Hydra provides a key for the assignment of ancestral and contemporary body axesSimple radial symmetric animals have only a single axis (oral-aboral) and a single opening of the gastric cavity, the blastopore. A central issue of evolutionary biology is how the two major body axes of higher organisms, anteroposterior and dorsoventral, evolved from such an ancestral body plan. In the patterning of the freshwater polyp hydra, molecules have been found that participate in higher organisms in the generation of both axes. Thus, hydra and other Coelenterates are assumed to provide information about the relation of the ancestral single axis and the two main axes of bilateral-symmetric organisms.
Problems with the conventional protostomia-deuterostomia conceptTwo conflicting hypothesis have been put forward. According to the protostomia - deuterostomia concept, a widening and deformation of the blastopore took place in one dimension, resulting in a slit-like geometry that forms the future midline (below left). In protostomia such as insects, the anterior part of the slit remained open as the mouth. In deuterostomia including vertebrates, a posterior part remained open forming the anus.
The system that was patterning once the bodies of radial-symmetric ancestors was later used to pattern essentially the head and the heart of higher organismsAs indicated by the expression pattern of Wnt - and Brachyury, the gastric opening in hydra, (hypostome) represents the most posterior part of the organisms although this part, the hypostome with the surrounding tentacles, is conventionally called the ‘head’. If this assigmment is correct, the antipodal (aboral) part, the so-called foot is homologous to the most anterior structures: Indeed, Nkx2.5 is expressed in the foot of hydra  and in the forebrain and heart of higher organisms (pink in the figure below). The tentacle/hypostome border has the same positional address as the midbrain/hindbrain border in vertebrates (posterior border of Otx-expression ). The recently observed highly conserved patterning in the brain of higher organisms is proposed to have its origin in the preserved body pattern of radial-symmetric common ancestors (for details and references see Meinhardt, 2002).
The anteroposterior axis consists of two parts; the trunk is evolutionary a later inventionWith the specialization of the anterior portion towards brain function, the trunk, the largest portion of higher animals, was added during further evolution, for instance, to enable an efficient locomotion. The corresponding zone where this took place can be precisely localized in hydra. In a narrow zone between the tentacles and the gastric opening Goosecoid (yellow)  and the Brachyury (red)  homologues are expressed in adjacent regions. Since in vertebrates Goosecoid is involved in head patterning, Brachyury in tail formation, the trunk evolved and differentiated next to the posterior end of the ancestral organism. This sequence is recapitulated in the ontogenetic development of many animals including vertebrates, short germ insects and annelids in which the head is formed first and the trunk is added subsequently. In hydra Goosecoid and Brachyury are not yet separated by the trunk. This agrees with the observation that the typical HOX genes characteristic for the trunk in higher organisms with their 3'-5' co-linear arrangement have not been found in hydra. This supports the view that hydra preserves an ancestral axial pattern  before trunk formation and bilaterality evolved.