Ultrastructure of the body cavity lining in a secondary acoelomate, Microphthalmus cf. Listensis westheide (Polychaeta: Hesionidae)

Abstract

The organization of the body cavity lining in selected regions of the juvenile and adult of the interstitial hesionid polychaete Microphthalmus cf. listensis is described. Tissues comprising the body cavity lining in the juvenile consist of somatic and splanchnic circular and longitudinal muscles and undifferentiated cells. Somatic and splanchnic cell layers exhibit epithelial ( = eucoelomate) organization in the pharyngeal region. In the midbody, some undifferentiated cells exhibiting mesenchymal organization persist among the epithelially organized somatic and splanchnic cells, forming a gradation between eucoelomate and acoelomate tissue organizations. A coelomic cavity is absent. Tissues comprising the body cavity lining of the adult consist of somatic and splanchnic circular and longitudinal myocytes and coelenchymal cells. Coelenchymal cells are shown from serial section analysis to be mesenchymal in organization and derived from the somatic peritoneum. A 30-65-nm coelomic cavity lies between the apices of somatic and splanchnic cell layers in the pharyngeal region. In the anterior setigerous segments, the coelom is reduced to a narrow cavity surrounded by coelenchymal cells lying midventrally between the paired ejaculatory ducts. There is a regional obliteration of the splanchnic musculature in the posterior segments so that apices of the coelenchymal cells lie in direct apposition to the basal extracellular matrix of the gut. The coeom is only present middorsally as a 0.7-μm-wide cavity. Although the coelomic cavity is highly reduced in the adult, the body cavity lining still reveals its origin from the epithelial ( = eucoelomate) organization. The findings of this study illustrate possible organizational intermediates in the evolution of the acoelomate from the eucoelomate condition in annelids.

Larval development of Myzostoma cirriferum (Myzostomida)

“…Here we show that no coelomic cavity appears during larval development, thus strongly suggesting that the female genital cavity of adults does not differentiate from such a cavity and cannot therefore be homologous to a coelom, or at least not to the coelom of polychaetes. The acoelomate condition of myzostomids can be considered as an apomorphic feature, as is also the case in some interstitial polychaetes (Fransen, 1980(Fransen, , 1987Smith et al, 1986). According to Smith et al (1986), secondary reduction of the coelomic cavity of polychaetes may arise either by the obliteration of the formed coelom by mesodermally derived cells or by the total absence of a coelom during ontogeny, which is the case in M. cirriferum (present study).…”

Mesodermal Gene Expression in the Acoel Isodiametra pulchra Indicates a Low Number of Mesodermal Cell Types and the Endomesodermal Origin of the Gonads

“…Miniaturization per se does not necessarily imply that coeloms are lost ( e.g., interstitial priapulids [99] or hemichordates [100]). All clearly secondary acoelomate conditions show at least a coelom-anlage (interstitial polychaetes, [101], [102]) or the reduction of the coelom can be traced during embryogenesis as seen for example, in the anterior somitomeres of Branchiostoma [6] or in the dwarf male of the echiuran Bonellia [103]. Alternatively, the complete absence of the coelomic remnants could indicate the independent origin of the coelomic cavities in hemichordates and echinoderms, a possibility which has been suggested previously (Fig.…”

The role of character loss in phylogenetic reconstruction as exemplified for the Annelida

“…Species of the interstitial polychaete taxon Microphthalmus (Nereidiformia incertae sedis) lack coelomic cavities (Smith et al. 1986; Rouse and Pleijel 2001) and a progenetic origin was proposed for this taxon (Westheide 1987).…”

“…Species of the interstitial polychaete taxon Microphthalmus (Nereidiformia incertae sedis) lack coelomic cavities (Smith et al 1986;Rouse and Pleijel 2001) and a progenetic origin was proposed for this taxon (Westheide 1987). Whereas polychaete larvae only possess primary body cavities, coelomic cavities can be found in most adults (Rieger and Purschke 2005), which means that the lack of a coelom in adult Microphthalmus is interpreted as a juvenile trait.…”