Fig. 2

Schematic representation of the apical and frontal organs in Nemertea based on morphological data of neurogenesis and ultrastructure in the studied species. a The apical organ of 2 d post-fertilization (dpf) larva of Quasitetrastemma stimpsoni consists of an apical plate, frontal gland, and apical neurons. Frontal gland includes mucoid (mc), bacillary (bc), and granular (gc) cells. Apical neurons (an) are located beneath bodies of gland cells and connected with the brain rudiment (br) and the apical plate cells (apc) via neurites of apical neurons. b In the apical organ of 3–4 dpf larva of Q. stimpsoni, some apical plate cells possess 5-HT-like immunoreactivity (5-HT-lir apc). The frontal gland has four glandular cell types: mucoid cell (mc), granular cell (gc), bacillary cell type I (bcI), and bacillary cell type II (bcII). The pairs of subapical-plate neurons (sapn) are visible. c In the apical organ of 7 dpf larva of Q. stimpsoni, all apical plate cells are 5-HT-lir. The apical neurons (an) processes are resorbed. d The frontal organ of the adult worm of Q. stimpsoni. e The apical organ of pilidium prorecurvatum larva (the scheme was created according to [36]. Abbreviations: 5-HT-lir apc–5HT-like immunoreactive apical plate cell; ac–apical cilia; an–apical neuron; apc–non 5HT-like immunoreactive apical plate cell; at–apical tuft; br–brain rudiment; bc–bacillary cell; bcI–bacillary cell type I; bcII–bacillary cell type II; gc–granular cell; mc–mucoid cell; ep–epithelium; sapn–subapical plate neuron