The lantern of Echinus esculentus (L.) III . The fine structure of the lantern retractor muscle and its innervation

Abstract

The lantern retractor muscles are attached to the calcareous pyramid structures by a diffuse connective tissue into which the proximal ends of the muscle fibres are inserted. The lantern musculature is invested by a compartment of the water vascular system, and the limiting layer is a thin sheet of ciliated cubical epithelium. The muscle fibres are about 8 to 12 μm in diameter and several millimetres in length. They contain many short myofilaments arranged longitudinally in the cell; these are 1 to 2 μm in length and 45 nm in diameter. The muscle cells show much folding of the surfaces, and there is interdigitation of the surfaces of two contiguous cells. The finger-like projections are often bulbous at the end, containing mitochondria and glycogen droplets. The attachment area of the muscles also contains many nerve fibres. These arise at the hyponeural ganglion, and pass in tracts up to the muscles. The fibres fall into two populations, one (85%) of presumed motor fibres being about 0.8 μm in diameter, the minority (15%) reaching only 0.25 μm. Each muscle cell is innervated by a single neuron at the proximal end. The neuromuscular junction is a short dilation of the nerve wrapped in wing-like process of the surface of the muscle cell. In this area the nerve terminal contains small synaptic-type vesicles, and the muscle cell is possessed of many mitochondria.