On the structure and innervation of the muscle bands of Doliolum (Tunicata: Cyclomyaria)

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Abstract
The ultrastructure of the muscle fibres composing the circular muscle bands of Doliolum is described; these muscle fibres are obliquely striated. Each fibre is elongate and multinucleate; peripherally the myofibrillar array borders the fibre, centrally there are long mitochondria with tubular cristae, and vesiculated sarcoplasm containing glycogen particles. Neither a sarcoplasmic reticulum, nor invaginations of the sarcolemma are present; in both gonozooid and oozooid stages myelin figures are frequent in mitochondria and sarcolemma. On atrial and external faces of the fibres there are nerve terminals containing electron-lucent vesicles some 50 nm in diameter. The junctional gap is ca. 10-20 nm wide and is devoid of the dense layer found in the vertebrate synaptic cleft. Each fibre probably receives a number of nerve terminals. The significance of the organization of the muscle fibres is discussed in relation to their role in the locomotion of the animal, and to the structure of other tunicate muscle fibres.