The vestibule is closed by a circular ring-like vertical membrane the velum. It is perforated by a central circular aperture the enterostome. The enterostome is erroneously called mouth .The true mouth opens intostomodaeum lined with ectoderm. The velum is provided with a sphincter to open or close the enterostome. The posterior border of velum is prouced into 10 to 12 slender , ciliated and sensory velar tentacles.They also serve to strain the water current entering through the enterostome.
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