ABSTRACT
The attachment complex of Antedon bifida consists of three epidermal organs (viz, the ciliary cap, the apical tuft and the adhesive pit) underlined by a thick subepidermal nerve plexus that encloses a subapical sensory organ. Three types of epidermal cells co-occur in the complex. These are non-glandular vibratile cells, mucous vibratile cells, and sensory-secretory cells. Non-glandular and mucous cells are unevenly distributed in the ciliary cap and the apical tuft; they occur sparsely in the adhesive pit. Sensory-secretory cells are restricted to the adhesive pit. The subapical sensory organ is formed from two to four invaginated epidermal cells (so-called ganglionic cells) whose apices define a closed microcavity containing cilia and microvilli. It is suggested that this sensory organ could mediate the inversion of larval tropism that occurs when doliolariae become competent. Behavioural observations of doliolariae in the process of fixation indicate that recognition of the substrate by the apical tuft can be achieved through the secretion of a thin mucous film that allows the larvae to remain close to the substrate without adhering to it. Larval fixation results from the secretion of a proteinaceous cement by the sensory-secretory cells of the adhesive pit. Early fixed larvae can detach from the substrate; this is possibly due to the secretions of the mucous cells of the adhesive pit.
