Sentence:  He was still prepared to go on collecting all that life could offer, like a chambered <WSD>nautilus</WSD> patiently adding new cells to its slowly expanding spiral. 
Meaning of the word marked with <WSD> tags:  A marine mollusc, of the family (noshow=1) native to the Pacific Ocean and Indian Ocean, which has tentacles and a spiral shell with a series of air-filled chambers, of which (noshow=1) is the type genus.