The development of the catecholaminergic (CA) nervous systems in the larvae of the starfish Archaster typicus and the sea cucumber Actinopyga echinites was revealed through glyoxylic acid-induced fluorescence. In both species, the blue fluorescence, indicating the presence of catecholamine, is distributed over the surface of the early embryo. The larval CA nervous systems in these two species differ in two aspects: (1) occurrence of primitive neurons: primitive CA neurons appear at the apical region of the starfish embryo and then extend processes downward, whereas they occur throughout the sea cucumber embryo, then connect with axon tracts and become continuous; (2) distribution of a circumoral ganglion: a circumoral ganglion appears at the transverse preoral ciliary band of brachiolaria (starfish larvae), but at the lower lip of auricularia (sea cucumber larvae). The post-metamorphosed juvenile starfish has a pair of axon tracts in each tube foot and an axon ring around the ridge of the visceral body. In sea cucumber auricularia, the number of ovalshaped neurons at the lower lip increases following the development of the digestive system. The ontogenic development of echinodermal larvae is summarized briefly as follows: (1) neurotransmitters, such as catecholamines, exist in the embryo before the differentiation of the nervous tissues; (2) CA neurons first appear in the apical plate of the embryo at the gastrula stage; (3) neurons and axon tracts are scattered throughout the early larvae, and (4) finally, the CA nervous system is completed with the formation of continuous axons and neurons.
Echinoderm, Glyoxylic acid, Fluorescence
