Celiaphlatus and forward movement. There is a few advantages of the bilateral symmetry. Some of the advantages are food, movement and animals.
capillary beds
bony fishes
sponges
protostomes
Amphibians
multicellularity
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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.
Sea squirts are classified in the same phylum (Chordata) as humans.
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