Planarians - These are aquatic flattened-out worms (flat worms) that have a rather "primitive" aspect but also feature some very important evolutionary aspects. They do not feature shells/
Annelids - These are worm-like organisms, that live under humid conditions, and do not feature shells.
Mollusks - These also have soft bodies, like the first two, and they're also invertebrates. They feature external calcium-based shells.
Hope it helped,
BioTeacher101
