Sentence:  Some species also possess an anteriorly projecting <WSD>rostellum</WSD>, which may or may not be armed with one or more rows of hooks. The <WSD>rostellum</WSD> can be retracted into the rostellar sac within the scolex. 
Meaning of the word marked with <WSD> tags:  A retractable protruding part at the anterior end of a soft-bodied tapeworm; the scolex from which it protrudes is often armed with hooks which serve to keep it in place attached to the host.