Anatomy of Umbraculum

March 11, 2001
From: Bill Rudman

Ross Willis's photos of intertidal Umbraculum out of water show a very good view of the organs around the head, so here is an enlarged view.

With the shell lifted up, the 'pallial' cavity or gap between the shell and the body, is clearly visible. It runs all around the body and is normally filled with sea water. On the right side, it houses the single gill - which still hidden in this photo. Also of interest in these photos are the structures around the head. There are a pair of short enrolled head tentacles or rhinophores, and between them a third organ which looks a bit like the siphon you would see in a sea snail. This is in fact the penis, which in Umbraculum sits in the midline just above the mouth. The dark brown pit at the base of the right rhinophore is a chemosensory organ, sensing chemicals in the water current being drawn down into the pallial cavity by the rhinophores. It is called the Hancock's Organ.

This is quite an interesting view of Umbraculum which we don't usually see until we are dissecting a dead preserved specimen.
Best wishes,
Bill Rudman

Rudman, W.B., 2001 (Mar 11) Anatomy of Umbraculum. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

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