egg capsule of Diphyllobothrium latum, the fish tapeworm.  This egg capsule often displays a button on the non-opercular end (seen to the lower portion of the image).  The enclosed embryo will develop into a coracidium, a ciliated, free-swimming larva which must be eaten be a copepod, such as Diaptomus or Cyclops in order to continue the life cycle.  400x