This hypothesis therefore implies that the one hole in one-holed animals corresponds to the anus in two holed animals. This single hole was ascribed to be the anus since it was on the trailing end. There is a sensory organ is on the leading end, which was interpreted as the 'head', and a single orifice on the the trailing end, which was interpreted as the 'tail'. The 'head' and 'tail' ends were assumed to correspond to the swimming direction of the larva. A single hole forms in the embryo, which then grows into a swimming larva. This hypothesis relies largely on observations of jellyfish embryos. Most invoke a hypothetical ancestor called the 'Gastrea' which was postulated to have a single opening to the gut at the tail end of the animal and a sensory organ on the head end. Some animals have one hole, others have two-how did this happen? Does the single hole in one-holed animals correspond to the mouth or anus of animals with two holes? This raises a couple straightforward questions. These two holes create a through gut: a tube that takes in food at one end (the mouth) and releases waste at the other end (the anus).

humans, fish, snails, and so on), which diverged from jellyfish a long time ago in the course of evolution, have two openings.

sea anemones and Hydra), have a single opening to their gut, so they eat food and release waste from the same opening. Many animals, including jellyfish and their relatives ( e.g. A new paper by Mark Martindale and Andreas Hejnol offers a hypothesis on the origin of these very important holes. I recently attended a meeting between the Dunn and Wessel labs about the evolution of the mouth and anus.

Annelid blastopore