STRUCTURE. Barnacles may be broadly defined as attached or parasitic crustacea, with an indis tinctly segmented body, surrounded by a mantle, which generally calcifies and forms more or less of a shell or case. As they are attached by the head end, the first pair of antennae (adhering organs) are very minute, and the second pair are reduced. There are generally six pairs (less fre quently three or four) of long, hiramons, tendril like feet, though in parasitic forms these are wanting. There are no heart and no blood-vessels, and gills and other organs of respiration are therefore naturally wanting. Barnacles are gen erally hermaphroditic, but in some species dwarf males occur, living in the mantle cavity of the normal individuals. In a few species the sexes are separate and dimorphic. In their develop inent, barnacles undergo a most remarkable and very characteristic metamorphosis. The eggs hatch as minute, somewhat triangular with three pair of limbs, and a projecting horn or process on each side anteriorly. On the under
side of the head is a prominent upper lip. This larva sheds its skin several times as it grows, and finally appears as a larger, but still actively swimming creature, provided with a bivalve shell. like that of a clam. The body is segmented and there are six pairs of biramous feet. The antenna- are modified to serve as organs of ad hesion, and a 'cement' gland is formed and opens in their second joint. This larva attaches itself by these :internee, and after a resting or pupa stage, becomes transformed into the adult barna cle, as a caterpillar changes into a butterfly. There is this important difference between the two proc esses, however, that a butterfly does not grow, its size being no greater at death than when hatched, while barnacles continue to grow throughout life.