Joined: Nov. 2006
Henry on my claim that snail shells probably evolved (and vanished) via saltation:
Re "but the shell can appear only in one saltation, because it only makes sense as a whole."
It doesn't have to make sense to you, it only has to reduce the amount of predation on the species. A partial shell would most likely do that.
So the shell developed in snails because it reduces the amount of predation. Shell protects the vital organs like heart, stomach, and kidneys.
Slugs can be find often at same places as snails. But obviously they do not need shells anymore. So they get rid of them - having them only as "rudimentary organs". Selective pressure or natural selection is an interesting force, isn't it?
The mantle cavity,also called the visceral sac,contains the heart,kidney and the other organs.It is closed in pulmonate snails except for the breathing pore,the pneumostome.
The roof is covered with a network of blood vessels and forms a lung.
Snail -I need to protect myself, you know:
Slug from the same graden - Because there was not selective pressure on me I don't need to protect heart and kidney anymore and I got rid of the shell and transformed complicated structre of mantle cavity. I have the shell only as a rudimentary organ, you know:
I could not answer, but should maintain my ground.-