There are three definitely differentiated regions forming the hindintestine. These regions are the ileum, the colon, and the rectum. Just posterior to the mesenteron the ileum bends forward dorsally,extending anteriorly about one-half the length of the mid-intestine. The colon begins at the posterior bend of the ileum in its position over the mesenteron. The colon enlarges toward the rectum, where it constricts abruptly. This constriction is more pronounced dorsally. The rectum is about two-thirds the diameter of the colon at their point of union. Anterior to the anal opening the rectum enlarges slightly.