1: dumbbell-shaped, dorsal part of the black-outlined nerve ring and two cerebral ganglia (knots in the nerve cord). Visible as in weakly pigmented, translucent juveniles. The ring encircles the oesophagus and is the most centralised part of the nerve cord with dispersed ganglia; the ring is sometimes referred to as the ‘brain’.
The mantle cavity, which functions as a lung for respiration, is sealed off from the exterior by a thick membranous mantle-collar 2 which fits closely round the body 3 as it extends or retracts. The upper specimen had been recently immersed, and water had entered and filled the respiratory mantle-cavity apart from a small bubble of air 4; dark area to left of 2 is pneumostome/respiratory-pore in the mantle-collar.

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