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Description
Here's a quick thing I did to illustrate a concept of mine to NocturnalSea [link] . And here's a description of the idea I originally wrote on the Speculative Evolution Forum [link] :

On a distant planet, the first multicellular life was a ball of cells, much like Volvox on Earth. The ball had two adjacent openings through which water flowed, driven by thousands of beating cilia, and was encased in a protective mineral/protein shell. As time went on, this ball of cells became larger and more complex, evolving an efficient filter feeding apparatus at the mouth of one of its siphons, and a real stomach to process its food. A further refinement came later; in a pocket of its stomach the ball developed a rod of solidified mucus, which cilia on the lining of the pouch slowly rotated in order to mix and sort food (this adaptation exists here on Earth in bivalves [link] ). The rod grew larger and became adapted to increase the water current flowing through the animal as it rotated. To aid it in this task it developed a spiraling ridge around its surface, like an auger. The ball grew larger, its shell thicker, and it sank to the ocean floor. Although it still lived a largely sedentary life, it was advantageous for the ball to move at least a little bit in order to position itself in the path of plankton-rich currents. To this end, the rod of mucus, already protruding slightly from the second siphon, became longer until, lying on its side, the ball could slowly spin its auger in order to rotate itself around its central axis. In this way it controlled which way its siphons faced.

As time went on, the cilia lining the pouch in which the auger rested lengthened and fused together, (like those of a comb jelly) and the screw developed little notches into which these cilia fit like gear teeth. The screw became an organ of locomotion, pulling the ball along the ocean bed to shallow water and back again. It also made a handy burrowing implement, allowing the ball to retreat into the sand when danger threatened. Time passed, and one member of the ball's lineage dramatically furthered the trend towards an evermore powerful screw: muscle cells in the pouches' wall became attached to the bases of the stiffened cilia, allowing them to rotate the auger faster and with more force. The rest was comparatively straightforward; the cilia became larger and less numerous, the muscles associated with them more powerful, and the screw more efficient at propelling its owners across the sea bed. (to picture what this might look like, see this video: [link] ) Members of this clade became swimmers and some eventually made their way onto dry land.

________

I didn't mention how the animal eats, but I suppose there's a scoop behind its auger that funnels sand into its gut for processing. I also refined the propulsion mechanism a little; two toothed rods alternate to rotate the style, and the muscles powering them are connected to an internal ridge of the creature's carapace.

I'm not sure about the last line, since this guy seems way too specialized to ever amount to much evolutionarily. An animal constrained enough by evolution to actually develop rotary locomotion has got to be, I guess (the trick with these creatures is that they can never lose their shell: if one ever develops with a hole in its carapace, its organs will be exposed).

EDIT: I noticed that the sutures on the animal's back are in the wrong places. FORGIVE ME!

Description
Here's a quick thing I did to illustrate a concept of mine to NocturnalSea [link] . And here's a description of the idea I originally wrote on the Speculative Evolution Forum [link] :

On a distant planet, the first multicellular life was a ball of cells, much like Volvox on Earth. The ball had two adjacent openings through which water flowed, driven by thousands of beating cilia, and was encased in a protective mineral/protein shell. As time went on, this ball of cells became larger and more complex, evolving an efficient filter feeding apparatus at the mouth of one of its siphons, and a real stomach to process its food. A further refinement came later; in a pocket of its stomach the ball developed a rod of solidified mucus, which cilia on the lining of the pouch slowly rotated in order to mix and sort food (this adaptation exists here on Earth in bivalves [link] ). The rod grew larger and became adapted to increase the water current flowing through the animal as it rotated. To aid it in this task it developed a spiraling ridge around its surface, like an auger. The ball grew larger, its shell thicker, and it sank to the ocean floor. Although it still lived a largely sedentary life, it was advantageous for the ball to move at least a little bit in order to position itself in the path of plankton-rich currents. To this end, the rod of mucus, already protruding slightly from the second siphon, became longer until, lying on its side, the ball could slowly spin its auger in order to rotate itself around its central axis. In this way it controlled which way its siphons faced.

As time went on, the cilia lining the pouch in which the auger rested lengthened and fused together, (like those of a comb jelly) and the screw developed little notches into which these cilia fit like gear teeth. The screw became an organ of locomotion, pulling the ball along the ocean bed to shallow water and back again. It also made a handy burrowing implement, allowing the ball to retreat into the sand when danger threatened. Time passed, and one member of the ball's lineage dramatically furthered the trend towards an evermore powerful screw: muscle cells in the pouches' wall became attached to the bases of the stiffened cilia, allowing them to rotate the auger faster and with more force. The rest was comparatively straightforward; the cilia became larger and less numerous, the muscles associated with them more powerful, and the screw more efficient at propelling its owners across the sea bed. (to picture what this might look like, see this video: [link] ) Members of this clade became swimmers and some eventually made their way onto dry land.

________

I didn't mention how the animal eats, but I suppose there's a scoop behind its auger that funnels sand into its gut for processing. I also refined the propulsion mechanism a little; two toothed rods alternate to rotate the style, and the muscles powering them are connected to an internal ridge of the creature's carapace.

I'm not sure about the last line, since this guy seems way too specialized to ever amount to much evolutionarily. An animal constrained enough by evolution to actually develop rotary locomotion has got to be, I guess (the trick with these creatures is that they can never lose their shell: if one ever develops with a hole in its carapace, its organs will be exposed).

EDIT: I noticed that the sutures on the animal's back are in the wrong places. FORGIVE ME!