Annelids are made of one or more body segments. Each segment has one or more rings. These rings are called "annuli" (singular: 'annulus' = 'little ring'). Annelids reproduce sexually by hermaphroditic cross-fertilization. They can regenerate cut off pieces of their body, unlike sponges, which can come back together if ripped apart. They have a hydrostatic skeleton, a one-way digestion system, and bilateral symmetry.