Check out page 611-612. Some evidence 1. Have lost bilateral symmetry and have not cephalization. Sessile life style selects against cephalization pg 608 on the right 3. Bottom feeding on sessile prey has selected against cephalization.
Every evolutionary adaptation has both benefits and costs.
Spend it on either survival or reproduction. 1. part of survival is getting food. Determine in this case where it was lost. Arose twice, independently in Protostomes and Deuterostomes and was lost in Mollusca. 2. unit of elongation for the body a. duplication of excretory organs allows for larger size.