Academic literature on the topic 'Pentaradial symmetry'

The formation of pentaradial symmetry in the evolution of echinoderms was based on the possibility of the middle–left coelom to terminally forward growth along the anteroposterior axis and the appearance of a second growth vector along the left–right axis during the replication of the formed ambulacra. Both growth vectors were realized into the pentamerism of modern echinoderms due to the development of coelom asymmetry and subsequent torsion associated with the attachment of the larva to the ground by the anterior end of the body. In this process, the molecular genetic mechanisms of anteroposterior growth and left–right regulation, common to bilateria, and associated with the genes of the Wnt, BMP, Nodal signaling cascades, and Hox system genes, were probably used together. In the process of replication of channels extending from the ambulacral ring, the emerging ambulacral system was the organizer of the symmetry of the skeleton and the nervous and muscular systems. Replication in many fossil echinoderms ended on the three channels extending directly from the ambulacral ring. In crinoids, sea urchins, sea stars, brittle stars, and holothurians, the second stage of the formation of a more perfect five-ray symmetry of the ambulacral ring with five radial canals extending from it appeared, associated with a shift in ontogenesis of the branch point to the early stages of hydrocoel development.

Echinoid development progresses through embryonic and larval stages to metamorphosis and the adult form. Despite vast differences in embryos and larval forms, including bilaterally symmetric echinopluteus larvae, ovoid non-feeding larvae and brooded embryos, all metamorphose into juvenile sea urchins with pentaradial symmetry. The adult sea urchin body plan is initiated as the juvenile rudiment. The rudiment has been called the phylotypic stage for the class Echinoidea, a designation that implies little variation at this midpoint in development (e.g. Raff et al., 1991; Richardson, 1995; Raff, 1996). However, right at metamorphosis (upon eversion of the juvenile rudiment), variations in test symmetry, shape and number of spines, and number of skeletal plates, podia and pedicellariae are present in juveniles. This variation suggests either that there is no phylotypic stage or that such a stage occurs earlier in rudiment formation. To distinguish between these possibilities, I explored the patterns by which the juvenile rudiment is formed as well as the variation among juveniles approximately 1 day after metamorphosis in 19 echinoid taxa covering a broad taxonomic range including cidaroids, diadematids, irregular echinoids (spatangoids and clypeasteroids), arbaciids, temnopleurids, echinometrids and strongylocentrotids. Most of the material for analysis of juveniles was obtained by the author. Additional information was gathered from classical studies of metamorphosis. Data were collected on the number and shape of dorsal pedicellariae, juvenile and adult spines, primary and secondary podia, and juvenile test shape. When possible multiple individuals within a species were examined, revealing no or only minor trait variation. These data were mapped on a well-resolved phylogeny established from adult characters.

AbstractPalaeoscolecid worms are widespread in the Palaeozoic period, and are of key importance to understanding the emergence of moulting animals (superphylum Ecdysozoa). However, palaeoscolecids lack a diagnostic set of morphological characters, and as such are unlikely to form a natural (monophyletic) group. Consequently, detailed anatomical study of individual taxa is necessary in order to evaluate the phylogenetic significance of palaeoscolecids. New specimens of Mafangscolex from the Cambrian Stage 3 Xiaoshiba Lagerstätte in Kunming, China, provide the first detailed account of a proboscis in Palaeoscoelcida sensu stricto, a core group of palaeoscolecids characterized by having a tessellating scleritome of phosphatic plates and platelets. The eversible mouthparts of Mafangscolex comprise an armoured, hexaradially symmetrical introvert, a ring of coronal spines and quincuncially arranged pharyngeal armature, with a range of tooth morphologies. Taken together, this configuration strikingly resembles the proboscis arrangement inferred for the ancestral ecdysozoan. The six-fold symmetry represents an important difference from the pentaradial priapulan proboscis. The retention of key aspects of the ancestral ecdysozoan body plan suggests that palaeoscolecids represent a useful window on the earliest stages of ecdysozoan evolution.

The crystallographic orientations of echinoderm skeletal elements can supplement standard morphological comparisons in the exploration of echinoderm evolution. At a coarse scale, many echinoderms share a crystallographic pattern in whichcaxes radiate away from the axis of pentaradial symmetry. Within this common pattern, however,caxes of different taxa can differ dramatically in their degree of variability, angles of inclination, and relationships to the external morphology of skeletal elements. Crystallographic data reflect a variety of taxon-specific influences and therefore reveal different information in different taxa. In echinoids, orientations ofcaxes in coronal plates correlate well with high-level taxonomic groupings, whilecaxes of apical plates record modes of larval development. In blastoids,caxes of radial plates have a structural interpretation, with thecaxis oriented parallel to the orientation of the surface of the radial plate during its initial growth stages. In crinoids,caxes do not correlate with taxonomic group, plate morphology, or developmental sequence, but instead correlate with relative positions of skeletal elements on the calyx. Although their full potential has yet to be explored, the varied crystallographic patterns in echinoderms have been used to clarify skeletal structure, characterize developmental anomalies, and infer homologies of skeletal plates both within specimens and between groups. A axes are less constrained in their orientations thancaxes and offer less promise of revealing novel paleobiological information.