ABSTRACT
In Stylometra spinifera neuromuscular junctions were only found at the very ends of the interbrachial arm muscles, the region of neuromuscular contacts lying in the first 10–20 µm of the adjacent ossicle. Here the muscle fibres decrease in diameter and form long thin cytoplasmic processes that mostly lack myofilaments. Among these processes nerve fibres of about 0.5 µm diameter are to be found; they contain abundant microtubules and clear vesicles of approx. 70 nm diameter. The appearance of the vesicles makes it likely that they contain acetylcholine. Between some nerves and muscles typical pre- and postsynaptic membrane specialisations were observed. In Crinoidea the aboral nervous system innervates the flexor muscles, this nervous system contains numerous probably motor nerve fibres with exactly the same morphology as in the neuromuscular junctions. In tne aboral nerve we found few synapses.
