ReviewGetting to the bottom of anal evolution

The digestive tract is an essential organ system that allows animals to efficiently digest food and take up nutrients to maintain growth and sustain the body. While some animal groups possess a sack-like gut, others possess a tube shaped alimentary canal with an opening for food uptake, the mouth, and an opening for defecation, the anus. The evolutionary origin of the hindgut with the anal opening remains unclear. Bilaterally symmetric animals (Bilateria) have a great diversity of gut architectures and also show a great variety in the development of this important organ system. In this review, we summarize current knowledge about gut morphology, its development, and the underlying molecular mechanisms of the alimentary canal of Bilateria. We focus on gastrulation, the fate of the blastopore and its connection to hindgut and anus development in different animal taxa. We evaluate scenarios about bilaterian evolution on the basis of recent insights of animal phylogeny and widening knowledge about animal developmental diversity.