Aspidosiphon (Akrikos) mexicanus (Murina, 1967)
This species does not have the typical aspidosiphonid appearance. The introvert arises in an atypical angle and it is up to 5 times the trunk length. The anal shield is almost nonexistent, and caudal shield is completely absent. The hooks are small, compressed unidentate, and scattered on the distal part of the introvert. Aspidosiphon mexicanus has been reported only in the Atlantic in sandy and muddy sediments in depths over 80 and up to 320 m.
Morphology
(From Cutler 1994, Cutler & Cutler, 1980, Cutler & Cutler 1989, Murina 1967)
Introvert is 4 to 5 times the trunk length, and arises at an angle of 45º to 60º with the main trunk axis.
Trunk in average from 3 to 25 mm length, 4 to 10 times the diameter. Many worms have fine transverse ridges in the skin (from species described by Cutler & Cutler, 1980).
On the distal introvert only scattered compressed unidentate hooks (< 30 µm tall).
The anal shield is weakly developed and composed of scattered, ill-defined platelets.
The caudal shield is absent.
Internal anatomy
The longitudinal muscle layer of the trunk wall is continuous.
Intestine attached anteriorly and posteriorly by spindle muscle.
Rectal caecum is present.
Nephridia are 35 to 75% of the trunk length and originate slightly posterior to the anus.
A pair of retractor muscles originates 70 to 90% of the distance to the posterior end of the trunk.
(From Cutler & Cutler 1980, Cutler 1994)
This species has been reported from the Southern Brazil, Cuba, the southeastern United States, and Azores.
(From Cutler & Cutler 1980, Cutler 1994)
This species lives in sandy and muddy sediments from 80 to 320 m depth.