   setTopic: ( x, y ) ->
     candidates = ( t for t in @thingsAt( x, y ) when t instanceof Animal )
     if candidates.length
+      @topic.topicColor  = null if @topic
       @topic             = candidates[0]
-      @topic.belly       = @outline
+      @topic.topicColor  = @outline
       @topic.inheritMark = false
       @draw()
   # highlight an organism by coloring its belly
   highlight: ( x, y, color, inherit ) ->
     for t in @thingsAt( x, y ) when t instanceof Organism
-      t.belly       = color
+      t.mark        = color
       t.inheritMark = inherit
       t.draw()
   # remove all highlights

  drawHunger: -> # show emptiness of belly
    h = @health()
    r = ( @radius - 1 ) * ( h - @hp ) / h
    @drawCircle @x, @y, r, @belly
    if @sickness?
      @drawCircle @x, @y, 3 * r / 4, @sickness.disease.color
  # maximum health points an organism can retain

    @radius = options.radius || 5
    @tick   = @universe.tick
    @babies = 0
    @mark ||= null
    @belly ||= 'white'
    @topicColor = null
  describe: ->
    description = super()
    description.health = @health()
