Celiaphlatus and forward movement. There is a few advantages of the bilateral symmetry. Some of the advantages are food, movement and animals.
Medusa
sea stars
Annelida
Pseudocoelomate is the word used for an animal with an unlined body cavity made up of a tube surrounded by organs.
Cnidarians
multicellularity
------------------------------------------------
Sponges are simple aggregations of single cells which come together for mutual benefit. If you liquidise a sponge, the cells it is made form will come together again to reform the sponge. Thus sponges (or something like them) were probably the first step in the evolution of multi-cellular organisms.
amphibians