Scalidophora

A small change to the way the muscles are arranged, and you gain the ability to pull your entire head down inside your body.  This keeps you safe when you are on the surface, although you spend most of your time down in the mud at the bottom of the sea.  You eat a variety of foods, from diatoms to other worms, and are effectively a wriggling mouth into the present day.

You have divided into three main types. Priapulida and Kinorhyncha, commonly known as penis worms and mud dragons, and Loricifera, which have only recently been discovered as they spend their entire lives buried deep in the mud at the bottom of deep seas.  This latter group has one very interesting characteristic: they are the first multicellular life form which can live entirely without oxygen.

Way back on our evolutionary tree, a life form acquired a special ‘organelle’ called a mitochondrium, which was once a captured bacterium.  They have their own DNA, and their own, continuing evolutionary story inside this one.  In Locifera, the mitochondria have mutated slightly, and no longer combine carbon with oxygen to create energy.  Instead they are able to metabolise carbon compounds by removing hydrogen.  As a result, some locifera species live their entire lives without needing oxygen.  In the event of a catastrophic change in our climate, they might be the start of the next phase of evolution.

That's as far as you can evolve here!  
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