2018 Ancestor's Trail Hike   

Protostomia (from Greek meaning "mouth first") are a clade of animals. Together with the deuterostomes and a few smaller phyla, they make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers. The major distinctions between deuterostomes and protostomes are found in embryonic development.

  The current pilgrimage, known as deuterostomes, is joined by a much larger group of animals, the protostomes, to meet the ancestor of almost all organisms in the kingdom Animalia, a worm. Just a single class in the joining sub-kingdom of protostomia, the Insecta, represents three quarter of all animal species on Earth. The great divide between protostomes (meaning 'mouth first') and deuterostomes (meaning 'mouth second') was devised by comparative embryologists based on the way animal embryos diverge after gastrulation where the blastula (a hollow ball of cells) indents to form a cup. In the sub-kingdom of protostomia, the indentation eventually becomes the mouth. In deuterostomia which includes humans, the indentation eventually becomes the anus; the mouth is formed later. An extremely large variety of animal phyla constitute protostomia, including annelid worms (e.g. garden earthworms), flatworms (e.g. tapeworms and flukes), molluscs (e.g.snails, oysters, ammonites and octopuses), and arthropods (e.g. insects, crustaceans, spiders and centipedes). Unlike thespecies, classes and genera of animals from the pilgrimage prior to this rendezvous point, joining animals from differentphyla have no obvious relationship to one another based on traditional anatomy. But modern molecular rangefinding has allowed molecular taxonomists to organize all phyla in the pilgrimage into a hierarchy, with the worm as the concestor of all animals in Bilateria who are bilaterally symmetrical, with left and right side, a dorsal and a ventral side, and a head and a tail end.