NERVOUS ORGANIZATION AND THE PROBLEM OF THE SYNAPSE INACTINIA EQUINA

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Abstract
The nervous system of Actinia equina was studied by routine histological methods and by metallic impregnation techniques. Some preliminary results from electron microscopy are included. The organization of the nervous system of this species is more complex than that of other anthozoans; it consists of two interconnected nerve plexuses which are developed to differing degrees in various parts of the body. These are: (1) a superficial (outer) plexus lying in the ectoderm, and (2) a deeper (inner) plexus constituting the main nerve net, lying in the mesoderm. The former is composed of bipolar and multipolar nerve cells, and the latter of multipolar cells. Receptor cells in the ectoderm make contact with fibers of the ectodermal plexus. Processes from the mesodermal plexus run out to the muscle fibers. Connections between the receptor cells and the nerve processes of the superficial plexus and between the processes of the cteeper plexus and the muscle fibers appear to be of the discontinuous (synaptic) type. In the nerve nets themselves, although some connections resembling synapses have been seen, most of the nerve elements stand in direct connection with one another, so that the system must be regarded as at least partly syncytial. Evidence is given for the growth of the nerve net, in step with the general growth of the animal, by division of nuclei followed by their movement apart within the syncytium. The distribution of the nerve elements in various parts of the body, the interconnections between these regions, and the cytological characteristics of the cells are described. Ways in which excitation could pass from one part to another are discussed.

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