The organization of the atrial nervous system of amphioxus (Branchiostoma lanceolatum(Pallas))

Abstract

The Acrania possess an histologically complex peripheral nervous system, the atrial nervous system, lying just under the epithelium lining the atrium and covering the various organs within it. The system contains both sensory and motor components, and is especially rich in peripheral sensory cell bodies. It is in connexion with the central nervous system by way of the dorsal root nerves. Most of the motor axons entering the system pass to the cross-striated pterygial muscle flooring the atrium, others pass to the cross-striated trapezius muscles, and there is also a large ciliarymotor component, which controls the action of the lateral ciliary tracts of the gill bars. Unipolar sensory neurons are abundant upon the surface of the pterygial muscle, and are also found upon the parietal walls of the atrium. Multipolar sensory neurons are abundant upon the foregut and its diverticulum. The hindgut (outside the atrium) is more sparsely innervated, but occasional multipolar sensory neurons occur there. The multipolar neurons of the foregut and diverticulum appear to be in connexion one with another asynaptically, but their axons pass to the central nervous system. Similar sensory neurons of several types are found in the richly innervated atrio-coelomic funnels. The function of the atrial nervous system is not yet entirely understood, but it is probable that it is mainly concerned with the regulation of the feeding process, and with spawning. It is concluded that the system is not evidently homologous with the ‘sympathetic’ systems of the craniates, and that it is unwise at present to attempt to homologize the visceral nervous systems of the two groups.