Osmotic Responses in the Sipunculid Dendrostomum Zostericolum

Abstract

1. The sipunculid Dendrostomum zostericolum demonstrates no ability to regulate osmotically. 2. Dendrostomum behaves superficially as an osmometer, but is actually more complex: (a) the worm shows volume control in concentrated and dilute sea water; (b) it is permeable to salts, mostly through the gut and/or nephridiopores; (c) it can release osmotically active particles from its body wall to the blood. 3. The body wall of Dendrostomum is highly permeable to water, but only slightly to salts. Permeability for both salts and water is greater inwards than outward. 4. Dendrostomum can tolerate a loss of 36% body weight by desiccation and recover when returned to sea water. The mechanism of this tolerance appears to be the removal by fixation in the tissues, of osmotically active particles from the body fluids.