The Ultrastructure of the Apical Organ of the Goette's Larvae of the Polyclad Flatworm Stylochus pilidium Indicates Homology Between Polyclad Larvae.

Polyclad flatworms exhibit both direct and indirect development, with various larval types observed, including Müller's larva, Kato's larva, Goette's larva and Curini-Galletti's larva. The different larval types are distinguished by shape, number of eyes and number of lobes. The Goette's larva of Stylochus pilidium exhibits a distinct six-lobed morphology, with one cerebral and one epidermal eye. The posterior half of the larva features a posterior tuft and four lobes, two of which are ventrolateral and two of which are lateral. Anteriorly, a larger lobe called the oral hood is situated ventrally, opposite a smaller dorsal lobe. The larval types share a distinct feature, known as the apical organ, which is located at the anterior tip of the larvae. Here, we investigated the ultrastructure of the apical organ in Goette's larvae of the polyclad S. pilidium. Six apical tuft sensory (ATS) cells are at the centre of the apical organ, encircled by a ring of apical tuft gland (ATG) cell type 1 necks. These cell necks merge into two distinct gland cells that extend dorsoposteriorly and terminate posterior to the brain. Two epidermal apical tuft anchor (ATAn) cells encircle the ATS cells and the central gland cell necks. Additionally, four ATG cell type 2 necks, which are distributed symmetrically around the ATAn cells, merge into a single cell and extend ventrally to the level of the cerebral eye. The third type of ATG cells is in a circular pattern around the anchor cells, with necks in the epidermal layer. The ultrastructural arrangements of the apical organ in Goette's larva of S. pilidium are very similar to those of previously studied polyclad larvae, supporting the hypothesis of a common origin of larvae within Polycladida.