Two types of bilateral symmetry in the Metazoa: chordate and bilaterian.

  title={Two types of bilateral symmetry in the Metazoa: chordate and bilaterian.},
  author={Richard P. S. Jefferies},
  journal={Ciba Foundation symposium},
          94-120; discussion 121-7
The chordate sagittal plane is perpendicular to the sagittal plane primitive for the bilaterally symmetrical metazoans (Bilateria). The earliest metazoans, when symmetrical at all, were probably radial in symmetry. The axis of symmetry was vertical and the mouth, when present, opened either upward or downward. The Bilateria evolved from the primitive metazoan condition by acquiring bilateral symmetry, mesoderm, a brain at the anterior end and protonephridia. Perhaps in the stem lineage of the… 

Acquisition of the dorsal structures in chordate amphioxus

The striking similarity in the gene regulations and their respective expression domains when comparing dorsal formation in amphioxus and the determination of the oral ectoderm in sea urchin embryos suggests that chordates derived from an ambulacrarian-type blastula with dorsoventral inversion.

A “rays-as-appendages” model for the origin of pentamerism in echinoderms

A new hypothesis concerning the evolutionary origin of pentameral symmetry in echinoderms is presented and it is proposed that a duplication of the uniray occurred and resulted in the addition of a pair of rays that followed Bateson's rules of symmetry to form a three-rayed 0-1-2, —A-BA, construction.

Amphioxus mouth after dorso-ventral inversion

The observations suggest that there are at least three types of mouths in deuterostomes, and that the new acquisition of chordate mouths was likely related to the dorso-ventral inversion that occurred in the last common ancestor of chordates.

Hydromechanical considerations on the origin of the pentaradial body structure of echinoderms

These evolutionary transformations were accompanied by certain histological modifications, such as the development of mutable connective tissues and skeletal elements that fused to ossicles and provided shape stabilization in the form of a calcareous skeleton in the tissues of the body wall.


It is confirmed that reduction in the number of marginals is a general trend in boot-shaped cornutes and that Phyllocystis is probably more closely related to these two clades than to scotiaecystids.

A cladistic analysis of the anomalocystitid mitrates

The anomalocystitid monophyly is supported by parsimony, although the instability of several basal taxa makes it difficult to ascertain the sequence in which characters were acquired or modified in the transition from the mi-trocystitids to the anomalocyStitids.

The oldest cinctan carpoid (stem-group Echinodermata), and the evolution of the water vascular system

Through the study of original specimens and ‘virtual fossils’ it was possible to reconstruct the palaeobiology of Protocinctus gen. nov.: cinctans are interpreted as pharyngeal basket feeders with a U-shaped gut, using their posterior appendage to aid stability on the sediment surface.

Is ventral in insects dorsal in vertebrates

The observations and arguments of earlier embryologists who tried to compare insect and vertebrate development by inverting the dorsoventral axis in one of the groups, whether just formally or — after Darwin’s Origin of species — in the assumed course of evolution are reviewed.

Is ventral in insects dorsal in vertebrates?

The observations and arguments of earlier embryologists who tried to compare insect and vertebrate development by inverting the dorsoventral axis in one of the groups, whether just formally or after Darwin's Origin, are reviewed.

Additional molecular support for the new chordate phylogeny

Maximum‐likelihood using standard models and Bayesian phylogenetic analyses using the CAT site‐heterogeneous mixture model of amino‐acid replacement both provided unequivocal support for the sister‐group relationship between tunicates and vertebrates (Olfactores).



The English Silurian fossil Placocystites forbesianus and the ancestry of the vertebrates

Placocystites forbesianus de Koninck, from the Silurian Dudley Limestone, is here interpreted as a primitive chordate with a calcite skeleton of echinoderm type, which agrees with earlier papers by the senior author and disagrees with the work of Ubaghs.

The development and certain points in the adult structure of the starfish asterias rubens, L.

The Asteriadse rank among the Cryptozonia of Sladen (87, xxiii et seq ., xxxiv), since they possess reduced marginal plates, while their papulae or gills are not confined to that part of the

Feeding Posture of Modern Stalked Crinoids

Observations and a review of deep-sea photographs indicated that modern crinoids favour an environment with moderate currents and are to some degree current-seeking (rheophilic)5.

The Vertebrate as a Dual Animal — Somatic and Visceral

In the study of vertebrate anatomy and embryology, one frequently encounters the terms “somatic” and “visceral”, which are merely descriptive, as regards external or internal position.

Earliest known echinoderm — a new Ediacaran fossil from the Pound Subgroup of South Australia

Arkarua adami gen. et sp. nov., a small disc-like animal with a pentameral arrangement of grooves on its oral surface, is preserved as an external mould on the base of sandstone beds within the


1. Living echinoderms are characterized by an extensive water vascular system developed from the larval left hydrocoel, a complex, multi‐plated endoskeleton with stereom structure, and pentamery.

VI—Caradocian Cystidea from Girvan

  • F. A. Bather
  • Biology
    Transactions of the Royal Society of Edinburgh
  • 1913
The Scottish specimens described in this memoir are all the property of Mrs Robert Gray, and I am exceedingly indebted to her, not only for entrusting them to me for description, but for allowing