With a shell above, and a shell below, you're safely protected, and can open and close to permit feeding, and to hide. When the shell is open, you filter feed happily.
As 'babies' dorsally shelled lophotrocozoa float around until they have accreted enough material in their shells to sink to the bottom. At the floating stage they are called plankton. Ideally, they eventually end up in places where there is a slight current to carry food to them.
If you land on a nice rocky shelf, you can build nice big shells and reproduce happily and safely for a long time. You don't want to get swept away though.
If you land on soft mud, this isn't going to work so well. You may get covered with silt, over time, which makes filter feeding difficult.
On rock, you can ensure you don't move by poking a sticky bit of your body - a 'pedicle' - out through your hinge and gluing yourself down. On mud, you need stilts! These take the form of extending your bottom shell into a long cone, and growing two long 'legs' near the front called 'helens'. Together these form a triangular prop, enabling you to lift your mouth up out of the mud, and continue to feed.
Obviously stilts on rocks are no good, and sticky pedicles on mud don't work either.
Helens or Pedicles?
Evolution starts with the Last Universal Ancestor.
You can find out what happened before evolution here.
You can also short-cut to Land plants, or Animals
No comments:
Post a Comment