With staining by antibodies, the general morphology and topography of serotonin-containing cells (SCC) and cells expressing high levels of actin have been compared between three developmental stages in ‘Echinopluteus transversus’ type larvae of sea urchins from genus Diadema. At an early pluteus stage single SC cells are found in the preoral buccal lobe of the pluteus as a component of the ciliary band. The cells expressing high levels of actin form the oesophageal muscles. At the middle pluteus stage, SC cells form two ganglion-shaped clusters in the anterior part of the larva, as well as a distributed neuropil running along the ciliary band; the bodies of the SC cells are included in the ciliary band epithelium. At this developmental stage the oesophageal musculature and little dilator muscles at the mouth were visualized. In late pluteus stage, the ganglion-shaped groups of SC cells merge into one in the anterior part of the larva; the distributed neuropil in the ciliary band is preserved. At this developmental stage, the cells expressing high levels of actin are in the oesophagus muscles and in three separate well developed muscles: ventral adductor muscle, dorsal adductor muscle, and abductor muscle. These muscles carry out movements of the larval arms.