In this case, as the union is very close, some of the adjacent structures may be united or represented by a single common structure. The two intestines may be united at the jejunum, but double above and below. The liver is usually double, but the two livers are generally united. The lungs are always double, and the heart usually so, but the two hearts may have coalesced at their borders and be externally single. There is sometimes a partial or complete coalescence of two of the arms, so that there are only three arms. The coalesced arm will arise from the left of the right twin, and the right of the left.