The Anatomical Organization of the Nervous System of Enteropneusta
Authors: Bullock Theodore Holmes 
Published: April 2021

Results of a detailed study of the nervous system of Saccoglossus pusillus, with comparative material of about two dozen other species of Enteropneusta, are presented. The primary feature of the enteropneust nervous system is its position within the superficial epithelium. Pertinent relations with non-nervous elements of the epithelium are described. The indifferent, ciliated cells elaborate supporting fibres in those regions where the epithelium is well developed and nervous tissue is concentrated. Such cells are considered to represent neuroglia in its most primitive form. The fibres appear in places to be continuous with the ciliary rootlet cones. Nerve-cells are distributed diffusely in all the epithelia of the body, with certain exceptions such as the intestine, gills, coelomoducts, and the non-glandular areas of the abdomen of some forms. Both sensory and connecting and possibly also motor neurons occur here; but the sensory cells greatly predominate, often outnumbering all other epithelial cell-types combined. However, but one morphologic type of sensory cell— a true primary sense-cell—and no sense organs seem to be present. The thesis of Hanstrom is borne out that the low order of complexity of the nervous system as a whole is correlated with a low order of development of sensory structures. This in turn is correlated with a sluggish bottom living habit of life. The nervous tissue is shown to be conspicuously undifferentiated. All nerve-cell processes are alike and resemble the most primitive nerve-fibres. A single exception is formed by the giant nerve-cell fibres, of which a few dozen exist in the nerve-cords. The absence of strata, tracts, and special neuropile-like regions as well as of elaborate nerve endings, ganglia, nerves, and ‘nuclei ‘, is impressive. Following the neurologic principle that complexity of function is reflected in complexity of structure, this is taken to mean a low degree of functional specialization. Indications of several kinds agree in suggesting that the relations between neurons are something other than proto plasmic continuity. In the sense that nerve-fibres from different neurons are discontinuous the enteropneust nervous system is tentatively to be regarded as synaptic. Experimentally, however, the plexus has been shown to function as a nerve-net. It is proposed that such physiologic behaviour be taken to indicate a net in the sense of diffuse conduction, but that it does not predicate anatomical continuity of the fibres of the net. Such a picture requires the assumption of unpolarized synapses and the facts derived from the present organisms are taken to be evidence for this assumption. Other primitive characters are described. The synapses are unlocalized, being scattered throughout the plexus. No special structural modifications have been developed at the synaptic endings. Connexions with the interior across the limiting membrane, heretofore unknown, are astonishingly difficult to demonstrate, but they must be assumed to exist and evidence is accumulated that they are diffuse. The widely scattered sense-cells, synapses, ganglion cells, and connexions with the interior are correlated with, and account for, the experimentally demonstrated autonomy of small pieces of the body-wall. The general plexus is locally thickened and modified (1) in the cords of the mid-dorsal and mid-ventral lines of the trunk, (2) circularly around the junction of the collar and trunk, (3) through the dorsal collar coelom as an internal, primitively hollow, medullary strand, and (4) on the dorsal side of the peduncle. These are primarily conduction paths and are only secondarily important as ganglionic or modifying regions. The ventral cord in the trunk is shown to be larger and more important than the dorsal. In the sense of an organ which is involved in all reflexes, which contains all the intermediate neurons, and to which pass all sensory nerve-fibres, the balanoglossid has no central nervous system. Internal to the limiting membrane no concentrations of nervous tissue are known with certainty to occur. No nerves, ganglia, or layers have been developed. As yet inadequately demonstrated, the internal nervous sytem can at most be only a sparse and diffuse system of cells and fibres communicating across the limiting membrane with the superficial plexus, at the least nothing but motor axons passing from cell-bodies in the integument inwards to the muscles. The histologic evidence supports the previously demonstrated physiologic picture placing the Hemichordata in respect to the level of complexity of the nervous system below all other groups of animals with nervous systems except the coelenterates and ctenophores. No evidence is adduced that this primitiveness is secondary rather than original. In numerous histologic respects the enteropneust nervous system resembles that of Echinodermata and Phoronidea, but is simpler than either. The chordate affinities of the balanoglossids are here accepted. But the strength of the argument from the nervous system is considered to have been overdrawn. No aspect of the general picture of primitiveness now demonstrated is, however, considered to argue against these affinities.