides (L.) has seven planktonic larval stages. The first six are nauplius larvae while the seventh is the cypris larva. The cypris larva is specially adapted to locate a suitable place for settlement. The structure of the nauplius larva is basically similar to that of the nauplii of other crustacean groups. During successive nauplius stages, however, the simplicity of its anatomy is progressively obscured by the development of the cypris organ systems. All the organ systems do not differentiate simultaneously, but development is closely related to the time at which the organ must start to function. The three pairs of nauplius appendages, antennules, antennae and mandibles, are used in locomotion and the latter two pairs are also used in feeding. The six pairs of cypris thoracic swimming appendages, and the first and second maxillae with their associated ganglia and muscles, develop from groups of ectoteloblasts and mesoteloblasts in the ventral thoracic region of the nauplius. The compound eyes develop as outgrowths of the lateral lobes of the brain. The paired cement glands develop pre-orally. The end sacs of the adult maxillary glands develop as cavities in the somites of the second maxillary segment. The cypris antennules develop within the nauplius antennules but differentiation of their intrinsic musculature is delayed until after the nauplius-cypris moult. The various muscles of the cypris carapace are fully formed by the time of the nauplius-cypris moult. During, and after, the moult, a number of morphological and histological changes occur. The antennae and mandibles regress, the intrinsic musculature and cement ducts of the antennules complete their development. At the same time all the nauplius muscles and the antennal glands histolyse. Until these changes are completed the cypris larva is probably unable to settle, and thus to initiate the changes leading to the completion of metamorphosis. Rudimentary adult mandibles, and first and second maxillae are incorporated into the oral cone. After the moult the digestive region of the nauplius mid-gut epithelium and other epithelial cells are sloughed off into the gut lumen and digested together with the remains of the food ingested by the nauplius. The oesophagus and hind gut are now closed and the cypris larva does not feed. The adult digestive glands develop at the junction of the oesophagus and mid-gut. In the cypris the nauplius frontal filaments are associated with the compound eyes and connected to the brain via the optic ganglia. The median eye is apparently unchanged. Paired antennular ganglia are present. Those neurons, which innervated nauplius structures which have histolysed, also degenerate. The nauplius antennal glands degenerate at the nauplius-cypris moult; the maxillary glands are probably the functional organs of ionic regulation in the cypris as well as in the adult. The conspicuous multinucleate oil cells of the cypris are probably a food reserve. The paired masses of yellow cells in the carapace, originate in the antennae of the nauplius and migrate into the carapace after the moult. During the 24 h between settlement and the moult to the young adult, all the cypris muscles histolyse. The muscles break up spontaneously into short fragments which are then ingested by phagocytic haemocytes. There is widespread histolysis of neurons in the nervous system and further cells are sloughed from the gut epithelium. The adult mantle muscles, which are recognizable in the free swimming cypris larva, complete their differentiation, and in the few hours preceding the cypris-adult moult the adult thoracic muscles develop. The nervous system assumes its adult form and adult neurons differentiate from cells which had previously lain dormant in the nervous system. The compound eyes, frontal filaments and optic ganglia degenerate, but the median eye persists apparently unchanged. The yellow cells disperse, but their function is unknown. The cement glands persist in the adult, but the adult gland cells differentiate from cells aroung the collecting duct of the larval gland while the larval cement gland cells histolyse. The median eye persists, but in the newly moulted adult the three components separate giving rise to the three adult photoreceptors: a pair of pigmented ocelli and a median unpigmented photoreceptor. Shortly after the moult the young adult resumes feeding. This study has shown that metamorphosis in Balanus balanoides must be thought of in terms of the change from the nauplius through the cypris to the young adult and not just as the changes taking place between settlement and ecdysis to the young adult.

In North Wales settlement and metamorphosis of Balanus balanoides occur in May and the animals breed for the first time in the following November. Three weeks after metamorphosis the oviducal glands appear as epidermal invaginations in the basal segments of the first pair of cirri. At first there is no lumen or external duct but these develop during the next fortnight. The wall of the gland is a columnar epithelium which increases in height as the gland increases in size. During mitosis this epithelium exhibits the pattern of cell movements characteristic of similar epithelia in vertebrate embryos.

INTRODUCTIONBalanus 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.

No abstract