The lantern of Echinus esculentus (L). II. Fine structure of hyponeural tissue and its connexions

Abstract

The innervation of the lantern muscles of Eschinus is accomplished via ten discrete areas of nervous tissue. These patches of nerve material have long been known as the hyponeural tissue. They lie one on either side of each radial nerve cord, but associated with the circumoral ring. They show some differentiation into specialized areas, the hyponeural spurce and the hyponeural boss. Two main efferent tracts arise at the hyponeural area and pass towards the lantern muscles. A tract of nerve fibres enters the hyponeural region from the circumoral ring, this then divides within the plaque to ramify among other fibres. The interaction of the fibres takes place in an area of entanglement of neuropile that has led to the suggestion that hyponeural ganglion is an appropriate name for the area. Nerve cell bodies are present in this ganglion. They are about 10 um diameter and the fibre size varies from 0.1 to 2.0 um. Glia is sparse in the ganglion. Vesicles have been described in the cell bodies and in the fibres. They fall into two types large membrane limited vesicles (70 to 100 nm diameter) with dense material centrally; and small (40 nm) vesicles. The former type are to be found in cell bodies, and in some fibres. The latter sort are typical of neuromuscular junctions, axo-axonic synapses and in a specialized type of neuron reminiscent of the synaptic frontal bag: amacrine cell system of Octopus. Axon-axon synapses are common in the region of the hyponeural boss and in the neuropile between the motor tracts. The motor tracts running to the muscles from the hyponeural ganglion contain two distinct fibre populations. These show mean diameters of 0.8 and 0.25 um respectively. It is suggested that these represent motor and sensory neurons respectively though there is as yet no definite evidence for this.