SEROTONIN-IMMUNOREACTIVE AND FMRFAMIDE-IMMUNOREACTIVE NERVE ELEMENTS IN THE CHITON LEPIDOPLEURUS-ASELLUS (MOLLUSCA, POLYPLACOPHORA)

The distribution of serotonin-like and FMRF-amide-like immunoreactive (5HT-ir and FMRFa-ir, respectively) neurons in the nervous system of t he chiton Lepidopleurus asellus (Mollusca, Polyplacophora) was studied using an immunocytochemical technique. The neurons were distributed i n characteristic patterns in the central nervous system, the 5HT-ir ne urons predominating in the ventral (pedal) cords and FMRFa-ir neurons in the lateral cords. In the body wall including the foot, a tight net work of 5HT-ir and FMRFa-ir nerve fibers is found, the former being mo stly attributed to the musculature whereas the latter seems to be asso ciated with the blood sinuses. Intraepithelial neurons of both types a re abundant in the fore- and hindgut. The presence and general distrib ution in the central and peripheral nervous system of the 5HT-ir and F MRFa-ir elements seems thus to be similar in simple and advanced mollu scs. The relationship between these neurons and their targets in the b ody also appears to be well conserved in molluscs.