ABSTRACT
Blood circulation in the thoracican Cirripedia (the typical barnacles) is more complex than in other crustaceans, and involves very high blood pressures together with vessel walls constructed of condensed connective tissue. In some species (e.g. Pollicipes) the system is essentially closed, but in juvenile stages and in small species of barnacles there is a tendency for the system to be more open and haemocoel-like. The term ‘closed haemocoelic’, rather than ‘closed circulatory’ system is proposed in order to differentiate the arrangement in barnacles from the very different circulatory system of other invertebrates and of vertebrates.
In the evolution of the thoracican circulatory system the heart appears to have moved anteriorly and then lost its musculature. Rhythmic pumping activity was transferred to the muscles of the rostral sinus and other somatic muscles. It seems probable that the closed haemocoelic system of barnacles was developed as a response to high blood pressures required to maintain turgidity of the peduncle of the lepadomorphs, and thus differs from the fully closed system of vertebrates where the need for high pressure arises from resistance in the peripheral capillaries.
