Shells are round, blue/black spheres fired by land vehicles in Civilization Stage that explode when they hit their target, causing damage. They also leave a blue trail through the air, like those left by Missiles.
Unlike their Earth counterparts, shells used in the Spore Galaxy behave like missiles, although it seems that they are initially fired out of a gun instead of making use of their own propulsion. They seem to be more like primitive ancestors of the Sea Vehicle's missile, even though missile-firing sea vehicles are usually developed only a short time later.
After being fired, they typically reach a maximum height and then fall down towards their target - most often a stationary target such as a building or turret. However, if the target is moving - for example, a vehicle - the shell will change direction and follow the target until it hits. They can follow fast-moving vehicles for hundreds of meters. Shells are even apparently capable of climbing beyond their normal maximum height when hunting flying epic creatures.
Shells are only used in Civilization Stage, later being replaced by the missile technology previously used only by sea vehicles.