The arrangement and ultrastructure of the musculature, nerves and epidermis, in the tail of the cercaria of Cryptocotyle lingua (Creplin) from Littorina littorea (L.)

Abstract

The ultrastructure of the caudal muscles of the cercaria of Cryptocotyle lingua is related to the rapid swimming movements. Dorsal and ventral striated longitudinal muscle cells extend in a straight line along the tail length each containing a single myofibril, U-shaped in transverse section and divided into sarcomeres. A-, I- and H-bands are recognizable and the fragmented Z-band consists of a row of dense bars to which the thin myofilaments are attached. A pair of proximal ventro-lateral striated muscles may be responsible for the figure of eight pattern of movement. Large mitochondria are abundant in the non-fibrillar region of the muscle cells. Tubular sarcoplasmic reticulum envelops the myofibril giving off branches at the Z-bands which extend into and across the myofibril alternating with the dense bars. T-tubules are absent but cisternae of the sarcoplasmic reticulum form numerous dyadic junctions with the sarcolemma. Dorsal and ventral motor caudal nerves arise from a nerve plexus behind the tail root; axo-axonal and neuromuscular synapses are frequent. Clear and dense synaptic vesicles occur in the presynaptic terminals.