A phylogeny showing when gill slits may have arisen. It is thought that gill slits were subsequently lost in echinoderms.
<span class="int-own-work" lang="en">Own work</span> by <a href="//commons.wikimedia.org/w/index.php?title=User:Zebra.element&amp;action=edit&amp;redlink=1" class="new" title="User:Zebra.element (page does not exist)">Zebra.element</a> (<a href="//commons.wikimedia.org/wiki/User_talk:Zebra.element" title="User talk:Zebra.element">talk</a>)
2010-05-16 18:34
Originally uploaded to the English language Wikipedia by <a href="https://en.wikipedia.org/wiki/User:Zebra.element" class="extiw" title="en:User:Zebra.element">en:User:Zebra.element</a> (<a class="external text" href="https://en.wikipedia.org/w/index.php?title=Special:Log&amp;page=File:Phylogeny_gill_slit.png">file log</a>).