Biologic evolution

Crow shellfish.
The crow shellfish is a point different from Asari.

The number of strings must be different.
There is Asari by one string and are about ten crow shellfishes.

The crow shellfish must cut the string for myself and move.
Strength of the string is different.

Crow shellfish's string is stronger.
Crow shellfish's string doesn't cut though the string easily cuts to Asari.

The purpose of the crow shellfish's putting the string is to fix the individual.
It is not proven though it is thought that Asari's string is also the same.

It is imaginable with arthropod's prototype it when thinking that crow shellfish's string evolved.
Act of cutting string for myselfAct that starts moving for myself.

How do it become it when the animal that can make the string that can move cutting in the string evolves?