FMRFamide IMMUNOREACTIVITY IN THE PERIPHERAL (ATRIAL) NERVOUS SYSTEM OF AMPHIOXUS (BRANCHIOSTOMA)

ABSTRACT

Many of the components of the atrial nervous system of amphioxus (Branchiostoma) are immunoreactive to antisera against the tetrapeptide FMRFamide. The atrial nervous system of the lancelet contains both motor and what are presumably sensory components, linked to the central nervous system via dorsal root nerves, but the role of many of these components is still unclear. There are large numbers of FMRFamide-like positive multipolar neurons over the gut and its diverticulum, separated from the gut by a system of thin coelomic spaces. Because their axons pass towards the central nervous system, they are presumably sensory. Again, although the rich gill plexus is presumably involved in ciliary control, the frequent immunoreactive cells on the gill bars do not seem to be ciliary-motor. There is also a rich subendostylar plexus containing FMRFamide-like material, perhaps secretomotor. In fact, the only immunoreactive fibers the function of which is evident are those on the pterygeal muscles flooring the atrium. The results are discussed in terms of the evolution of the vertebrate enteric nervous system.