Sentence:  There are sharks with round <WSD>pupils</WSD>, sharks with slitlike <WSD>pupils</WSD>, and some with <WSD>pupils</WSD> that expand and contract with the amount of light available. As unimpressive as this might sound to people who are used to having their <WSD>pupils</WSD> dilate and contract regularly, realize that "no bony fish" has this modification of the eye. 
Meaning of the word marked with <WSD> tags:  The hole in the middle of the iris of the eye, through which light passes to be focused on the retina.