The nervous system is embedded in the epidermis, and the pairs of ganglia are separated as in Serpula, &c.; each pair has a longish commissure between its two ganglia.
(d) Univalvia absque spira regulari: Patella, Dentalium, Serpula, Teredo, Sabella.
The calcareous tube worm Serpula vermicularis forms twisting masses of tubes on the shell occupied by the hermit crab.
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