Sentence:  An extra-salty <WSD>siphuncle</WSD> naturally absorbs water from the phragmocone chambers, and the empty space left behind is filled with gases diffusing out from the blood. 
Meaning of the word marked with <WSD> tags:  A strand of tissue passing longitudinally through the shell of a cephalopod, used primarily in emptying water from new chambers as the shell grows.