Sagaia is something of a scaled down version of Darius which has only 8 zones, and no choices on them. However, it's got its interesting remix of levels and its exclusive bosses Big Merman and Ancient Helm.

Stages and Bosses

Stages/Zones Bosses
Zone 1 King Fossil
Zone 2 Red Crab
Zone 3 Strong Shell
Zone 4 Big Merman
Zone 5 Cuttle Fish
Zone 6 Green Coronatus
Zone 7 Ancient Helm
Zone 8 Great Thing