Siphonosoma arcassonense (Cuénot, 1902)
This species has scalelike chitinoid papillae on the introvert arranged in many rings. Internally the presence of many villi, well developed, along the contractile vessel and the posterior fusiform bodies distinguish this species from its congeners.
Morphology
(From Cutler and Cutler, 1982; Cutler, 1994)
External anatomy
Introvert shorter than the trunk. Scalelike chitinoid papillae, without sharp tip, tubular shape, attached to the skin except for the tip, arranged in up to 120 rings.
Trunk up to 240 mm in length. In preserved specimens the posterior tip of the trunk may be withdrawn 2-3 mm.
Internal anatomy
Longitudinal muscle anastomosed.
Ventral retractor muscles originate anterior to the dorsal retractor muscles.
Contractile vessel with many crowded, well developed, digitiform or shorter clavate villi.
Ventral nerve cord with numerous transverse nerves forming a plexus near the posterior end of the trunk.
Three or four thin fusiform bodies, 3-7 mm long.
Total length (trunk plus introvert) up to 540 mm (Stephen and Edmonds, 1972).
Stephen and Edmonds (1972) report that one of them found a live specimen of S. arcassonense while digging for Sipunculus nudus.
This species occurs rarely in beaches along the Atlantic coast of France and Spain (Cutler, 1994).