Abstract
The fine structure of the soft tissues at the bases of spines in the sea-urchin Echinus esculentus has been examined with particular reference to the innervation of these appendages. The basal nerve ring encircling the spine contains many somata of neurones, and circumferentially directed elements, as well as tangled neuropil. The smooth muscles that bring about spine-pointing movements are innervated by terminals that contain two different types of vesicles, suggesting dual innervation by neurones containing different neurotransmitters. The neuromuscular junctions include apparent synapses between nerve cell bodies and muscle fibres. There are also neural elements that may be involved in the control of the catch apparatus of the spine. The complexity of the nerve ring and effector innervation implies that coordination of spine movements is more sophisticated than has been previously supposed.
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