![]() ![]() Two photographs below show sperm removed from the testis of a mature barnacle and a developing embryo taken from an ovary belonging to the same specimen. Fertilized eggs are brooded for a while and the resultant larvae (Nauplii) are released into the plankton where they develop into a Cyprid larval stage (see below) that eventually leaves the plankton and attaches to the rock surface where it develops into a young barnacle. The barnacle Semibalanus balanoides is a hermaphrodite that internally fertilizes the eggs of a neighbor. Because the base is not as wide as those in less crowded barnacles, they are easier to dislodge.When this happens, more space becomes available for other organisms to occupy. When barnacles are crowded together, they often grow upward from a small base as shown below. ![]() When retracted the cirri come together, enclosing any food particles and upon further retraction, food is brought into the mantle cavity where it is scraped off the cirri and transferred to the mouth by appendages called maxillae.Cirri have small lateral extensions which help in holding food particles. ![]() When protruded from the shell and spread out, the cirri form a net with which the barnacle scoops water immediately above the shell opening. The volcano shaped shell consists of a basal part cemented to the rock a wall composed of six non-movable plates and one or two pairs of movable plates at the top that allow the barnacle to seal itself from the outside environment during periods of stress or alarm.The movable plates can be separated to allow protrusion of the thoracic limbs, and the cirri, for feeding. Adult acorn barnacles Semibalanus balanoides are enclosed in a calcareous shell. The barnacle zone, situated just below the blue green zone, is composed of a horizontal band of numerous white barnacles. ![]()
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