Ultrastructure of sensory receptors in ascidian tadpoles
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An analysis of fine structure and function was conducted on three types of receptors in the cerebral vesicle of two species of ascidian tadpoles (Ciona intestinalis andDistaplia occidentalis). Theocellus is composed of one pigmented, cup-shaped supportive cell, an estimated 15–20 sensory cells, and three lens cells, each with a large body of granules (glycogen ?). The outer segments of the photoreceptoral processes are modified cilia, one per sensory cell, consisting of many lamellae, formed by infoldings of the ciliary membranes, and axonemes of 9 + 0 doublets of microtubules. The lamellae are homologous to retinal disks of vertebrates.Hydrostatic pressure (?)receptors are modified cilia containing tubules which open to the lumen of the cerebral vesicle. These receptors closely resemble the globular, ciliary processes of coronet cells in the saccus vasculosus of fishes. Thestatocyte is a onecell gravity receptor. The part extending into the lumen of the brain contains the nucleus and a large black body which is thought to function as a float. The foot-piece of the cell is firmly anchored in the brain wall; the neck is probably the site of generation of signals.
Key-WordsInvertebrate Tunicate Ocellus Statocyte Ultrastructure
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