Echiura is one of the most intriguing major subgroups of phylum Annelida because, unlike most other annelids, echiuran adults lack metameric body segmentation. Urechis unicinctus lives in U-shape burrows of soft sediments. Little is known about the molecular mechanisms underlying the development of U. unicinctus. Herein, we overviewed the developmental process from zygote to juvenile U. unicinctus using immunohistochemistry and F-actin staining for the nervous and muscular systems, respectively. Through F-actin staining, we found that muscle fibers began to form in the trochophore phase and that muscles for feeding were produced first. Subsequently, in the segmentation larval stage, the transversal muscle was formed in the shape of a ring in an anterior-to-posterior direction with segment formation, as well as a ventromedian muscle for the formation of a ventral nerve cord. After that, many muscle fibers were produced along the entire body and formed the worm-shaped larva. Finally, we investigated the spatiotemporal expression of Uun_st-mhc, Uun_troponin I, Uun_calponin, and Uun_twist genes found in U. unicinctus. During embryonic development, the striated and smooth muscle genes were co-expressed in the same region. However, the adult body wall muscles showed differential gene expression of each muscle layer. The results of this study will provide the basis for the understanding of muscle differentiation in Echiura.

Coleoid cephalopods are excellent models for evolutionary and developmental studies due to their centralized nervous system, short life span, and sophisticated sense organs. Arm suckers, essential for predation, manipulation, and locomotion, have been studied in Decapodiformes, but little is known at the cellular and molecular levels. Here, we investigated sucker development in Octopus minor from the embryo to the juvenile stage using morphological, histological, immunostaining, scanning electron microscopy (SEM), in situ hybridization, and transcriptomic analyses. SEM revealed that suckers initially form symmetrically and later become asymmetrical through an embedding process. Histology showed progressive structural differentiation, while immunostaining with acetylated α-tubulin and phalloidin visualized nerve fiber and muscle development. Transcriptome profiling of embryonic stages 12, 14, and 20 identified 2,349 differentially expressed genes (DEGs) linked to arm and muscle development. Among them, Omi-Gata4, Omi-Mef2A, and WNT signaling molecules (Omi-Fzd9, Omi-Wnt2, Omi-Wnt5) exhibited increased expression in arms and developing suckers, consistent with in situ hybridization results. These findings suggest that WNT signaling contributes to sucker muscle development in O. minor, paralleling its conserved role in vertebrates. This study provides new insights into the embryonic morphogenesis of arms and suckers, highlighting the molecular and cellular mechanisms that shape cephalopod appendages.