The evolution of the serotonergic nervous system

The pattern of development of the serotonergic nervous system is described from the larvae of ctenophores, platyhelminths, nemerteans, entoprocts, ect oprocts (bryozoans), molluscs, polychaetes, brachiopods, phoronids, echinod erms, enteropneusts and lampreys. The larval brain (apical ganglion) of spi ralian protostomes (except nermerteans) generally has three serotonergic ne urons and the lateral pair always innervates the ciliary band of the protot roch. In contrast, brachiopods, phoronids, echinoderms and enteropneusts ha ve numerous serotonergic neurons in the apical ganglion from which the cili ary band is innervated. This pattern of development is much like the patter n seen in lamprey embryos and larvae, which leads the author to conclude th at the serotonergic raphe system found in vertebrates originated in the lar val brain of deuterostome invertebrates. Further, the neural tube of chorda tes appears to be derived, at least in part, from the ciliary band of deute rostome invertebrate larvae. The evidence shows no sign of a shift in the d orsal-ventral orientation within the line leading to the chordates.