diff --git a/src/ComponentDepot.cs b/src/ComponentDepot.cs
index cfe0e65..18057fe 100644
--- a/src/ComponentDepot.cs
+++ b/src/ComponentDepot.cs
@@ -55,16 +55,19 @@ internal class ComponentDepot
 			entityComponentMap.Add(entityID, new HashSet<Type>());
 		}
 
-		var alreadyExists = entityComponentMap[entityID].Add(typeof(TComponent));
+		var notFound = entityComponentMap[entityID].Add(typeof(TComponent));
 
 		// update filters
-		if (!alreadyExists)
+		if (notFound)
 		{
 			if (typeToFilterSignatures.TryGetValue(typeof(TComponent), out var filterSignatures))
 			{
 				foreach (var filterSignature in filterSignatures)
 				{
-					CheckFilter(filterSignature, entityID);
+					if (CheckFilter(filterSignature, entityID))
+					{
+						filterSignatureToEntityIDs[filterSignature].Add(entityID);
+					}
 				}
 			}
 		}
@@ -93,7 +96,10 @@ internal class ComponentDepot
 			{
 				foreach (var filterSignature in filterSignatures)
 				{
-					CheckFilter(filterSignature, entityID);
+					if (!CheckFilter(filterSignature, entityID))
+					{
+						filterSignatureToEntityIDs[filterSignature].Remove(entityID);
+					}
 				}
 			}
 		}
diff --git a/src/System.cs b/src/System.cs
index 125de07..12e8dfb 100644
--- a/src/System.cs
+++ b/src/System.cs
@@ -10,8 +10,18 @@ public abstract class System : EntityComponentReader
 		MessageDepot = messageDepot;
 	}
 
+	public System(World world)
+	{
+		world.AddSystem(this);
+	}
+
 	public abstract void Update(TimeSpan delta);
 
+	public virtual void InitializeFilters()
+	{
+
+	}
+
 	protected Entity CreateEntity()
 	{
 		return EntityStorage.Create();
diff --git a/src/World.cs b/src/World.cs
index 2eddb2a..9b53b8f 100644
--- a/src/World.cs
+++ b/src/World.cs
@@ -2,25 +2,40 @@
 
 public class World
 {
-	private readonly List<System> Systems;
-	private readonly List<Renderer> Renderers;
-	private readonly EntityStorage EntityStorage;
-	private readonly ComponentDepot ComponentDepot;
-	private MessageDepot MessageDepot;
+	private readonly List<System> Systems = new List<System>();
+	private readonly List<Renderer> Renderers = new List<Renderer>();
+	private readonly EntityStorage EntityStorage = new EntityStorage();
+	private readonly ComponentDepot ComponentDepot = new ComponentDepot();
+	private MessageDepot MessageDepot = new MessageDepot();
 
-	internal World(
-		List<System> systems,
-		List<Renderer> renderers,
-		ComponentDepot componentDepot,
-		EntityStorage entityStorage,
-		MessageDepot messageDepot
-	)
+	internal void AddSystem(System system)
 	{
-		Systems = systems;
-		Renderers = renderers;
-		ComponentDepot = componentDepot;
-		EntityStorage = entityStorage;
-		MessageDepot = messageDepot;
+		system.RegisterEntityStorage(EntityStorage);
+		system.RegisterComponentDepot(ComponentDepot);
+		system.RegisterMessageDepot(MessageDepot);
+		Systems.Add(system);
+	}
+
+	internal void AddRenderer(Renderer renderer)
+	{
+		renderer.RegisterEntityStorage(EntityStorage);
+		renderer.RegisterComponentDepot(ComponentDepot);
+		Renderers.Add(renderer);
+	}
+
+	public Entity CreateEntity()
+	{
+		return EntityStorage.Create();
+	}
+
+	public void Set<TComponent>(Entity entity, in TComponent component) where TComponent : struct
+	{
+		ComponentDepot.Set(entity.ID, component);
+	}
+
+	public void Send<TMessage>(in TMessage message) where TMessage : struct
+	{
+		MessageDepot.Add(message);
 	}
 
 	public void Update(TimeSpan delta)
diff --git a/src/WorldBuilder.cs b/src/WorldBuilder.cs
deleted file mode 100644
index d184f1c..0000000
--- a/src/WorldBuilder.cs
+++ /dev/null
@@ -1,59 +0,0 @@
-namespace MoonTools.ECS;
-
-public class WorldBuilder
-{
-	private ComponentDepot componentDepot;
-	private EntityStorage entityStorage;
-	private MessageDepot messageDepot;
-
-	private readonly List<System> systems = new List<System>();
-	private readonly List<Renderer> renderers = new List<Renderer>();
-
-	public WorldBuilder()
-	{
-		componentDepot = new ComponentDepot();
-		entityStorage = new EntityStorage();
-		messageDepot = new MessageDepot();
-	}
-
-	public void AddSystem(System system)
-	{
-		system.RegisterEntityStorage(entityStorage);
-		system.RegisterComponentDepot(componentDepot);
-		system.RegisterMessageDepot(messageDepot);
-		systems.Add(system);
-	}
-
-	public void AddRenderer(Renderer renderer)
-	{
-		renderer.RegisterEntityStorage(entityStorage);
-		renderer.RegisterComponentDepot(componentDepot);
-		renderers.Add(renderer);
-	}
-
-	public Entity CreateEntity()
-	{
-		return entityStorage.Create();
-	}
-
-	public void Set<TComponent>(Entity entity, in TComponent component) where TComponent : struct
-	{
-		componentDepot.Set(entity.ID, component);
-	}
-
-	public void Send<TMessage>(in TMessage message) where TMessage : struct
-	{
-		messageDepot.Add(message);
-	}
-
-	public World Build()
-	{
-		return new World(
-			systems,
-			renderers,
-			componentDepot,
-			entityStorage,
-			messageDepot
-		);
-	}
-}