A 40-year-old man from India presents with a liver abscess. Ultrasound shows a single large abscess in the right lobe. Aspiration yields 'anchovy sauce' (chocolate-brown) pus that is odourless. Gram stain and bacterial culture are negative. Stool microscopy shows Entamoeba cysts. What characteristic feature distinguishes E. histolytica cysts from E. coli cysts on microscopy?