Solutans (Echinoderms): Evolution Frozen between Torsion and Pentaradiality.

The variability of the location of the mouth along the theca in different genera of solutans indicates the presence of torsion in their ontogeny and is explained by a paedomorphic delay or overdevelopment of this process. The plane of change in the location of the mouth with the feeding appendage completely covering it corresponds to the larval plane. The flattening of the theca of most solutans is a secondary result of the transition to lying on the substrate. Larvae or young individuals of different taxa lay down on the substrate on their right or left side. This explains the opposite direction to the ground of adult animals of some taxa. Solutans branched off from the main stem of echinoderms before the origin of pentaradial symmetry, but after the appearance of torsion in their ontogeny. In their level of organization, they appear to be neither directly related to hemichordates, nor to be strongly modified Blastozoa. The appearance of the torsion in the ontogeny of echinoderms phylogenetically precedes the emergence of pentamerism but does not necessarily lead to it. The inversion of the anterior Hox-genes is a consequence, but not the cause, of torsion and pentamerism. [ABSTRACT FROM AUTHOR]