Aspidosiphon (Paraspidosiphon) laevis De Quatrefages, 1865
This species has only compressed unidentate hooks arranged in rings, well defined anal and caudal shields with extensive grooves or furrows, and two retractor muscles united for most of their length Aspidosiphon laevis is a widespread species, but is usually present in low densities in tropical shallow waters.
(From Cutler 1994, Saiz Salinas 1984)
External anatomy
Introvert commonly approximately as long as the trunk.
Tentacles up to 24 surround the nuchal organ. They are short and fused laterally.
On the distal introvert compressed unidentate, sharply, pointed or blunt hooks are arranged in many rings. The height of the hook (20-80 µm) is correlated with trunk length. Some scattered compressed hooks are present.
The anal shield is solid with 10-15 longitudinal grooves.
The caudal shield is conical and grooved. Saiz Salinas (1984) described 24 radial grooves for the type material.
Internal anatomy
The trunk wall has 25-35 longitudinal muscles bands, which often anastomose and be usually can be seen through the trunk wall. The circular muscle layer is subdivided in bands, but these are not always distinct.
Intestine attached anteriorly and posteriorly by spindle muscle. A distinctive feature of the latter structure is the bifurcation near its anterior end. The contractile vessel of many specimens has vesicular pouches or swellings along the portion united with the retractors.
Rectal caecum present and can be single or complex with lateral branches.
Nephridia are usually more than half of the trunk length.
A pair of retractor muscles, fused for most of their length, originates in front of the caudal shield about 65-80% of the distance towards the posterior end of the trunk.
Haldar (1991) reported one worm measuring 138 mm in length by 42 mm width.
(From Cutler, 1994)
Commonly present in low densities in tropical waters, this species is considered to be widespread. There are records in the Indo-West Pacific from Durban to Red Sea, in the Andaman Island, Malaya to southern Japan, Indonesia, the Great Barrier Reef, and islands east to Hawaii. It is also present in the western Atlantic Ocean from the 20ºS to 31ºN, the Caribbean, and in the eastern Atlantic from Canary and Cape Verde islands to the Gulf of Guinea.
This species is an inhabitant of coral rubble in shallow waters.