Sentence:  The membrane of the <WSD>vestibule</WSD> in this animal is thrown into three folds. The margins of these folds, looking towards the <WSD>vestibule</WSD>, are approximated, and, following the law which is now known to regulate the formation of hollow tubes, doubtless unite and coalesce in the next higher species of fish. 
Meaning of the word marked with <WSD> tags:  (body cavity) Any of a number of body cavities, serving as or resembling an entrance to another bodily space. (from the 18th c.)