Fine‐Structure and Evolutionary Implications of the Frontal Organ in Turbellaria Acoela. 1 Diopisthoporus gymnopharyngeus sp.n.

Abstract

Although a variety of larval and adult invertebrates possess an anterior ‘frontal organ’, ‘frontal gland’ or ‘apical organ’, the degree of functional or phylogenetic relatedness of these various organs is largely unknown. The frontal organ in primitive turbellarians is important for phylogenetic analysis both within and without the Turbellaria, a group whose phylogenetic position in the Metazoa is of crucial importance to many current theories of bilaterian origins. In the present paper, the ultrastructure of the frontal organ in Diopisthoporus gymnopharyngeus sp.n. is studied, in connection with a description of this new acoel turbellarian species from North Carolina, U.S.A. The frontal organ of D. gymnopharyngeus comprises two identical gland cells that open at the exact apical pole of the body. There are no specific sensory associations with the opening of the frontal glands and, therefore, the frontal organ in this species is best regarded as purely glandular. This constitutes a major difference between the frontal organ in D. gymnopharyngeus and the glandulo‐sensory apical organs of the Müller's and Götte's larvae of polyclad turbellarians, as well as the purely sensory apical organs of the larvae of other invertebrates investigated to date. Similarities between the body wall glands and the frontal glands in D. gymnopharyngeus, coupled with the generally low level of differentiation of the frontal organ in this species, are in accord with von Graff's hypothesis that the frontal organ in acoels evolved from an anterior accumulation of body wall glands, and support the relatively primitive position of the Diopisthoporidae within the Acoela postulated from earlier studies of the parenchyma.